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Nad 83 - Wgs 84

#1 User is offline   DaveD 

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Posted 03 May 2005 - 11:29 AM

The question was posed; arenít NAD 83 and WGS 84 the same? The answer is yes and no. First letís define a horizontal datum. All horizontal datums are composed of 8 components:

3 Ė Define the origin of the coordinate system
3 - Define the orientation of the coordinate system
3 Ė Define the size and shape of the reference ellipsoid (mathematical model of the Earth).

Letís start with the easy part. The ellipsoids, GRS 80 for NAD 83 compared to WGS 84 for WGS 84 are for all practical purposes identical. While there are small numerical differences between them in a practical sense they amount to less than 0. 1 mm across all of North America.

There is also a Federal Register Notice --
http://www.ngs.noaa.gov/PUBS_LIB/FedRegist...doc95-19408.pdf, that states they can be considered identical for all mapping and charting applications at scales of 1:5,000 or smaller.

In real geodetic terms, NAD 83 (CORS96) which is the highest accuracy realization of the datum differs from WGS 84 (G1150), which is what WGS 84 is technically referred to, by about 1 m each in the horizontal and ellipsoid height. This is because WGS 84 (G1150) uses a more contemporary realization of the location of Earth mass center than is used in NAD 83 (CORS96). For purposes of users of hand-held GPS this means they are identical. Virtually all GPS receivers use the transformation parameters defined by the U.S. Defense Department in their technical report 8350.2 ďWorld Geodetic System 1984 Ė Itís Definition and Relationship to Local Geodetic Systems,Ē -- http://earth-info.ng...G/tr8350_2.html, page B.6-9 which shows the differences as 0 in each component (+/- 2 m). Over the last 10 years, the Defense Department has been attempting to bring the various iterations of WGS 84 closer in alignment with the International Terrestrial Reference Frame (ITRF) which is the best global set of coordinates and used by NGS as the foundation for NAD 83. The current ITRF, referenced as ITRF 2000 or ITRF00, relates to NAD 83 (CORS96) as indicated in the tables prepared by NGS -- http://www.ngs.noaa....RS/coordinates/. The bottom line, if youíre positioning capability is worse than 3 m then they are the same, if youíre better than 3 m then they are not.

#2 User is offline   Klemmer & TeddyBearMama 

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Posted 03 May 2005 - 11:52 AM

Interesting. Thanks for the straight info, Dave.

#3 User is offline   evenfall 

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Posted 03 May 2005 - 01:29 PM

Thank you Dave,

That supports what I have been explaining a great deal. I appreciate you weighing in on this.

Rob

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Posted 04 May 2005 - 07:43 AM

DaveD -

Interesting topic and reference. According to the ellipsoid parameters, the WGS84's ellipsoid's pole-to-pole distance is 0.21 millimeters longer than the NAD83 ellipsoid. This is about like 2 sheets of paper sitting on the North Pole versus no sheets of paper. The equatorial diameters of WGS84 and NAD83 are the same as each other, and are about 42,769 meters larger than the pole-to-pole distance (an oblate spheroid).

Once again, though, THE QUESTION hasn't been explicitly answered.

What is THE QUESTION? An example of it, using GH0669, whose coordinates are:
NAD83: 36 42 47.13094(N) 097 05 12.28316(W),
the question is - how far from that point, in millimeters, is:
WGS84: 36 42 47.13094(N) 097 05 12.28316(W)
?

(I realize the answer in millimeters would be different in different parts of the country, so I picked somewhere vaguely in the middle.)

I recall trying a couple years ago, and could not find a converter program between WGS84 and NAD83. All such programs said there's no appreciable difference between them.

#5 User is offline   evenfall 

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Posted 04 May 2005 - 01:55 PM

BDT,

I realize you are addressing Dave here, but I have attempted to explicitly answer these questions in several different posts I have made. Let me try one more time, and if you like, you can look at my old posts here to research through.

As I have said in the past, WGS84 and NAD83 are from different agencies who have different missions. They use different standards because in part WGS84 is trying to be a best fit for the earth, and NAD83 is trying to be a best fit for North America. You could say it is a harder look at just one continent, to describe it as close as it can. As you might expect, there are different folks working at each agency and they are going about this each in the way they think is best, so it is safe to say that there is a differing opinion between them as to what the best geodetic solutions are, and you would be right.

From this I am hoping that you understand that comparisons between NAD83 and WGS84 are not routinely performed. Dave has mentioned and so have I that there is an approximate difference 1 meter max, between the datum. Problem is, that this is only true at the earth center interpretation so if you imagine two ellipsoidal reference spheres that are the same size sitting in the same place but with earth centers that are their own and only 1 meter apart then the surfaces (known as the ellipsoidal heights) would not be in the same place, they would overlap slightly, so only a few spots on the globe would be the same from one to the other and all the rest of the places would be between 0 and one meter apart, but mostly somewhere in between the two extremes. the orthometric heights will differ all over as well, and that is not proactive either. Bring two different Geoidal models to the table and compare them and you just have something that is close to mind numbing on a point by point basis.

Two compare the one spot like you want is doable I suppose, but the comparison would be unique to that location. Professionally it isn't done because the civil engineering world does not consider the WGS84 Datum as relevant to North America. The Standards are not as high as NGS for North America and it is a Commerce thing not a Defense thing. So it is a sort of quasi political - engineering thing.

Due to the quasi thang, nobody has a comparison calculator out there on a web page, because the plan is to stick to the plan and the plan is to use either one Datum or the other, and not compare the two. Partly political, partly engineering. NAD27 and NAD83 Datum Shifts are Compared due to Propriety reasons and due to NAD83 replacing the NAD27 as a standard datum. WGS84 And NAD83 do not supersede each other, they are just two different Datum for different reasons. WGS84 is not a close look at North American Geodesy nor is it used for civil work, Mostly it is a non player in the game.

Bottom line, the appreciable differences are there though small, they are mainly only interesting to geodetically related things, that is a focus on things very small and most of the world is focused on bigger. This is one of the scientific studies that fit in the box with superconductivity and absolute zero. The closer you come, the harder it gets.

I have tried to answer your questions, I hope to your satisfaction. Maybe Dave has more he can add, but that is pretty much what most of us in the field need to understand to do the work we do. I didn't do the math for you because I don't really have a way to frame both references available to me in order to compare them at the one spot. Calculating ing Datum shifts at a specific place is a lot of number crunching, is not for the meek and besides, Bleh!

Rob

This post has been edited by evenfall: 04 May 2005 - 02:03 PM


#6 User is offline   Klemmer & TeddyBearMama 

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Posted 04 May 2005 - 03:12 PM

A tiny comment:
WGS84 was also designed to work for aircraft & spacecraft.
I rather doubt that was much considered in designing NAD83 <_< :huh:
As Rob said, different mission.
:blink:

#7 User is offline   Black Dog Trackers 

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Posted 04 May 2005 - 03:24 PM

I've heard plenty of non-numeric rhetoric on this subject. I skimmed, saw no numbers. <_<

I am still waiting for a real, numeric, answer with just one PID or other example using a pair of real coordinates.

#8 User is offline   evenfall 

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Posted 04 May 2005 - 04:26 PM

BDT,

You can cross your arms and expect the ocean to wash an answer up in front of you if you like, But I won't be joining you! :-D I have already answered this exhaustively. What I am saying is that there are no datum shift conversions bro, Nobody bothers. Use the appropriate Datum for the job is the answer. But I clearly stated that in every essay I ever wrote to help you understand this. :-) I just hope that you will get it.

Unless someone goes out of their way for you, and I wouldn't hold your breath to long. If it does happen for you, and it could, Please Consider that someone went out of their way just for you and only you, because that is what it will be. Be Honorable towards them for doing you such a favor.

Oh and what I told you are truths which you can easily corroborate, nothing rhetorical there. I tend to steer clear of rhetoric in my conversations and communications. I work in the field but that is no reason why you should believe me. I have no credential that means anything to you, nor do I need to. You are a Skeptic and I am used to those, I see them every day :-)

Nobody out there has convincing you as a part of their mission, and I think you have the best answer you'll get unless you roll up your own sleeves and do the figuring. :-)

Be my Guest. :-D

Good Luck BDT!

As Always,

Rob

#9 User is offline   bicknell 

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Posted 04 May 2005 - 04:51 PM

Based on an earlier conversation, if I understood what was said about the professional (eg, 1 cm) survey gear was that it was "datumless" and that the datum was post processed by a computer. If that understanding is basically correct, what I would ask is this:

Next time one of the professionals here survey a triangulation station (or any high precision horizontal control) and have the precise data, can you post process it to NAD27, NAD83, and WGS84, and post the full coordinates in each datum here?

I'm hoping that's the more correct way to ask the quesiton.

#10 User is offline   Black Dog Trackers 

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Posted 04 May 2005 - 07:33 PM

bicknell -

Yep, that would work too!

#11 User is offline   evenfall 

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Posted 04 May 2005 - 07:50 PM

Bicknell,

You can do this between NAD27 and NAD83 already with the NADCOM program that NGS has online. But it does not give you the formula to make the datum shifts. There are many factors used in developing a datum and each Datum has it's own. it is a lot to compare.

Nobody adjusts between NAD and WGS. To be honest it is pointless to do it. I mean you use a shovel to shovel and a rake to rake, it is almost that clear, We use one for this and not the other.

I could get you that number for a location of my choosing, but I won't. Maybe someone will. The Instrument would give you a difference, but the number you get would be a result for that location, NOT a number you could reverse engineer into a formula, because between the Datum there are virtually few places where the ellipsoidal heights would compare due to different earth centers. Depending on where you compare, the difference could range from zero to 1 meter, and this is not going to take in to account orthometric differences that are not allowed for by the WGS in North America if I am not mistaken. See, CORS Data is watching this all the time. But even at any two different CORS stations, the Datum Shift would be different, and dependent on the Latitude Longitude and elevation, etc. It would be wrong to think that number is a rule, as in a hard fast rule, but a skimmer for numbers is likely to misinterpret it as a hard fast rule and not the difference in datum shift for one location. In the end I feel most people would misunderstand what the number means and globalize the result as an answer. Just my opinion but I have seen it happen with less.

There are a lot of geodetic equations in play. That is in the realm of Geodesy, and Sorry, I am not a geodesist. It is a bit more than I need to know. Interestingly there is a lot about Surveying that Geodesists don't know too. We are all a bunch of specialists in the end. In fact even at NGS you might have to speak with a geodesist who specialized in Horizontal Datum shift work to get a person who is up on that information. That is 8 components to compare, datum to datum and it is only right for one spot on earth. Sorry. My Advice is Use NAD 83 and be happy there are people who enjoy Geodesy!

Thanks for the question.

Rob

#12 User is offline   Z15 

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Posted 05 May 2005 - 03:14 AM

Here you go, for $198 you can buy Software to do this, or try the free download.... 24+ mb



Quote

Our datum conversion product DatumPro converts between a host of datums and projections, including Gauss Kruger, UTM, and Lambert Conic grids - all with an easy to use user-interface. With over 600 datum conversions, over 450 Grid Projections each way, hundreds of specific TM and Lambert conic conformal grids and all the latest EPSG transformations - DatumPro is the most comprehensive and accurate package available for co-ordinate conversion. While DatumPro is ideal for conversion to and from WGS84, it also directly converts between other datums.

The integrated DatumPro Editor enables a user to set up their own custom transformations if available.

Over 600 Datum Conversions - plus inverses
Over 450 Specific Grid Projections
Generic Grids: TM, UTM, Gauss Kruger 3/6 Degree, Lambert Conic Conformal 1/2 SP
Output: File / Clipboard - e.g. to Excel, Word etc / Screen
Single Point: Keyboard or File / Clipboard Processing
Multiple Points: ASCII Text Lists (NMEA etc)
Colour Plots of Datum Differences.
Reads and writes virtually any format eg D, DM, DMS, DDMMMSSS, N, E etc
OS: Windows 95 / 98 / ME / NT4 / 2000 / XP



All our GPS projects started out as WGS84 positions (we were setting control for others to use on projects). Once we had a solution in state plane, we never would go back to WGS84 as it would not give us a solution we could use. We mapped many projects using WGS84 (default datum for GPS) but as soon as the PS solved the network, it was converted to state plane by SkiPro/Leica software, a simple matter takign a nano-sec with the software. No projects were ever completed and built in WGS84.

This post has been edited by Z15: 05 May 2005 - 03:30 AM


#13 User is offline   Prof. Y. Lupardi 

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Posted 05 May 2005 - 01:58 PM

The coming Galileo system will not use WGS84 but international standards like ITRF.

Positions in your local system were measured in the past by triangulation.
Now we can measure with GPS long baselines with very much more accuracy so all errors made in the triangulation process became measurable. Those are (in my small country at least) less than 1 meter. Whole areas are decimeters shifted wrongly. Your country will have the same kind of problem.
It is a no-go to give everything a new position in the local grid.
There are nice mathematics (7-parameter Helmert transformation, seven-parameter Bursa-Wolf transformation, Molodensky ) to convert between the local mapdatum and worldwide mapdatums with very good accuracy (centimeter wise). That is not the problem.
But when the errors in the grid are known you can interpolate from tables what correction you have to use for your GPS-measurement to fit in with the environment.
I use some 'official' sanctioned software (with the error-tables inside) to go from ETRS to local RD-Grid with centimeter accuracy.

In the case under discussion: a mathematical conversion from a worldwide mapdatum to NAD with an accuracy of <1 meter will make only sense when the local grid-errors from the past are known.

#14 User is offline   evenfall 

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Posted 05 May 2005 - 04:32 PM

Hi Professor,

You are correct in your thinking and the process. The United States National Geodetic Survey already has all this geodetic work well in hand. Thanks for you insights. Like you say, all the people who decide these things have to choose what their standards will be, or use.

What some of the people want to see here is a direct comparison of WGS84 and NAD83 in a couple of specific places just to see what the difference is between reference systems. The stickler is that the Datums though very closely related were developed by different government agencies here in the US to satisfy different primary objectives. It has been noted by Geodesists who were writing thesis papers that there are differences and similarities but no one has really bothered to publish a paper on all the nuances. For the most part, it probably isn't considered proactive when based against all the other things they want to know so it has not been done. In addition, with both NGA (DOD) and NGS (NOAA/DOC) always upgrading their editions of the Datum toward higher accuracy, the freshness of the information if published would have a relatively short shelf life and quickly become misleading to those who are not aware of all the little details involved which affect these sorts of calculations.

If your mind is begging the question, why would one government allow two different agencies within it's oversight to develop two different systems that do nearly the same thing only slightly differently? Well It is a good question, and I have two answers. First answer is that the government is big and the right hand does not know what the left hand is doing, Partly true because one of the hands belongs to the military and they like secrets. Second answer is Politics and Agency Culture. Government has branches and funding comes from different places and for different reasons. They all have their own way of doing things.

In the most basic way of comparing the two datum, the problem is that some things about it are apples and oranges, You have 2 frames of references that utilize differing frames of references so you have to agree on how you want to compare them. ie; You are using two different models to define earth center so what way should you compare them? Ok from one to the other looking from both directions, the other is 1 meter away in both the horizontal and vertical components. But it does not end there. Both datum use different mathematical models to define things further, so how do we define a term or a baseline for reference? One person said assuming the GPS system has it's own references to which the formulas are applied after the radio work is done, that would show a difference right? Well I suppose so, but it then becomes a 3 way comparison. We would not be directly comparing Datum, we would be comparing the Datum against the Satellite Triangulation methodology. Probably close enough for the answer they want, but the answer is not be all end all because it will vary all over the globe no matter where you measure it, yet they are so closely matched that it would be like comparing 2 BMW M3 Sedans in a full on spec for spec race, with the only major difference being the Stereo Package, and that one is owned by a Man in New York, and the other is owned by a Lady in L.A. They are so similar in what they do that no one bothers to do a comparison, they just know that they need to use one for some things and the other one for other things and leave it at that.

So far, and I have checked into it, no one I have spoken to so far is aware of a comparison they feel will give a Certified answer. They know there are programs that calc datum shifts, but they refuse to endorse them because they cannot certify the accuracy of the algorithms being used as up to the specs they would expect. To survey it properly there seems like it would be a problem as well. You spend 2 days surveying an NGS Tri Station with RTK, but that is not your comparison. The NGS grid is adjusted through a Least squares method, so I suppose we need to find a way to adjust our survey to WGS84. So now you are comparing two things that can argumentably be hard to define terms for, because there is more than one camp here and they see this different ways. Bleh. For that matter we could give the question asker a reasonable answer, and they could still play devil's advocate and say How do you know, or how can you be sure? Or what did you use as a reference model? It could go on forever.

Quick and dirty you can take a snapshot of the 2 datum in one place and wait for the accuracy to need to see the difference, but the scientists could still ask the question, as compared to what?

For most usability, it is enough to know that there is enough of a difference that we should use the one that is recommended for the purpose. Are you bombing or engineering a civil works project? That is the criteria we use for determining which Datum to use.

Rob

#15 User is offline   holograph 

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Posted 06 May 2005 - 12:46 PM

Black Dog Trackers, on May 4 2005, 06:24 PM, said:

I've heard plenty of non-numeric rhetoric on this subject.  I skimmed, saw no numbers.  :ph34r:

I am still waiting for a real, numeric, answer with just one PID or other example using a pair of real coordinates.

In the original NOAA professional publication titled "North American Datum of 1983" edited by Charles Schwarz, 1989, there is a paper "Relation of NAD 83 to WGS84" by Charles Schwarz.

In that paper, he explains that fundamentally NAD83 and WGS84 are computationally identical for all intents and purposes. The coordinate systems are identical. The ellipsoids are slightly different. The error introduced by doing calculations in one datum or the other amount to approximately about 1/10 of a millimeter at latitude 45 degrees.

However, he does explain why there are often differences of a meter or more in the two datums, and it has nothing to do with the coordinate system or the way calculations are performed or the ellipsoids that were used, it is almost entirely due to the errors in the observations that were used to obtain control points in either datum.

He explains that a datum is more than a mathematical construct, it is always fixed to a set of physical control points. Therein lies the difference. Each datum used a different set of control points, and each datum used somewhat different techniques to determine the position of those control points. Ultimately all the other measurements in a datum are made with relative to a datum's control points.

NAD83 control points are located mostly in North America, while WGS84 used control points across the world. The "expected" difference between the two datums worldwide is zero. That means that if you managed to extend NAD83's control around the world, and then measured a lot of points with respect to both NAD83 control points and WGS84 control points, on average the difference would be zero.

However, because the control points are always determined by observations that are susceptible to error, there will be some random variation. Until such time as someone ties the NAD83 control to the WGS84 control, no one will know what the variation is at any given spot. At a few points, the measurements have been made, and it is known that there are variations of about a meter. Neither datum is "more correct" or "more accurate" than the other. If you are a surveyor, and you are required to tie into the NAD83 datum, they you are well advised to make observations and measurements relative to NAD83 control.

Any effort to tie the two together would be a project on the order of the NAD83 project, which took 12 years and cost about $37 million at the time. The NADCON program that converts from NAD27 to NAD83 (which I have studied and ported to other languages) really does nothing more than look up locations from a large set of tables and interpolates the difference between NAD27 and NAD83. The tables were computed from measurements and statistical techniques that minimized the conversion error. There is no "formula" that converts from one system to the other.

In order to do similar conversion from NAD83 and WGS84, someone would have to do a lot of observations and then perform a best fit just as was done for the NAD83 conversion. Apparently no one has been motivated to do that.

edit: P.S. the short answer to your question about what the difference in position would be if you measured any particular point in both datums is "nobody knows," nevertheless, it would be less than about a meter. It may be zero, it may be a meter, it may be somewhere in between, and there is no way to predict what it would be at any particular point.

This post has been edited by holograph: 06 May 2005 - 01:05 PM


#16 User is offline   Black Dog Trackers 

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Posted 06 May 2005 - 08:36 PM

holograph -

Very interesting excerpts you obtained from that paper! Fascinating that there is up to a meter's difference because of adjustment differences! Raw measurements do have error and even when adjusted, the result can be like a grid viewed through slightly wavy glass.

Of course if the two different nets do get a co-adjustment, then the observed differences will likely go down to the 1mm level or less. (The differences between the two ellipsoids amounts to only difference is 1/10 mm in the distance from the center of the Earth to a pole. Comparitively nearby points would have a far less difference.)

I had not realized that the two nets were so independently adjusted. It sounds sorta like two separate football leagues, or looking through two different pieces of wavy glass at the same grid.

Here is a nice reference that is also uses the Schwartz paper as a source.

The paper's value of 'less than a meter' is still well below the detection level of our handheld GPS units.

#17 User is offline   holograph 

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Posted 07 May 2005 - 03:50 AM

I think it implies something else: that your GPS is really using WGS84, even when it says it is using NAD83. Since you are not using differential GPS, you cannot tie into any control other than the satellites themselves. The positions of the satellites are relative to WGS84 control, and your GPS can do nothing else but calculate its position relative to WGS84. It will then attempt to mathematically convert to other Datums based on the recommended conversion formula, but that formula says there is no mathematical correction between WGS84 and NAD83, so the result will be the same.

In other words, our handhelds are incapable of giving us the NAD83 coordinates. They are happy to display the words "NAD83" when you set them to a datum, but they can never in fact tie into NAD83 control, so they cannot display "real" NAD83 coordinates.

If we submit an NGS recovery report and say we've used NAD83 simply because that's the way we've set our handhelds, we're actually lying. The only datum we can honestly say we're using is WGS84.

edit: You can compute the differences at the locations of the CORS stations, since NGS publishes dual positions for the stations. For the station nearest to me, the error that would incur if I measured in WGS84 and used it as NAD83 would be a 0.924 meter error in the horizontal position.

This post has been edited by holograph: 07 May 2005 - 05:41 AM


#18 User is offline   Klemmer & TeddyBearMama 

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Posted 07 May 2005 - 07:33 AM

I've been following this thread with interest. Thanks, guys. I enjoyed it. I'm not a suveyor or any sort of expert on datums. But - I am an experienced engineer and do know a fair amount about the GPS system, used as originally intended, and I actually do some engineering on and sell sell military "boxes" that use GPS receivers inside.

I had the same sort of thought tickling around in my head:

"If there is no conversion formula from WGS84 to NAD83, then what is my GPS doing when is shows me NAD83"??

There sure ins't memory space in it for a worldwide (or even US) lookup table. No formual? Huh. What if you are in Europe & select NAD83? Meaningless.
So, at best, it is just a "guess" of some sort. A very simplified formula that Magellan or someone else cooked up.

Bottom line for me: For a handheld GPS with (at best) several meters accuracy, it really doesn't matter if I use WGS84 or NAD83. I may switch to NAD83 when hunting benchmarks - just for fun. I like the "gadget value". But it really doesn't matter for our purposes.

P.S. See my signature line below. How appropriate!!

This post has been edited by Klemmer & TeddyBearMama: 07 May 2005 - 07:35 AM


#19 User is offline   Bill93 

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Posted 07 May 2005 - 07:59 AM

Can you post coordinates of the CORS station or other point with a known 0.9 meter or larger difference? I want to see if my handheld displays any difference for a waypoint in this vicinity when I switch datums. For the locations I've tried so far (on Garmin 76S) I can see a shift maybe up to 25 meters for NAD27 but never any difference for WGS84 vs NAD83.

DD.DDDDD format, the one with greatest resolution for most handhelds, is just good enough that we should see a change of 0.9 meter. if it is mostly in either latitude or longitude but maybe not if it is split between them.

I don't find any menu item to switch vertical datums. Looking at some data sheets around here there doesn't seem to be even a foot difference for NGVD29 (associated with NAD27) vs NAVD 88 (associated with NAD83). I wonder if for horizontal datum selections where the vertical system most commonly associated does have a significant difference the software applies any conversion.

#20 User is offline   holograph 

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Posted 07 May 2005 - 08:50 AM

Bill93,

Here are a couple:


PID AI0952 Anchorage, AK
ITRF00/WGS84(G1150).... 61 11 14.38134 N 149 53 41.90314 W
NAD 83................. 61 11 14.37495 N 149 53 41.81980 W
Difference............. 1.261 meters

PID DF9223 Los Angeles, CA
ITRF00/WGS84(G1150).... 34 36 12.66234 N 118 05 02.00539 W
NAD 83................. 34 36 12.64749 N 118 05 01.96477 W
Difference............. 1.132 meters

PID DF9219 Denver, CO
ITRF00/WGS84(G1150).... 40 11 14.29552 N 105 07 37.99705 W
NAD 83................. 40 11 14.27224 N 105 07 37.96036 W
Difference............. 1.126 meters

PID DF5765 Chicago, IL
ITRF00/WGS84(G1150).... 41 43 47.44273 N 087 32 18.37413 W
NAD 83................. 41 43 47.41453 N 087 32 18.35359 W
Difference............. 0.991 meters

PID DF4064 New York, NY
ITRF00/WGS84(G1150).... 40 47 03.58007 N 073 05 49.78556 W
NAD 83................. 40 47 03.55019 N 073 05 49.78137 W
Difference............. 0.927 meters

PID DF9225 Miami, FL
ITRF00/WGS84(G1150).... 25 49 28.60270 N 080 19 09.07673 W
NAD 83................. 25 49 28.58580 N 080 19 09.06695 W
Difference............. 0.587 meters


#21 User is offline   Klemmer & TeddyBearMama 

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Posted 07 May 2005 - 08:58 AM

Bill:
I doubt very much your handheld GPSr knows what a vertical datum is. Vertical accuracy of consumer grade GPS signals is WAY worse than horizontal. I have never seen a consumer GPS with vertical datums. Military ones - maybe - not sure. Survey units - I bet they do (?).

#22 User is offline   Black Dog Trackers 

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Posted 07 May 2005 - 12:54 PM

Let me see if I got this right.

The CORS WGS84 and NAD83 values you see are adjusted values; i.e. the WGS84 adjusted geoid position and the NAD83 adjusted geoid position.

Our GPS units are not showing adjusted coordinates based on geoids. Instead our GPS units see distances to satellites and convert their distance observations to coordinates based on which ellipsoid you set it to use.

The GPS units translate from one ellipsoid to another based on the parameters of the different ellipsoids.

The handheld GPS companies probably don't bother to have a real calculation for WGS84 vs. NAD83 because the difference is way too small. They probably just use one or the other ellipsoid for both; probably the WGS84 as holograph suggests.

#23 User is offline   seventhings 

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Posted 07 May 2005 - 01:17 PM

All -

Great discussion! I think I should get three college credits, just for reading.

Well, it looks like the conclusion (or, at least, one of the conclusions) is that I shouldn't waste my time ensuring that my handheld is set to NAD83 everytime I turn it on (it defaults to WGS84).

In the past, I've taken readings (averaged several readings each) in both NAD83 and WGS84 at several stations with adjusted coordinates. I thought I detected an average difference of about .001 min (which I interpreted to be a bit less than a meter). But the discussion above suggests that what I saw was more likely just the normal volitility of the hobbyist-level handheld, and the "pattern" I detected just the normal result of the brain's propensity to detect patterns in all sequences.

Thanks, again, to all for the excellent lesson. Y'all must have stayed at a Holiday Inn Express last night.

Will

This post has been edited by seventhings: 07 May 2005 - 01:18 PM


#24 User is offline   evenfall 

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Posted 07 May 2005 - 03:54 PM

Hello to all,

I have read every word of this and I do not concur with the conclusions being drawn. To say there is no difference between WGS84 and NAD83 Derivation Schemes in a Handheld unit is simply postulation at this point. I would strongly caution those reading this thread to keep in mind that the words "implies" and "probably" are words used to describe best guesses in the face of a lack of information or proof and not facts. I have seen no hard data from Garmin or Magellan, nor anyone else who manufactures GPS units for sale to the general public stating that their GPS does not use _ACTUAL_ NAD83 Datum Parameters for conversion in their units. This would be hard to determine on a Saturday. I am saying that those are unsubstantiated claims. Implication and Probability should not be enough to persuade any position. In the real world it is what it is, so since we don't know what really is, I feel it is not safe to assume anything. I would invite anyone to come forth with hard facts with supporting source material from the Manufacturer's that support the idea that some Datum are not carefully converted when very similar.

My reasoning behind this is pretty easy to check. In a Garmin eTrex Vista, there are 110 Datum loaded for conversion when used against, and in addition to the WGS84 reference ellipsoid. There are 11 separate iterations of NAD27 loaded in it, with specific names for specific areas in which it is meant to be accurized for. Yes, I did say that Garmin's eTrex Vista claims it can position you accurately (within specified accuracy) of 11 separate iterations of NAD27. Using this as an example, do you really think it is safe to assume that since NAD 83 is only one meter different than WGS84 that the GPS manufactures would go to all the trouble they have and then throw that particular baby out with the bath water? Do you think they actually would and not tell the public? Do you think that if it could be proved they did as such that they would be misrepresenting the facts? That the devices they sell cannot actually do as is claimed they will do? Boy, That is not how I would sleep well at night. I would hope they can prove that there is a basis for all 111 Datum loaded in that little device.

If someone were to prove that GPS Manufacturers were actually omitting the conversion to NAD83 on the basis that it is so similar to WGS84 yet selling their equipment to the public touting that they are offering such a Datum conversion, is it plausible that I or any of the rest of us would be able to Sue them for misrepresenting that to me? Do you think the NGS could take action for proving that the datum is not being represented properly and no note of this has made the public aware? The Datum, all of them in some way are Proprietary and there has to be permissions to use them, even in the public domain. Something tells me the Legal departments of these companies would not go so far as to open themselves up to such claims. If the GPS says it can convert to the Obsrvtoria '66 Datum, then it should have a program loaded in there that knows how this is to be done within reasonable accuracy when based on the WGS84 reference ellipsoid it uses for positioning itself. If not, then it is not true in what it claims it can do and that would be a legal liability.

I realize that 1 meter is not enough to be revealed in your handhelds. I realize that many of you want to use WGS84 because you can. Some I get the feeling may even feel that though the fields of Survey and Geodesy are old fields with time honored standards and practices, these practices are not so honorable to them and they would rather do what they want, perhaps even in spite of those traditions, and interpret anything as they want for the sake of making things easier on themselves. It is an inkling of an idea, but some I sense would rather just do things however they want.

I am not sure that is a great path to travel, but who am I to change the minds of those who are looking for ways to get out of having to believe a scientific construct when their minds are already made up? I have tried on many occasions to explain this as based on the science of it. You know, Here is the way Science presets the situation? Not what I want it to be, but on the basis of what it is. I have watched while many in the geocaching community have tried to find a way to support killing NAD 83, it is as if many of you hate the fact that the Datum even exists. So a way has to be found to prove it is nothing more than a Crock. I have got to tell some of you guys, this scenario is looking a lot like a chapter out of the book Animal Farm by George Orwell.

It is a Fact that as Datum on the Geodetic level, NAD83 is not WGS84. I will say this very carefully one more time. You are not doing yourselves any favors, nor anyone else the same, by claiming it is. You would be wrong. Worse, people will read what you are saying and draw some very incorrect conclusions. Seems to me it would be better to keep them separated, something I have advised all along... It is so easy to do that too. In the end I suppose it simply does not matter. The construct of what science believes or what is generally thought to be held as most true is simply just a construct. No one has to do anything in any other way that the way they wanna. Of course there would be no way or methodology to compare one another in this line of thinking either but hey? <shrugs>

I think if any of you wanted to sit down and pick up the phone, Call NGS and ask to speak with a Horizontal survey specialist, a Geodesist or something, They would spend a while trying to explain what the differences are and why they observe them in different ways than WGS84. I can tell you that in 25 years they have not adopted WGS 84 and it's methodology over their own. I wonder why.

Rob

#25 User is offline   bicknell 

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Posted 07 May 2005 - 04:32 PM

A CORS station is right down the road from me, I pass it all the time. So I checked: ZDC1.

If you select coordinates and then submit, you can get the details. To wit:

ITRF00 POSITION (EPOCH 1997.0)
Computed in December, 2003 using 22 days of data.
X = 1069125.958 m latitude = 39 06 05.74307 N
Y = -4839598.629 m longitude = 077 32 33.87636 W
Z = 4001126.172 m ellipsoid height = 79.614 m

NAD_83 POSITION (EPOCH 2002.0)
Transformed from ITRF00 (epoch 1997.0) position in Dec. 2003.
X = 1069126.500 m latitude = 39 06 05.71496 N
Y = -4839600.091 m longitude = 077 32 33.86748 W
Z = 4001126.309 m ellipsoid height = 80.899 m

Of interest to me, was the X Y and Z in meters. I get differences of:

X = .542
Y = 1.462
Z = .137

Also note that the NAD83 position has moved a bit (there are velocity numbers on the page), the NGS data sheet for DF9217 shows:

DF9217 EPOCH DATE - 2002.00
DF9217 X - 1,069,126.500 (meters) COMP
DF9217 Y - -4,839,600.092 (meters) COMP
DF9217 Z - 4,001,126.310 (meters) COMP

Notice that Y is different by .001, and Z by .001 meters.

Facinating.

#26 User is offline   evenfall 

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Posted 07 May 2005 - 05:52 PM

Bicknell,

If you look at the CORS on the West Coast, primarily West of the Rockies, and most specifically California, you will see a lot more velocity. Geodesists have learned that North America Plate movement affects Survey Accuracy and they try to keep their database appraised of this. WGS84 does not adjust it's own accuracy for this movement in the minute ways that NGS does by the way. In fact it doesn't even try to do it on this scale. They are more Basic and Global in their thinking. The Mission is different.

Interestingly many of these real time stations are operated by County Seats and Municipalities. They use them to keep their own civil measurements as accurate as they can, and tied into NGS Sponsored Datum. It is pretty cool the way it is all networked. It means that I can use a city monument, or a DOT Marker, in that if it has been tied in, I can be pretty happy with it's geodetic accuracy when away from actual NGS Survey.

Generally speaking, Though Many Countries in the world are using WGS84 as a basic Datum, they have to apply their own local methodologies to accurize the datum for use in their Locale. NGS does this for their NAD and NAVD Datum as well, but the Methodologies though similar, are different in the most basic ways than what NIMA/NGA used to formulate the WGS Datum. The NGS Datum are not rip offs of the WGS, they are truly developed differently and different enough to be called by a different name. Until the scientist decide different, they are not alike at the Geodetic level.

Has anyone ever noticed this? Maybe it is not anything more than a Rhetorical Coincidence. :-D

Rob

This post has been edited by evenfall: 07 May 2005 - 05:53 PM


#27 User is offline   2oldfarts (the rockhounders) 

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Posted 07 May 2005 - 10:22 PM

After reading this thread in it's entirety, and having found several hundred benchmarks (having the GPSr set to show both WGS84 & NAD83). I find it is NOT worth worring about which datum I use (the readout has always been identical).

Using WGS84/NAD83 I can go right to any adjusted benchmark I choose. If it is a scaled benchmark, then it doesn't matter which datum you use, you are still going to have to hunt for that mark.

If the GPSr is reading the same for both datums, then it is irrelevent which one it is using.

Pick one and go find some benchmarks! :)

John

#28 User is offline   Klemmer & TeddyBearMama 

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Posted 08 May 2005 - 06:46 AM

Bottom line FOR ME, as an engineer and always interested in how and why thinks work the way they do:
1) Yes, the datums are different. We need to remember that, especially for the future.
2) Yes, consumer handhelds do the math and display NAD83 properly, as best they can. They wouldn't not to, these days.
3) I appreciate all the education in the above discussion. I really do. 1 credit hour at least!
4) Because of the the inherent limits in consumer grade GPS, even with WAAS, you will not see any significant difference bewteen WGS84 and NAD83 in units ON THE MARKET NOW. You MIGHT see a .001 minute difference (which is very close to 6 feet of latitude).
5) Who knows what the future will bring? If you are around (I HOPE to be) for the planned GPS II system, or the European "pay to play" Gallileo system, we may need to pay more attention to datums.
6) WGS84 will get you to an adjusted benchmark as well as NAD83. Just try to remember you SHOULD be using NAD83.

#29 User is offline   holograph 

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Posted 08 May 2005 - 06:54 AM

Evenfall,

You have valid points, and certainly no one disputes the fact that NAD83 and WGS84 are different (it is easy to see the difference when NGS publishes dual coordinates such as for the CORS stations), and so it's extremely important for professionals to distinquish between them.

And you have another valid point that no one knows the exact calculation used by the GPS makers (more on this later). However, there are a lot of reputable sources that describe the Garmin algorithm as based on the Molodensky transformation, which is the technique described in the NIMA publication on WGS84. That transformation is approximate at best, with errors ranging up to 25 meters or more for some datums. Even the NAD27 to NAD83 conversion is most likely done using the Molodensky transformation, and the results can have errors of about +/-5 meters on average, depending on the region of the US.

The Molodensky transform is attractive for inexpensive GPS units because it does not require large lookup tables, and the parameters for over a hundred datums have been published by NIMA (which not coincidentally is the same number of datums most GPS units advertise themselves supporting).

The only thing that is certain is that all GPS units use WGS84 internally, and they convert to other datums with some unknown algorithm having an unknown degree of error. Since we don't know the degree of error in their conversion, the way we can best serve the professional community is to provide unadulterated WGS84 coordinates whenever we publish coordinates, and to say they are WGS84.

That way, anyone who does have the ability to do a high accuracy conversion, and for some reason wants to use our numbers at sub-meter precision, is better served by doing their own conversion from the most accurate coordinates we can provide, the WGS84 coordinates, rather than relying on us to convert to other datums with some unknown conversion accuracy.

edit: typos

This post has been edited by holograph: 08 May 2005 - 07:28 AM


#30 User is offline   Bill93 

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Posted 08 May 2005 - 01:15 PM

I haven't had time to finish the job, but I tried a couple of waypoints set to the CORS station coordinates, as well as noting the lat/long of nearby features in the base map of my Garmin 76S.

The disappointing news is that I have yet to find any difference between the values displayed for WGS84 vs NAD83. I would have expected some of those with a meter of difference to show up as 0.00001 degree difference in the rounded values as I shift between datums. I'm suspecting that they decided the difference was small, and the easy transforms were not accurate enough to improve the accuracy of the unit so they just entered null values in the transformation for NAD83. In general they do the right thing for transformations within their accuracy, and there certainly is a difference displayed for NAD27.

I heartily endorse Klemmer's observations, with my interpretation of "best they can" to mean what I discussed in the previous paragraph.

Has anyone found a set of coordinates for which their handheld shows a difference?

#31 User is offline   evenfall 

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Posted 08 May 2005 - 03:22 PM

Bill and all,

Not to put a fork in this But Consumer grade gear isn't going to give anyone the accuracy you are trying to see, but I feel you are correct in the general thinking that not all modes of handheld accuracy is currently created equal. In the last 5 years we have seen Accuracy improve with the removal of Selective Availability and I remember reading in the past that the hopes for consumer grade accuracy were going to be in the 1-3 meter range in the future.

The Crux of what I have tried to maintain here is that there is a danger in simply saying NAD83 = WGS84. The problem is that on a blanket basis, that is not a fact and simply is not true. It is important to know when it is similar enough to matter and not matter. Many people have a misconception, not one that will get them killed or harmed, but since we are working in the realm of geodesy here in the Benchmark hunting side of geocaching, we are looking for Geodetic monuments, and so it can matter. It can matter because the future is alleged to be bringing everyone higher accuracy, and if it does, the more accuracy we use now will make it easier for users in the future.

It is safe to say in 2005, that for any navigational purpose that a human can use as transportation including their feet, WGS 84 and NAD83 are similar enough to be used interchangeably. When you get on your hands and knees with a tape measure near an object that is meant to represent Geodesy, that last meter, or yard if you will is within the grey area that would be better defined by using the Datum which has been asked for. I contend that you and I may see accuracy like this on consumer grade gear in the not too distant future so I advocate being careful. Just do yourself a favor and use the correct Datum. It may not seem to make sense but the future could reveal a difference and that will make it easier for all concerned.

The Big deal is that for the purposes of this discussion, We are working at a level with objects where they are part and parcel of what defines a Datum. If I were looking for a latte stand on my GPS via the software that helps me find local business, and I was in an area of unfamiliar territory, I am sure several different Datum could get me close to my beverage. But since we are looking for object which represent high locational accuracy, and with the intentions of improving on that accuracy as time and technology permits, it is important to think of the Datum in an appropriate way.

It is a hard concept to get our minds around, I know, and I have the unfortunate advantage of being accustomed to accuracy, which most people are unable to reveal. It is hard to convince you all of this when I know you have not seen it, but I can tell you, and with repeatable results that I can check the value of survey I have set on a daily basis, and the accuracy seems higher than the published values. I am talking about being right on the nut, day after day, same exact spot, same exact numbers and in little time to derive it. I can hop on a D-9 Caterpillar Bulldozer with a Huge "U" blade , equipped with GPS and cut grade at 4 miles per hour to better than 1/10th of a foot accuracy in real time. I could surpass that accuracy to 2/100 of a foot on a Cat 140 H Road Grader, but that higher ability is inherent in the Machine and the material being graded, not the onboard GPS equipment. 2/100ths of a foot is about the width of a paperclip if you were wondering. They will then come behind me and pave that road. The same can be said for an excavator digging a ditch and laying any kind of a pipe, and Pipe is laid to such accuracy. I am saying that I observe the system as having what seems more inherent accuracy than they publish that it has, and I believe that most of us here see better accuracy out of our consumer gear than the published values claim for it. The seems to mean that the GPS system itself is operating with seemingly more accuracy that the Government publishes for it as well. I said seems. I am leaving all the Manufacturers an out, because I cannot speak for their claims. and no Manufacturer is going to claim higher accuracy than the Government provides...

I will agree with Holo, in that the cheapest, or should I say cost effective way to convert datum in consumer grade GPS is likely a Molodensky transformation, and I cannot refute that either. (pro equipment often uses Helmert amongst other schemas) I have tried quizzing people in customer support at GPS manufacturers but unfortunately, I have not reached a person who could speak to a good many of my questions, and have not found them yet. To further make it painful the GPS is still too much of a Black Box for my liking, too many of the answers we seem to find are over simplified for the use we have for them. But, it is what it is. Datum Conversions are post processed by either the GPS or a software in a GIS system where any transformation can be made form raw data. Basically all a GPS can get from a satellite is X,Y,Z, and current network time coordinates and post processing has to apply what is known about that location as based on the selected datum, after the fact.

In the end, I want to say that if we were not dealing with geodetic survey markers as a point of discussion, a comparison of the WGS84 and NAD83 Datum would not have been necessary and I likely would not have pressed that we carefully define the terms.

NAD27 is a good ways different from either NAD83 or WGS84, or any other Datum. It is obvious that if called for, we must observe that datum as radically different from any other. Just the same, it is important to stick to the time honored rule. Not many people know Datum from David. We may think we know better but "always use the Datum being called for by whatever it is you are working with", Use the Datum on the Map, Use the Datum being specified, even if you think you know what you can get away with and you will be inherently more accurate in the end. That trumps a 10,000 word discussion in one sentence and it is correct. It is a safer way to go, it requires zero over thinking, and It really is not that hard to do.

Rob

#32 User is offline   seventhings 

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Posted 08 May 2005 - 05:31 PM

Rob -

Your last point is well-taken. Earlier, I said that I concluded that it wouldn't make a difference if I set my handheld to NAD83 or left it in WGS84. Well, upon further consideration, I must re-conclude that it does make a difference. If you're looking for or describing the location of a mark that is defined with reference to the NAD83 datum, then you should work in that datum. The other datum may get you there physically, and there is nothing improper in it (GPS, datasheet, dowzing rods, chicken entrails - whatever works), but once you get beyond the purely recreational approach to benchmark hunting, I think it best to hunt and document in NAD83.

By the way, I think BDT's use of the word "rhetoric", above, was in the classical sense of "a well-reasoned but non-mathematical/non-scientific discourse". His point being (here I am, speaking for BDT!!) there was presented by all thorough examinations of the difference between NAD83 and that other datum, but there was (initially, at least) no straight-foward calculation of the conversion from one to the other. Well, I understand now that such a conversion is a bit more problematic than feet to meters. I would have thought that I could get from WGS84 to NAD83 by adding .001 minutes to the latitude and subtracting .003 minutes from the longitude. It's never that simple, is it?

At any rate, thanks, again, to all for the excellent lesson.

Will

p.s. Happy Mother's Day to all you benchmark hunters of the maternal persuasion.

#33 User is offline   Black Dog Trackers 

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Posted 08 May 2005 - 09:23 PM

Here is an interesting reference - Charles Schwarz on the difference between NAD 83 to WGS 84.

The difference between the two ellipsoids is 0.1 millimeter in the polar axis of the Earth. The mathematical conversion between NAD 83 and WGS 84 is based on the minute difference in these ellipsoids. According to the Schwarz paper, the difference in longitude is zero and the difference in latitude is 0.1 millimeter at 45 degrees latitude. The paper goes on to say "It is assumed that most users will ignore this very small difference."

What about the differences in CORS data and other comparisons between NAD 83 and WGS 84 that are much larger than 0.1 millimeter, one may ask. The larger observed difference is NOT due to the intrinsic mathematical conversion from WGS 84 to NAD 83. Instead, the difference is in adjusted values. WGS 84 points are adjusted independently from NAD 83 points. Adjustment is a powerful mathematical technique of (detecting and) distributing observational error, but it does not get rid of error. The distributed error in these independent adjustments results in observed coordinate differences that are much larger and mask the true (extremely minute) mathematical difference between the two systems.

For this reason, the Schwarz paper says that it is inadvisable to compare the coordinates of a point adjusted with the set of WGS 84 points to the coordinates of a point that was adjusted to the set of NAD 83 points.

The NGS database is made of points adjusted to the set of NAD 83 points, obviously. Therefore it is important not to try to measure the distance between one of those points (a PID) and a point adjusted to WGS 84 or you might see a discrepancy of up to a meter.

When we are using a handheld GPS unit,:
1. we are not measuring the distance between two differently adjusted points
2. we are making an unadjusted observation (neither adjusted to NAD 83 nor to WGS 84)
3. we are remembering that our GPS unit is statistically blind to differences of less than a meter

So it does not matter at this time if you set your GPS unit to WGS 84 or NAD 83.

If you have a GPS handheld unit built in the year 2010 (not possible) that has an accuracy of 1/1000 second, or if you own a $10,000 GPS unit and don't mind watching it all day at a PID, then it will matter. If you don't have either of those, then it doesn't matter, so just don't worry about it. Set it to NAD 83 to feel better if you like - I do.

#34 User is offline   Prof. Y. Lupardi 

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Posted 09 May 2005 - 10:58 AM

When in the future there are GPSr's capable of centimeter accuracy for an unassisted standalone absolute position in the WGS84 frame then there is a problem with height: it varies because of tidal effects in the crust of the earth you are standing on.

#35 User is offline   GEO*Trailblazer 1 

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Posted 09 May 2005 - 06:39 PM

Prof. Y. Lupardi, on May 9 2005, 12:58 PM, said:

When in the future there are GPSr's capable of centimeter accuracy for an unassisted standalone absolute position in the WGS84 frame then there is a problem with height: it varies because of tidal effects in the crust of the earth you are standing on.

More than centimeters.

#36 User is offline   evenfall 

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Posted 10 May 2005 - 04:10 AM

Professor,

It is interesting to see how tidal effects and plate movement seem to change things with each new Geoidal model they derive.

More interesting, WGS84 makes this sort of thing difficult to track in terms of Orthometric Heights, as NGA does not offer this Data. Only the ellipsoidal and Geoidal references for the Datum. In the US, NGS is the keeper of Orthometric heights but they are not for use with the WGS84 Datum. So for us in the US, the WGS Datum is highly uncheckable. We have no local control for use with it.

Rob

#37 User is offline   Prof. Y. Lupardi 

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Posted 10 May 2005 - 06:24 AM

Living in a very flat country we have the important question: "The water, where does it want to go to?" We made the NAP (New Amsterdams Peil) that gives us the 0-plane.
With GPS we can also measure height above the ETRS89 ellipsoid but how to tie this in with the NAP?
It was done by measuring the gravity in and around my country. A very big job but last year it came to fruitation. We now have the NLGEO2004 model and we hope it is accurate enough (0.35 cm sigma) to last for the next 10 years. After a policy decision all the NAP markers were adjusted to the new heights. The max differences found were 6 cm. They did not want the hassle with tables, interpolation and errorcorrection.
Here is a picture where you can see what the height above the GRS80 ellipsoid is of the 0-level NAP plane. As you also can see: a 'gradient' of 1 cm/km is not exceptional.

Posted Image

#38 User is offline   caseyb 

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Posted 10 May 2005 - 11:59 AM

Better late than never I suppose.....

This thread, in all its technical details, is frankly way over my head. I am neither a surveyor or a Geodisist. So I went to somebody who was. I asked one of the big-wig, PhD in Geodecy guys (most of the books and papers referenced in the thread were actually at his desk!) around here to take a look at this thread and see what he thought of it. I was unable to convince him to join the forum here (he has too much to do he says), but I did at least get him to write up a little reply and I learned a lot from just going through this thread and asking questions. It is important to keep in mind that he is responding to some of the original posts, not the entire thread. The thread got a bit (maybe that is an understatement) too long for him to read and respond to everything.

Please don't take this as any kind of attempt to re-invigorate the debate. Its just one guys take. But it does do something nobody else did.

Anyways... below is his take on this.


-Casey-




On Dave Doyle's original post:
------------------------------

Dave is dead-on. The only clarification
I'd add (as an addition, not a correction) is that one
could argue that defining a horizontal datum also
implies defining its scale (that is, "how long is
a meter"?)

Example: Two points, A and B, have coordinates given in
2 different datums, call them D1 and D2.

D1 and D2 have defined their origins, orientations
and ellipsoids to be identical.

Yet if one computes (from the coordinates as given
in the two datums) the distance from
point A to point B one might get
100.000 meters based on the D1 coordinates,
while the distance between A and B in the
D2 system might be 100.001 meters.
Which is right?

Well, both can be correct within their own context. A
"meter" in D1 might be different than a "meter" in D2,
causing a "scale factor" between the two datums of
(100.000 - 100.001)/100.000 = 1 x 10^-5.

Ok, I only began with this example to weed out those
who don't wanna hack with me through the NECESSARY
math and geodesy needed to fully appreciate Dave's
clear post.....


Black Dog Trackers first post:
------------------------------

A direct answer to your question: "How far apart, in
millimeters, are points with identical numerical values
for their geodetic coordinates, but referenced to WGS-84
and NAD-83 respectively?"

Here's your answer:
1) Because WGS-84 has had numerous realizations, and because
NAD-83 has had numerous realizations, and because the
latest realization of WGS-84 is very close in origin,
orientation and scale to the latest realization of the
ITRF (which is ITRF2000), then the best transformation
available from NAD83 to WGS84 would be:

[X83]= s * {[[R]] . [X84] + [T]}

I won't consider MOTION of the datums (due to tectonics)
as that will just confuse things. The values for the
R matrix, T vector, and s scalar are found from
Dave Doyle's original post. Also, you didn't specify
the ellipsoidal height of the points, so I'll assume
that h(83) = 0 and h(84) = 0 for the purposes of
this.

Additionally, when you ask "how far apart", you inherently
ask "how far apart, using a meter defined in a specific
way". Unfortunately, NAD83 and WGS84 have a scale factor
difference, meaning that a "meter" in NAD83 isn't the same
as a "meter" in WGS84. Do you want the answer in NAD83
meters or WGS84 meters? (This makes a difference
at the mililmeter level!)

Aaaaaanyway, here it is, in "NAD-83 meters"

Point 1 (where you gave the coordinates in
lat/lon for WGS84), as given, has:

X1(84) = -631552.023124...
Y1(84) = -5079983.658788...
Z1(84) = 3791917.04765...

Point 1, scaled, rotated and translated into
NAD83:

X1(83) = -631551.4868661...
Y1(83) = -5079985.0513089...
Z1(83) = 3791917.1378866294....

Point 2 (where you gave the coordinates in NAD 83)
has:
X2(83) = -631552.0231277...
Y2(83) = -5079983.6588179...
Z2(83) = 3791917.047547...

As such, in NAD-83 meters, the physical separation
between your points 1 and 2 would be:

d = SQRT(dx**2 + dy**2 + dz**2)
= 1494.914 millimeters


Like Dave Doyle said, if you're positioning at better than 3 meters,
this matters. If not, it doesn't.



I think the BASIC lesson to take from this is that Geocaching is
very helpful to give NGS 2 pieces of information:

1) Did you find the mark?
2) Where, in general, is it located?

This will help surveyors FIND the mark, and help us keep track
of missing, moved or damaged marks.

What NGS will NOT get from your handheld receiver is:
3) What are the exact geodetic coordinates of the mark?

#39 User is offline   Black Dog Trackers 

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Posted 10 May 2005 - 12:54 PM

caseyb -

Thank you! Very interesting. I figured a conversion could be done. I'm assuming that "realizations" means "observations included in the adjustment".

Concerning the statement:

Quote

What NGS will NOT get from your handheld receiver is:
3) What are the exact geodetic coordinates of the mark?

we benchmark hunters definitely realize this and are providing handheld GPSr-level coordinates ONLY as part of the to-reach information, thereby improving upon the horizontal coordinates of those PIDs with SCALED horizontal coordinates. For those PIDs with ADJUSTED horizontal coordinates, we are not submitting any handheld GPSr readings since that would be of less than zero value.

#40 User is offline   evenfall 

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Posted 10 May 2005 - 06:31 PM

Black Dog Trackers, on May 10 2005, 01:54 PM, said:

caseyb -

Thank you!  Very interesting.  I figured a conversion could be done.  I'm assuming that "realizations" means "observations included in the adjustment".


BDT,

Just remember what I said early on in the thread, "Unless someone goes out of their way for you, and I wouldn't hold your breath to long. If it does happen for you, and it could, Please Consider that someone went out of their way just for you and only you, because that is what it will be. Be Honorable towards them for doing you such a favor."

This is what happened for you and you got real lucky. This is a Golden BB. Casey, in quotes; "asked one of the big-wig, PhD in Geodecy guys (most of the books and papers referenced in the thread were actually at his desk!) around here to take a look at this thread and see what he thought of it."

So please realize that you basically got a favor equal to a Holy Grail sort of answer here. Someone had to go to an extreme to do this for you. And for the most part, everything we tried to tell you is the truth about it. I hope you can finally accept this answer. You even got past the bunny rabbit guarding the Cave.

Realizations means that each time they set out to re observe WGS84 and NAD83, (separately and by different agencies) they took what they felt were newer and improved reference frames, (and other ways of looking at this problem) which were used as baselines to observe it with. Each time they got a new, different result and they named it WGS84. Yup, the same old name. Both in the case of WGS84 and so on with NAD Datum. (they have had reasons for not renaming too) Each realization used different reference frames so the results were different, Hopefully better, yet each with a different purpose in mind. That Purpose in mind is in the name of each Datum. World Geodetic System is trying to describe the World, and North American Datum is trying to best describe a Continent.

WGS84(G873) (G873 being the GPS Date when that particular realization was made) is the latest realization of that Datum being used I believe, but since it's initial inception, the changes made to it from one realization to the next has the WGS Datum becoming closer to the ITRF than it had originally been, but it is still called WGS84. So to the Geodesist, it becomes necessary for him to know what to compare. Without going into it, the NGS has similar realizations and that is why you see the year they did that in parenthesis next to the NAD83 designation, such as NAD83 (1993) as a for instance. This too is important to a Surveyor, as they have to use the version, or realization which is there. If I tie a survey into NGS control and that control is NAD83 (1993) control then my work will Jibe with that. If I go to another part of the country where a lot of the latest adjustment in that area is (1999) then that is the control. Yes our instruments and GIS systems will know of these different iterations. We get the Data From where you might imagine.

The realization in the end is the affected adjustment. It can include new observations but it does not have to. They each are continually trying to improve their Datum, but they do not agree upon doing it the same way and they are not trying to meet in the middle.

Again, I just want to remind you that NGA does not provide adjusted Orthometric data to the Public, as they have not unclassified it. So on the point of that fact, Beyond what you have seen NGS give for a CORS Station, which is basically providing an orthometric height to WGS84 in a Back door way via the ITRF conversions, Most Surveyors would have no way of comparing datum at any arbitrary station. It simply isn't done. Also keep in mind that this Geodesist did one point for you and his work is true for that one point only. His result is not uniform, and will not fit for every other place on the Continent or Globe. Please don't read that answer and go away thinking you have a global answer, applicable to all locations on the continent. It is very specific to the coordinates used to solve for it. That is the only place in the world that the result is true, as based on the terms or realizations used.

Rob

#41 User is offline   kc2ixe 

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Posted 12 May 2005 - 03:35 AM

The part that BUGs me in all of this is the difference in the definition of the metre - folks, it's had a fixed length for a LONG LONG time, and the fact that survey work messes around with non standard meters at all is just WRONG, IMHO

#42 User is offline   Z15 

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Posted 12 May 2005 - 04:52 AM

That is a common comment from people who have no understanding of surveying and geodesy. Even our experienced highway constuctions engineers could not understand this, so its hard for the aveage person to.

Mathamatics may be perfect science, only problem the earth is not...

#43 User is offline   caseyb 

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Posted 12 May 2005 - 05:28 AM

kc2ixe, on May 12 2005, 03:35 AM, said:

The part that BUGs me in all of this is the difference in the definition of the metre - folks, it's had a fixed length for a LONG LONG time, and the fact that survey work messes around with non standard meters at all is just WRONG, IMHO

I said the exact same thing when I was talking with the guy here at work. But I learned that I was wrong.

Yes the meter is a standard length. It is universally accepted. But what we are talking about is the "realization" of a meter, which can (actually it must) differ. What the heck does that mean? As I understood it, in say WGS84 the distance from point A to point B is 1000 kilometers (dont think measuring tape.... think of using the given coordinates of A and B and then doing the math to determine distance). But in NAD83 that same points A and B are (again in the math sense) are 999.999 kilometers apart. (dont quote me on the number of decimal places. I am speaking hypothetically). So there is a scalar difference. Of course, points A and B are not two different distances apart, that is not physically possible. As such, 1 meter in NAD83 is different from 1 meter in WGS84.

If you took a perfect tape measure and measured the distance, it would be the same. Again, a standard meter exists. But the realization of that in the model that is NAD83 or WGS84 or whatever will have some minute scale difference.


Hope that makes sense.

-Casey-

#44 User is offline   Prof. Y. Lupardi 

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Posted 12 May 2005 - 09:22 AM

Could this expanation also be valid for the following?:
I have an Indian map (NC44-9) in Transverse Mercator projection with mapdatum spheroid Everest. It has a grid in British Yard I thought. But I forgot the scalefactor. (realisation) I am told. The normal yard is 0.9144m but the Indian yard on these maps seems to be 0.9143985m :laughing:

#45 User is offline   evenfall 

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Posted 12 May 2005 - 10:19 AM

kc2ixe

I know it is a PITA but if you carefully re read most of what I have posted in this thread, you will get a good idea of what all the different parameters and reference frames are, believe me, it is not just the meter. I am pretty much done explaining this. I have had my work fact checked and It is correct. Trust me, I know Surveyors who have more formal training than I who have a hard time with this concept so don't feel alone. It is a super difficult subject to explain and understand. There are many, many things you have usually got to have a really good grasp of before this particular concept begins making sense.

In the clearest language I can use. WGS84 Is what your GPS and Most all GPS uses to derive basic locational Parameters, Primarily known as Ellipsoidal Heights. The Ellipsoidal Height is an Ellipsoidal Height is something that is a smooth surfaced 3 dimensional object that closely resembles the size and basic shape of the earth.

For Civil Survey Work In the United states of America, Mostly it ends there. I will repeat. It ends there. WGS84 goes in the trash right there. Kaput.

What we do with Pro GPS at this point is after we derive ellipsoidal triangulation with GPS via WGS84. Three components, Latitude, Longitude and Height above or below reference ellipsoid, then we post process it into NAD83. First, A different reference Ellipsoid that the Geodesists at NGS use because they feel that their Ellipsoidal Model is a Better fit for North America. Then we apply a different Geoid (gravity) Model than WGS84 uses because the Geodesists at NGS feel it is a better fit than the one used with WGS84. Then we apply the sum or difference of our observed ellipsoidal height above or below the reference frame and compare it to the Geoid Height to derive Orthometric Heights. Of course Orthometric Heights are also compared to Other known and documented orthometric heights which are in a look up table, and the highest accuracy ones are part of the NSRS. WGS84 Does not give Orthometric Data for any Country. That is Military info in that Datum. It happens to turn out that the scientists who develop all these reference frames have Meters that differ in length too. It is because of what happens when the meter is scaled by the various reference frames. Yes because of Scaling there Are Differing models for the length of a Meter too. Oh and not to mention the Length of a foot too. It all works out in the end when you use the models that were used before.

The 2 datum are close but they are not the same. Further no Datum is the best Datum in the world, and WGS84 is not I repeat NOT more accurate nor considered to be More Bettah than NGS's NAD83 especially in this Continent. WGS is a datum which is trying to be a best fit for the entire earth, NAD83 is trying to be a best fit for North America. WGS84 is not the so called king of all Datum. It is not more right that other Datum. (My Datum and your Datum were hanging out the clothes, My Datum punched your Datum right in the nose...)

Your consumer grade GPS uses a lot more of the WGS84 Schema that Pro Gear has to to derive it's location. It uses all of WGS84 and then applies a Moleninski Transformation. Pro Gear can do that too, but there are other, more accurate ways available and for real survey work, those are used.

If you want to know more, Google will happily bring you tons of Technical Documents to read! Really! :-)

In the end, We in the survey world are told to Use NAD 83 and everything will be just fine, so we do, and it is. My Best advice as someone who has made a buck or two off this stuff is if you are doing something in the Behalf of NGS, just follow their direction and advice and don't worry about the other stuff. You will be fine and well taken care of. If you choose not to believe me or trust in my advice. Ok. Don't.

All I have attempted to show everyone is that there is a difference, what the difference is, and why it is important. (P.S, it may seem insignificant but at this level, the geodetic Millimeter level, it is) I think I have accomplished that, and so have the others who helped out in this thread. If someone want's to not follow industry standards from there on, they are on their own! I wish them well! :-)

Good Luck!

Rob

This post has been edited by evenfall: 12 May 2005 - 10:22 AM


#46 User is offline   Black Dog Trackers 

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Posted 12 May 2005 - 10:34 AM

Here again is the word "realization" ("realisation"). I doubt it means a conversion factor between different kinds of metric standards.

As we know, the geoids of NAD 83 and WGS 84 are verrrrrrry slightly different in one of the two radii. If you draw on paper one ellipse inside another, both ellipse's right and left sides touching but one ellipse smaller and not touching either the top or bottom of the other ellipse, and then draw a section like a pizza slice from them, the arcs of the 2 crusts are different in length. Perhaps that's what this 'realization' is. The 2 points where the knife slices across the ellipses' edges are like two geodetic points on the surface of the Earth - same 2 points, but the distance along the crusts between them is different.

Or, it's the "adjustment of a whole bunch of WGS 84 points" vs. "adjustment of a whole bunch of NAD 83 points" thing.

This post has been edited by Black Dog Trackers: 12 May 2005 - 10:37 AM


#47 User is offline   evenfall 

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Posted 12 May 2005 - 10:46 AM

Black Dog, Please. You are tilting at windmills here. Splitting hairs. WGS 84 offers us no Physical Survey Orthometric reference. I said that many times. They do not publish a database. So there is no way to compare that as such on a point to point basis without a Geodesist to figure it out for each point which has already been done for you on, One point.

Go do your research eh? Go learn all the various definitions for yourself, Not for what you want to take them to mean, but for what the accepted meanings are. Then go ask a geodesist if your understanding is indeed correct.

:-)

Rob

This post has been edited by evenfall: 12 May 2005 - 10:49 AM


#48 User is offline   Black Dog Trackers 

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Posted 12 May 2005 - 11:41 AM

I'm wondering what the term "realization" specifically means in the posts given by caseyb.

#49 User is offline   evenfall 

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Posted 12 May 2005 - 12:41 PM

evenfall, on May 10 2005, 07:31 PM, said:

Realizations means that each time they set out to re observe WGS84 and NAD83, (separately and by different agencies) they took what they felt were newer and improved reference frames, (and other ways of looking at this problem) which were used as baselines to observe it with. Each time they got a new, different result and they named it WGS84.  Yup, the same old name.  Both in the case of WGS84 and so on with NAD Datum.  (they have had reasons for not renaming too)    Each realization used different reference frames so the results were different, Hopefully better, yet each with a different purpose in mind.  That Purpose in mind is in the name of each Datum.  World Geodetic System is trying to describe the World, and North American Datum is trying to best describe a Continent. 

WGS84(G873) (G873 being the GPS Date when that particular realization was made) is the latest realization of that Datum being used I believe, but since it's initial inception, the changes made to it from one realization to the next has the WGS Datum becoming closer to the ITRF than it had originally been, but it is still called WGS84.  So to the Geodesist, it becomes necessary for him to know what to compare.  Without going into it, the NGS has  similar realizations and that is why you see the year they did that in parenthesis next to the NAD83 designation, such as NAD83 (1993) as a for instance.  This too is important to a Surveyor, as they have to use the version, or realization  which is there.  If I tie a survey into NGS control and that control is NAD83 (1993) control then my work will Jibe with that.  If I go to another part of the country where a lot of the latest adjustment in that area is (1999) then that is the control.  Yes our instruments and GIS systems will know of these different iterations. We get the Data From where you might imagine.

The realization in the end is the affected adjustment. It can include new observations but it does not have to. They each are continually trying to improve their Datum, but they do not agree upon doing it the same way and they are not trying to meet in the middle.


So BDT, I think I covered that.

A geodetic Realization is the frames of reference(s) used to make the latest determination, as it the terminology used to refer to older iterations or determinations.

It would be ok with me if you would like to refer back and re read some of the things I covered. I know I covered a lot of ground and it would be easy to overlook, or not remember what I had tried to convey.

In another frame of reference:

In order to have a part fit a particular car you may need to know the make, model year, Build Date, Transmission Type, If it has A/C... Same diff. You own a car? It is a realization of all the particular options you got on that model.

Rob

This post has been edited by evenfall: 12 May 2005 - 12:43 PM


#50 User is offline   Black Dog Trackers 

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Posted 12 May 2005 - 01:13 PM

That's the concept I was referring to with

Quote

Or, it's the "adjustment of a whole bunch of WGS 84 points" vs. "adjustment of a whole bunch of NAD 83 points" thing.
. Thank you for reminding me which concept was meant by "realization", I was hoping someone would answer one way or the other.

Interesting that this gets worded as a difference in how long a meter is when it is really a difference in distance because of a difference in WGS 84 vs. NAD 83's 'opinion' on where 2 points are. I certainly agree with kc2ixe's reaction to that.

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