Problems With Accuracy

[Pittfan]

Hey all. Sorry if this topic has been approached before. I am new to this board and I was having a problem with searching for this topic. If anyone can help, it would be much appreciated!!

 

I went out geocaching this week, and my gps consistantly read about 30-45 feet away from the coordinates given. My gps has been doing this for about a month now. I have the map send direct route software loaded onto the gps, and I have recently updated the gps (3.35v), and the software (for the patch). Last night, the gps was telling me that I was on a different road, and would tell me that I had already completed a turn before I was even to the intersection.

 

So, I know there are a lot of people out there that say it could be the satellites. But, this has been going on longer then 1 day, and I know that I had about 8 satellites on my gps. Also (I don't know if it is related), I found the "re-calibrate compass" feature in the set-up menu, and I have had to do this quite a few times since I bought the unit last sept. Is there some type of keep up that I need to do on the gps for the accuracy?

 

Also, does it matter what type of software is on the gps? I used to have the streets and destinations loaded on my gps, until I bought the direct route. I don't ever remmeber having an accuracy problem as bad as this on my gps.

 

Thanks for the help, and sorry if this has been brought up before!

Jason

(Pittfan)

[+welch]

GPS is never totally accurate, 30-45ft is not that uncommon.

Remember to try and keep the gps in the position recommeded by the maker. IIRC vertical for quad helix antenna. You should receive better reception in open places with a clear view of the sky, and worse in closed spaces and areas with lots of things overhead (trees etc etc).

How far are these other roads it was saying you were on? If they were nearby and/or the maps loads are off a little bit could easy have shown you on the wrong road.

Recalibrating the electronic compass should keep it knowing with direction your moving and/or the select waypoint is from you (vs units without which only know your direction when moving). And may need to be done everytime you change the batteries.

[Pittfan]

The locations yesterday (while geocaching) were in open fields and parks. It was a mid-afternoon day, and here in the St. Louis area, we had perfect skies. Last night, while near downtown, it was saying that I had already gone through an intersection, when I wasn't even to it yet. It seems to be getting worse. I don't know the mechanics inside a gps, thats why I am a little unsure whether there is something going wrong with it, or something that can be recalibrated. It used to always give me a better reading then 30 feet away. I used to be right on top of the geocache, and now it just seems to be sending me on a wild goose chase. Yesterday, it wanted me to stand in the middle of a stream for a geocache. I know that the maker said something like, expect it to be up to 3 meters accurate. I always thought that meant somewhere near 9-10 feet.

 

Thanks for the tips! I enjoy geocaching, I just find it difficult when the coordinates seem to be way off.

 

Jason

(Pittfan)

[+JohnnyVegas]

3 (9-10feet) meters is the best accuracy you can get. it is not what you are going to get all the time. With my Meridian Gold I get around 15-20feet most of the time.

After a while you will learn what to look for(some cachers hide there caches the same way all the time). After awhile 20-30 feet will not seem like much anymore.

 

Last night, the gps was telling me that I was on a different road, and would tell me that I had already completed a turn before I was even to the intersection.

 

By telling you the You were in a different road, do you mean that on the map screen it looked like you were on a road that was parallel to the road you were on?

 

It was a mid-afternoon day, and here in the St. Louis area, we had perfect skies.

 

THe condition of the sky will not effect the GPS accuracy.

[+MedicP1]

This might be obvious to most but I'll assume from your tadpole status that you're bordering on a newbie, if not sorry upfront. Are you using the right datum on your GPSr, that can give you a consistant offset on your unit from where you actually are. If you leave the unit sitting in the open, say at an intersection of roads, does it average out to the proper position?

[+embra]

Some other possibilities: the caches you have been hunting lately may have some error in their coordinates; if the downtown area you were in had tall buildings, you may have had some signal bounce.

 

If you want to check you accuracy against a known coordinate set, look for a benchmark that has been found in your vicinity that has "adjusted" (not scaled) coordinates. (If you're not sure how to do that, tell me your zip code and I'll give you a suggestion or two).

[Pittfan]

Hey. Thanks for the replys. They're helping.

 

Okay, so to answer a few questions all at once here. Yes, I am relatively a "newbie". And, it doesn't hurt me to admit it. I have to start somewhere, and I would admit to it if I felt I didn't fully understand something. That's kind of why I am here now, to ask questions. The road that it was telling me that I was on. I was inbetween two other roads. All three were very close together (city streets), and all three were parallel to each other. At one point, the software was trying to tell me to turn off of one road when I wasn't even on it.

 

I'm not too sure about the datum. Not too sure what it is, and what it should be on. I have heard that cloudy skies affect the ability of the gps to pick up the satellite singnals. That is why I mentioned the weather conditions. When I hold the gps still, it does move. Like, when I stand still, it will read that I am moving around a little bit. Like trying to zero in on me.

 

As for the zip code that I am in. It's 63301.

 

By the way, what is the "baud rate", and what should it be set at?

 

Thanks!

Jason

(Pittfan)

[+welch]

I'm not too sure about the datum.  Not too sure what it is, and what it should be on.  I have heard that cloudy skies affect the ability of the gps to pick up the satellite singnals.  That is why I mentioned the weather conditions.  When I hold the gps still, it does move.  Like, when I stand still, it will read that I am moving around a little bit.  Like trying to zero in on me.

 

By the way, what is the "baud rate", and what should it be set at?

 

Thanks!

Jason

(Pittfan)

I still think you should look for someone with another unit and do a side by side comparsion. check settings (datum, waas on?), handling, everything. then if yours is still acting just whacky consider it may have corrupted alamanc or somthing. rare, but you never know maybe you'll win the lotto too?

it moving it not uncommon, actually since its a magellan it may have went into avegeraging or something when you stopped. As long as it eventually figured out you stoped, couple minutes maybe, and comes back to a relatively stable coord, dozen feet either way, then don't worry.

 

e/

almost forgot. baud rate (maybe someone can give a techinal explaintion?) is how fast the data is being transfered between pc and gps. you should use the hignest one possiable that works.

Edited January 26, 2005 by welch

[Z15]

I have heard that cloudy skies affect the ability of the gps to pick up the satellite singnals. That is why I mentioned the weather conditions.

 

Totally false.

 

Datum,

 

Go thru the menu and look for datum. Select WGS84 or NAD83, those are what you want to use in the USA.

 

The fact you are not on the road the GPSr is saying your on is the fault of the mapping software in the GPSr and not the fault of GPS. e.

 

I am going to suggest you check your unit to a known survey mark below. The bold are is the Lat and Lon

 

Degrees, Minutes, Seconds and Decimal Degrees Latitude/Longitude Conversions

 

DATABASE = Sybase ,PROGRAM = datasheet, VERSION = 7.12

1 National Geodetic Survey, Retrieval Date = JANUARY 26, 2005

AA8570 ***********************************************************************

AA8570 DESIGNATION - CSL 10

AA8570 PID - AA8570

AA8570 STATE/COUNTY- MO/C OF ST LOUIS

AA8570 USGS QUAD - CLAYTON (1993)

AA8570

AA8570 *CURRENT SURVEY CONTROL

AA8570 ___________________________________________________________________

AA8570* NAD 83(1997)- 38 39 13.88921(N) 090 16 55.47164(W) ADJUSTED

AA8570* NAVD 88 - 154.5 (meters) 507. (feet) GPS OBS

AA8570 ___________________________________________________________________

AA8570 X - -24,554.165 (meters) COMP

AA8570 Y - -4,987,455.118 (meters) COMP

AA8570 Z - 3,962,458.621 (meters) COMP

AA8570 LAPLACE CORR- 1.00 (seconds) DEFLEC99

AA8570 ELLIP HEIGHT- 123.35 (meters) (02/17/00) GPS OBS

AA8570 GEOID HEIGHT- -31.13 (meters) GEOID03

AA8570

AA8570 HORZ ORDER - FIRST

AA8570 ELLP ORDER - FOURTH CLASS I

AA8570

AA8570.The horizontal coordinates were established by GPS observations

AA8570.and adjusted by the National Geodetic Survey in February 2000.

AA8570

AA8570.The orthometric height was determined by GPS observations and a

AA8570.high-resolution geoid model.

AA8570

AA8570.The X, Y, and Z were computed from the position and the ellipsoidal ht.

AA8570

AA8570.The Laplace correction was computed from DEFLEC99 derived deflections.

AA8570

AA8570.The ellipsoidal height was determined by GPS observations

AA8570.and is referenced to NAD 83.

AA8570

AA8570.The geoid height was determined by GEOID03.

AA8570

AA8570; North East Units Scale Factor Converg.

AA8570;SPC MO E - 313,029.921 268,968.399 MT 0.99993776 +0 08 10.0

AA8570;UTM 15 - 4,281,871.340 736,511.922 MT 1.00028890 +1 41 54.4

AA8570

AA8570! - Elev Factor x Scale Factor = Combined Factor

AA8570!SPC MO E - 0.99998065 x 0.99993776 = 0.99991841

AA8570!UTM 15 - 0.99998065 x 1.00028890 = 1.00026954

AA8570

AA8570 SUPERSEDED SURVEY CONTROL

AA8570

AA8570 NAD 83(1997)- 38 39 13.88987(N) 090 16 55.47174(W) AD( ) 1

AA8570 NAD 83(1986)- 38 39 13.90434(N) 090 16 55.46784(W) AD( ) 1

AA8570

AA8570.Superseded values are not recommended for survey control.

AA8570.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.

AA8570.See file dsdata.txt to determine how the superseded data were derived.

AA8570

AA8570_U.S. NATIONAL GRID SPATIAL ADDRESS: 15SYC3651281871(NAD 83)

AA8570_MARKER: DD = SURVEY DISK

AA8570_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT

AA8570_STAMPING: CSL-10 1990

AA8570_MARK LOGO: MODNR

AA8570_MAGNETIC: M = MARKER EQUIPPED WITH BAR MAGNET

AA8570_STABILITY: C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO

AA8570+STABILITY: SURFACE MOTION

AA8570_SATELLITE: THE SITE LOCATION WAS REPORTED AS SUITABLE FOR

AA8570+SATELLITE: SATELLITE OBSERVATIONS - 1990

AA8570

AA8570 HISTORY - Date Condition Report By

AA8570 HISTORY - 1990 MONUMENTED MODNR

AA8570

AA8570 STATION DESCRIPTION

AA8570

AA8570'DESCRIBED BY MO DEPT OF NAT RES 1990

AA8570'STATION, AZIMUTH MARKS AND REFERENCE TIES THE STATION IS IN THE NE

AA8570'ANGLE OF THEE INTERSECTION OF DELMAR BLVD. AND AT THE SW CORNER OF THE

AA8570'ST LOUIS REGIONAL MEDICAL CENTER GROUNDS. IT IS 27.80 FT (8.47 M) SW

AA8570'OF A CUT (X) ON THE SOUTH SIDE OF THE SOUTH POST OF THE EMERGENCY

AA8570'ENTRANCE SIGN, 24.33 FT (7.42 M) SSE OF A CUT (X) ON THE EAST CURB OF

AA8570'CLARA AVE, 23.3 FT (7.1 M) EAST OF THE CENTERLINE OF CLARA, 19.31 FT

AA8570'(5.89 M) NW OF A CUT (X) ON THE NORTH DELMAR CURB BY A CONCRETE LIGHT

AA8570'POLE AND 2.5 FT (0.8 M) WEST AND 1.6 FT (0.5 M) NORTH OF EDGES OF THE

AA8570'SIDEWALK, AND 2 FT (0.6 M) SOUTH OF A CARSONITE WITNESS POST THE

AA8570'AZIMUTH MARK IS 0.4 MILES (0.6 KM) WEST ON THE SOUTH SIDE OF DELMAR IN

AA8570'THE SW ANGLE OF THE T-JUNCTION OF LAUREN AVENUE (ONE-WAY NORTH) AT THE

AA8570'WEST END OF BI-STATE COMPLEX. IT IS 30.7 FT (9.4 M) WEST OF THE

AA8570'CENTERLINE OF LAUREN, 29.13 FT (8.88 M) EAST OF A FIRE HYDRANT, 16.81

AA8570'FT (5.12 M) WSW OF A CUT (X) IN THE WEST CURB RETURN FOR LAUREN

AA8570'AVENUE, 12.9 FT (3.9 M) NW OF THE CORNERSTONE OF THE BUILDING IN THE

AA8570'SW ANGLE, 10.35 FT (3.15 M) SE OF A CUT (X) ON THE STONE CURB OF

AA8570'DELMAR, 3.2 FT (1.0 M) SOUTH OF BACK OF CURB AND 1.3 FT (0.4 M) WEST

AA8570'AND 2.5 FT (0.8 M) NORTH OF EDGES OF THE SIDEWALK. STATION AND

AA8570'AZIMUTH MARK TO REACH TO REACH THE STATION FROM THE CENTER OF THE US

AA8570'40 HWY INTERCHANGE WITH KINGSHIGHWAY IN ST LOUIS, GO NORTH ON

AA8570'KINGSHIGHWAY FOR 1.45 MILES (2.33 KM) TO TRAFFIC LIGHT INTERSECTION

AA8570'WITH DELMAR BLVD., TURN LEFT AND GO WEST ON DELMAR FOR 1.0 MILES (1.6

AA8570'KM) TO TRAFFIC LIGHT AT CLARA AVENUE AND STATION SITE ON RIGHT (NORTH)

AA8570'IN NE ANGLE OF INTERSECTION AS DESCRIBED. TO REACH THE AZIMUTH MARK

AA8570'FROM THE STATION CONTINUE WEST ON DELMAR FOR 0.4 MILES (0.6 KM) TO

AA8570'AZIMUTH MARK ON LEFT (SOUTH) SIDE JUST BEYOND THE BI-STATE COMPLEX AND

AA8570'OPPOSITE A MCDONALDS RESTAURANT ON THE NORTH SIDE OF DELMAR AS

AA8570'DESCRIBED. DATE OF REPORT 06-06-1995

 

 

*** retrieval complete.

Elapsed Time = 00:00:00

Edited January 26, 2005 by elcamino

[Pittfan]

AA8570* NAD 83(1997)- 38 39 13.88921(N) 090 16 55.47164(W) ADJUSTED

AA8570* NAVD 88 - 154.5 (meters) 507. (feet) GPS OBS

AA8570 _________________________________________________________________

 

Ok. Sorry, I got lost there. Guess I am a bit more of a newbie then I thought

 

From what I understand, you are suggesting that I eneter into my gps a waypoint, where the coordinates (in latitude and longitude) are the same as a survey marker already placed. I think I understand that part. What I guess I don't understand is the list that was posted. What does the "AA8570" stand for? It looks like after some of them, they are giving a brief description of where some of them are located at. Also, the bold coordinates. I know they're just a bit longer then the normal coordinates that I am used to. Is this where I need to use the link that you supplied?

 

Thanks for the help!

 

By the way. I don't know anyone out here that has a Meridian gps. I just moved out here recently, because of my job, and just recently had some free time to start geocaching again. I would like to compare my findings with another gps'r, but can't right now.

 

Jason

(Pittfan)

[Dale_Lynn]

The "AA5870" is just the name/record/file/marker designator... It has nothing to do with the actual datum for lat./ lon. you will enter for a waypoint. BUT you could use it for the "name" of the way point of your "search"........

 

For conversion of Lat/lon cordinates .... Go here as elcamino stated.

 

http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html

 

Dale

Edited January 26, 2005 by Dale_Lynn

[Pittfan]

AA8570* NAD 83(1997)- 38 39 13.88921(N) 090 16 55.47164(W) ADJUSTED

AA8570* NAVD 88 - 154.5 (meters) 507. (feet) GPS OBS

 

Okay. I think I almost have it. I went to the web site, and tried to input the data. Now, I'm not too sure what I am suppose to be inputting. Neither of the two conversions have a large enough space for the numbers that were given earlier. 1 question would be, am I trying to convert the coordinates to decimal degrees, or from decimal degrees to degrees, minutes, seconds? If I was going to guess, I would think that it would be to the last one (to degrees, minutes, seconds). But, after entering that information, I am getting a weird result.

 

I do not have enough space to enter the entire "38 39 13.88921 (N) 090 16 55.47164 (W)" into either of the two conversion fields that were at the web site. I have enough room to input the 13.88921 (N) 55.47164(W) into the convert the decimals into degrees, minutes, seconds conversion, but what do I do with the 38 39, and the other remainding numbers.

 

I like trying to figure this out, and I'm not asking for someone to do the conversion for me, but it's a bit confusing right now for a newbie with the given information.

 

Thanks!

Jason

(Pittfan)

[+JohnnyVegas]

Try this-this should be an easy test.

On the main Geocaching page look up Benchmarking, these are survey points that are very accuralty placed. Inter your zipcode, there should be one close to were you are. Enter those coordinates and see how closed your GPS gets you to the Benchmark

 

As far a streets, if you are on a main hwy and the GPS shows you on a frontage road next to the highway it could be that you are in the right hand lane and no GPS will be 100% correct, being in most cases you are only getting 20 feet in accuracy this can happen. There is a section on Highway that I drive a couple of times a month that has 6 lanes in each directio and a medium strip that was around 20 feet wide. If I drive on an inside lane my GPS shows me on the road. If I drive on the outside lane it shows me off the highway. I have seen this on 4 lane roads also.

[Pittfan]

Thanks for the response. I found a benchmark that is nearby, and I'll go see what my gps reads at this site. Not too sure what I will find, but I hope that my gps is accurate. I don't want to get into what I do next if it is off by 30-45 feet again, right now. But, I'll be sure to report back what I find out!

 

Thanks!

Jason

(Pittfan)

[+JohnnyVegas]

Good luck with the bench mark

[Z15]

what I do next if it is off by 30-45 feet again

 

Many people think GPS is perfect when it it not, what you are seeing is normal.

That may be the extent of what you can expect from your GPS. Do you know if it has WAAS capabilites? If so then that will get you within 3-5 meters, if not 10 meters (<>30 ft) is about what you can expect from a low priced unit wihtout differential GPS capabilities.

 

I should have been more clear. Yes, do input that lat and lon into you receiver and let it lead you to the mark. You can truncate the 5 decimal places, not to worry about that. The point I gave is within 1-2 CM of being exact. If you input those lat and lon, when you reach the survey mark it will tell you how far off you are. Thats how good your GPS unit it.

 

You've heard the term WAAS, seen it on packaging and ads for Garmin® products, and maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.

 

The origins of WAAS

 

The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

 

How it Works

 

WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

 

Who benefits from WAAS?

 

Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.

 

Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

Edited January 26, 2005 by elcamino

[+embra]

Thanks for the response. I found a benchmark that is nearby, and I'll go see what my gps reads at this site. Not too sure what I will find, but I hope that my gps is accurate. I don't want to get into what I do next if it is off by 30-45 feet again, right now. But, I'll be sure to report back what I find out!

 

Thanks!

Jason

(Pittfan)

As I mentioned before, make sure that the benchmark you are looking for has *adjusted* coordinates. The *scaled* coordinates are just estimates from reading a map, and can be hundreds of yards off.

 

Here is an example of a BM with adjusted coords in your zip: AA8592. Like the one elcamino gave you, lots of pictures to help be sure. This one might be easier to locate, since it seems to be in a U-Haul lot.

[Pittfan]

Here is an example of a BM with adjusted coords in your zip: AA8592.

 

Well, I will give that one a try as well!

 

I just got back from trying the one that I already knew where it was. It was at a Church that I drive by a lot. Some people had posted pictures of the benchmark, and I knew where it was by that. I didn't know that it was there until I started looking for benchmarks today! Well, went there, and I walked right up to the benchmark. Had my gps on for a while when I was driving there. The gps said that I was 69' off. I held the gps right over the top of the benchmark (for about a minute, until it stopped moving the distance scale) to get a good reading. And, then saved my findings. Walked away, and then told my gps to give me a direct route to my new findings. It pointed me right to the new findings, and I held it again right over the top of the benchmark, and it read within 6'.

 

I'll give the one that embra posted about a try. See if I have any luck with that one. The last post on that becnhark though said that they have done some ditch work at that site and couldn't find it. So, I'll see if I have any luck.

 

Oh, and yes. My gps has WAAS enabled. It's a Magellan Meridian Platinum. The only reason that I have concern over the false readings now, is because up until about a month ago, it never gave me such a far off reading. I had always had great luck with it, and finding things under 15' away.

 

Thanks!

Jason

(Pittfan)

[Pittfan]

Sorry, just to add. The one that I just looked for was a scaled location, not adjusted. BM "JC0051"

 

Thnaks!

Edited January 26, 2005 by Pittfan

[+embra]

I'll give the one that embra posted about a try. See if I have any luck with that one. The last post on that becnhark though said that they have done some ditch work at that site and couldn't find it. So, I'll see if I have any luck.

Whoops...I was hasty in overlooking that. Happycycler knows what he's looking for, too, so that one may be gone. I'd like to have you looking for something that you have a very high probability of finding.

 

You can click on the "Find nearest benchmarks" link on that page and get a list of candiidates. Look for one with a "!" icon on the left and a smiley-face on the right. (For example: AA8595) Then make sure that the coords are adjusted. That signifies that the disk was placed using professional grade GPS equipment that is extremely precise.

 

Sorry for the bum lead...but glad I caught you on the scaled coords you ran down. They won't tell you much about your accuracy.

[+EScout]

Some good points earlier about going to the "adjusted" benchmarks.

The one you went to and it showed you 6 feet away must have been adjusted, because scaled BMs are sometimes hundreds of feet off their coords.

To search for one: USGS Datasheets

Pick your state, get county. Choose GPS sites only, get marks. Resort by Lat or Long., choose a few near you (not CORS, look for standard disks.)

 

You will have to convert the coords from seconds to decimal minutes, and round to the one thousandth of a minute. This rounded waypoint can be up to 3 feet away from the disk. If you are good at math, calculate the distance and direction, and stand over the waypoint and read your distance to the waypoint. You might say this is the accuracy of your GPSr at this place and time. I think you will also be very pleased how accurate your GPSr is.

[Z15]

fyi

 

Scaled positions are not accurate for GPS use. All that means it someone out in the field plotted that BM on a map and then scaled the Lat and Lon. You can be anywhere up to 6 seconds off, anywhere up to 500 ft or more depending in the quaility of the plotting.

 

Do use adjusted.

 

Another thing, there are times during the days when the statelites are not in good geometry and thus the position solution will not be as accurate.

 

Dilution of Precision Values

Position Dilution of Precision (PDOP)

The DOP value for the three dimensional position solution.

 

Dilution of Precision (DOP)

The geometrical effect on GPS accuracy. The position of GPS satellites in orbit determine the DOP values for a given site. The higher the DOP value, the less accurate the position solution.

 

This should confuse..

 

 

US Coast Guard - Navigation Center

 

GPS OPERATIONAL ADVISORY

Edited January 26, 2005 by elcamino

[Pittfan]

Okay. So, to reply real quick to EScout. When I said that I was over a benchmark disc, and that my gps was reading 6 ft away, that was only after I had marked that spot as a new waypoint. Walked away, and then asked my gps to find that waypoint again. That is when it found it within 6 feet. But, as stated before, that BM that I went to, was a scaled position. Not an adjusted.

 

I just got back from going to two adjusted BM's. Here is what I found.

 

The first BM (AA8595) I found only because I printed out the description, and found it under some grass. I held my gps over the BM for a minute, and it said that the BM was 76 feet away.

 

The second BM (AA8622), I found quite easily. There was one of those "witness" markers in blazing orange. I walked right up to it, held my gps over the BM for a minute. It said that the BM was 85 feet away. At this location, I could spend a little bit more time at, so I continued to hold the gps over the BM for about 5 minutes, the gps got all the way up to 126' away.

 

I did mark what the gps said was the coordinates at each of the BMs. Not too sure that they will end up doing me any good, but thought that I would while I was there.

 

So, this is kind of why it is taking a lot of the fun out of looking for geocaches. Having to search in a very large radius.

 

Thanks!

Jason

(Pittfan)

[+embra]

Go thru the menu and look for datum. Select WGS84 or NAD83, those are what you want to use in the USA.

 

BTW, are we getting you hooked on benchmarking yet?

OK, Jason, you've gathered some useful information. You've established that your GPS is consistent, getting back to within a mark you've made within expected tolerance. And you've verified that you've got some sizeably different readings from known good points. My suspicion now is that you may have some datum setting that's other than what elcamino suggested.

 

Can you verify what your settings are? I don't recall your saying what kind of a GPS you have. On Magellans it's accessed through menu/setup/map datum. I'd go with WGS84.

 

Map datum refers to the coordinate system you are using. It's a little like metric vs standard measures...you can convert between the two, but if I tell you to walk north 100, and you're thinking meters while I'm thinking feet, you'll end up someplace other than where I intended.

 

There are map datums that will have you off by 100 feet or more from what WGS84, the most commonly used system (around here, anyway) would have you at.

[+EScout]

I think embra has the cause of your problem.

 

Also, please make sure you are converting correctly.

 

If it lists this as the BMs lat. 33 22 13.12785

 

which is in 33 degrees, 22 minutes, 13 point 12785 seconds.

 

converted to standard usage and rounded: 33 22.219

which is 33 degrees, 22 point 219 minutes.

 

When I have gone to these BMs, it usually shows me to be within 2 to 10 feet of the waypoint I entered (rounded coords.)

Edited January 27, 2005 by EScout

[Pittfan]

I have a Magllan Platinum. I did double check it when (I think) elcamino first mentioned it, and before I went to all 3 BMs. The primary map datum is set at WGS84, the secondary is set at NAD83.

 

As far as what escout posted. When I did the search for the BMs, I was using the benchmarks that are listed through a link on the geocaching web site. They have the coordinates listed there, so that I didn't have to convert the numbers to anything. But thanks for the tip. If I did overlook that, I know that I would be off by quite a bit

 

Thanks!

Jason

[+jacques0]

Referring back to your first couple posts, where it sounded as though you were driving, I have noticed that if I'm zoomed in on my GPS' base map, that my position is often shown one side or the other of the road. In other words, if I'm going 60 mph and I have the map zoomed way in (to the 500', or 250', or 100' scale, for example) then my location appears to be off.

 

If I zoom out to a more reasonable level for driving (.5, or 1 or 2 miles, e.g.) then it's right on the money. When walking or biking, it doesn't make a difference.

 

I'm not sure if this is applicable in your case, but perhaps worth a look. Maybe that's why you had good results with the benchmark.

[Pittfan]

Maybe that's why you had good results with the benchmark.

 

To be honest, I didn't have any luck yet with the BMs. I mean, I found them, but my gps only got me within 70 feet of all 3 BMs. The thing that did work for me, was when I set a mark, walked away from it, and asked my gps to return to that point. It got me back to that point within 6 feet. It just so happend that it was over the top of the 1st BM that I went to.

 

Where I am having problems, from what I gather, is that if someone were to give me a set of coordinates, and I punched them into my gps, I would only be able to get to about 70 feet of that point. Which in turn, I think is a little bit outside of the norm.

 

Not too sure if it has anything to do with the software that I am running. Maybe that is what I have to look at next. I used to be doing ok when I had the streets and destinations loaded on the gps. I put the direct route on over the summer, and just recetly I have started up again doing geocaching. But, something tells me it isn't the software.

 

Thnaks!

Jason

[+Warriorrider]

You might want to try this:

Menu>Setup>Clear Memory>All

Then reinitialize your MeriPlat and let it sit out in the open for about 20 minutes or so. This may or may not work, but it might be worth a try at this point.

Mine has acted funny a couple of time and this cured it.

[+embra]

A few other loose ends to rule out:

 

Jason, can you confirm that in Menu/Setup/Coor System you are using deg/min.mmm?

 

WGS84 and NAD83 are virtually identical...the error you are seeing is not due to any confusion between those two.

 

Do you recall what kind of EPE reading your GPS gave you? That should be viewable from the position screen. If you got within 6' of a previous mark, I doubt that you were having problems with something like poor satellite positions.

 

Warriorrider's suggestion may be the last thing I can think of to try.

 

A question out of curiousity that may or may not have relevance: do you send waypoints over to your GPS from the computer, or enter them manually?

 

Edit: I'd agree with you on the software issue. You might find differences in the map displays, but if anything the DirectRoute should be more accurate than Streets & Destinations.

Edited January 27, 2005 by embra

[Pittfan]

Do you recall what kind of EPE reading your GPS gave you? That should be viewable from the position screen. If you got within 6' of a previous mark, I doubt that you were having problems with something like poor satellite positions.

 

I'm not too sure. Unless the EPE stands for something like how many satellites my gps was picking up. I don't know what "EPE" stands for, but from that last sentence, it sounds like "how many satellites were you picking up". I was paying attention to that, and it was somewhere around 8.

 

Jason, can you confirm that in Menu/Setup/Coor System you are using deg/min.mmm?

 

Yes. It is just as you asked.

 

A question out of curiousity that may or may not have relevance: do you send waypoints over to your GPS from the computer, or enter them manually?

 

No. I don't send user waypoints to my gps from the computer.

 

Warriorrider's suggestion

reinitialize your MeriPlat

may be the last thing I can think of to try.

 

I think I am going to upload all of the waypoints that I have currently saved on my gps, up to my computer before I re-initialize my gps. I am also going to try out something before I do that. I'm going to go outside (just outside my house), mark a waypoint out in the open over top a mark. Write down the coordinates that it gives to me, then after I re-initialize my gps, see if it takes me back to that exact spot again.

 

Thanks for the tip about the software. I wasn't too sure what to think about the 2 compared to each other.

 

I'll let you know what I find out in a little bit!

Jason

[Z15]

The Maps are rarely as accurate as the GPS. I have several map programs on my notebook which is hooked to my Garmin GPS V with an extermal antenna on my truck. My location shows on the notebook and I can drive righ to my survye locations. None of them show my survey marks is the exact posistion there are in the ground. The maps are off. The only one that come close if Delorme TopoQuads.

 

For instance, one survey mark I have used is on the highest point of a small hill yet all my computor maps using the precise position of the point, plot it off the top of the hill, aboout 100 ft from where the map shows the top of the hill. So the map are close but not always perfect in all locations but many people assume the are perfect. They are not, the will tell you the general area but thats about. If you want more precise maps, be preparted to spend a great deal more money.

 

FYI- No surveyor will use these hand-held GPS units for anything more than general locations. Like I mentioned before, there are times when accuracy is not as good, some time during a given day will be worse depending on your Latitude (not attitude). If you have to many birds all in one general sky locations, you position will not be as good as when the birds are all over the sky in better geometry.

 

Do you have a screen with shows all the birds you are locked onto? The more the better. For instance I notice I get my best posistions when I have 6 or more birds and a WAAS bird. Also Note-While you may have WAAS, it does not mean you can track those birds all the time. My GPS V will show a 'D' in the bar graph when WAAS has corrected the location. So check that out, wish we were close by, we could compare receivers (I have 5, all Garmins, 2 WAAS enabled).

Edited January 27, 2005 by elcamino

[Pittfan]

Hey all.

 

Sorry that I haven't written/updated on this for a few days. I have been busy with other projects.

 

Just wanted to let you in on one of the experiments that I tried today.

 

I took the advice of Warriorrider, and reinitialized my gps. I think it worked. If you remember in one of my previous posts, I had marked a spot outside of my house as a waypoint. Prior to clearing the memory on my gps, I wrote down the coordinates. After I reinitialized my gps, I let it sit by my window for 20 mins. Took it outside, and went right to the spot on the ground (at my waypoint). The gps said that I was 54' away. I had 10 satellites, and 2 W. Which I think is what some of you are calling "birds".

 

I'm going to go out while there is still some daylight right now, and try one of those geocaches that I couldn't get to. I might try one of the 2 BMs that are adjusted, but both of the ones that I used the other day are about a 30 minute drive for me. I'll see if I have the time. But, I will make it to one of those sometime soon.

 

Thanks, and I'll update again after I find out my findings.

 

Jason

[Pittfan]

Well, I had a chance to do some more work. I went to one geocache that I was unable to find, and I still couldn't find it. This particular cache, anohter person posted that they were getting a reading that wasn't very close to the cache, but that they found it anyways. So, I decided to let this cache go for a little while, and I went to one of the BMs. At the BM, my gps was still reading off. I went to the BM (because I had been there before, and knew where it was), and the gps had a lot of movement while I was holding it over the BM. So, I stood there for about 3 minutes, and the gps got to about an average of 56' away. I had used the magellangps.com website to e-mail about this problem about a week ago, and this is part of the e-mail that they sent to me.

 

For the extended calibration, with the GPS off, hold down the NAV button and the power button at the same time until a box with 00 appears. Enter 21 using the arrows on the keypad; then press ENTER.

 

So, after I only got to 56' away, I went back to my car, and tried this trick. It basically went through a routine just as if I wanted to do an extended calibration of my compass. But, I did it anyways. And, then went back over to the BM. Bingo. It said that I was within 9' of the BM, and it didn't take very long for it to say that I was that close.

 

I went back to the previous geocache, to see if I could go to it now that I had performed the calibration. No luck. I plan on trying a different geocache. Maybe the one that I was going to today is not set up properly.

 

Thanks!

Jason

[+Warriorrider]

Interesting. I'm glad to hear that the problem is fixed. It slipped my mind to ask if you had done a full calibration of the compass after the firmware upgrade. I have always needed to do one after an upgrade. You will usually know right away by looking at your weather screen. If it's not working it's because a full calibration is needed. A little late now, but that's the easiest way to tell after doing a firmware upgrade.

[peter]

Seems very odd that a compass recalibration would have any effect on the GPS position accuracy. Has anyone else observed this correlation, or have an explanation for any conceivable mechanism?

Certainly it makes sense that it would be needed for good results with the magnetic compass, but the GPS positions should be entirely indipendent of the magnetic fluxgate sensor.

[+BillyBean]

It's funny you ask Peter. I have a Gold & Platinum. While both sitting side by side on a picnic table (in the open) both will read identical or very close. As soon as I start walking or moving, the Platinum goes wacky. My heading was ranging from 54-97, while heading NORTH. The gold was reading a steady 356. I turned and walked South and the Platinum was still reading the same and the gold was about 190. I have to do a recalibration, but I can not figure out why they read the same marked location, but the heading is so far off. Although, while both units were on teh table, the headings were about the same off. If anyone has suggestions, please let me know!

 

BillyBean

[peter]

I can not figure out why they read the same marked location, but the heading is so far off.

It makes sense that the heading would be way off in the Platinum if it hasn't been recalibrated. Say the batteries (or anything else in or near the unit) became fairly strongly magnetized since the previous calibration. Then the magnetic field of the earth would be competing with the field from the batteries/etc. and you might see about the same heading no matter which way you're actually going. Recalibrating lets the unit know how much of the magnetic field is due to such nearby disturbances so it can subtract them off and be left with just the effect of the earth's magnetic field.

 

However, in your observations it sounds like the actual GPS position given by the Platinum was not affected by the recalibration which is what I'd expect - it should only affect the compass headings, *not* the lat/long position. But Pitfan reported a position shift upon recalibration which is what I find hard to understand.

 

I'm still wondering if others observe that kind of behavior with the Platinum and if there's any known reason for the recalibration to affect measured GPS positions.

[+embra]

I'm with you in wondering about this, Peter, for the reasons you give. All I can think of is that if a full compass calibration is needed to get things like the weather features clicking, that this is a deep level process that ties in with other basic functions that may not have been obvious to us.

 

Since I've always calibrated fairly soon after a firmware upgrade, I've never noticed a problem. Guess this is an empirical quesion that would stand up to an experiment or two...

 

Edit: I, too, am glad your problem seems resolved Jason (well, except for that elusive geocache ). If you did your full calibration on the car or even close to it, you may want to repeat the process well away from metal that would affect your magnetic compass settings.

Edited February 1, 2005 by embra

[Pittfan]

I might try and do the rcalibration away from my car again. I haven't been able to get away and do any more work with it, so I don't know what to expect. There are several more geocaches around here to check out. I was just happy to have my gps get me to within 10' of that one BM. It just seems to me that is one giant step in the right direction.

 

On a side note, I set up a SD card reader so that I can download to it, instead of my gps. I think things are going to go a little bit quicker the next time I load a map onto it.

 

I might try and make time tomorrow for doing a test run on another geocache. I'll post to let you know how it goes.

 

Thanks for the help!

Jason

[+Warriorrider]

I might try and do the rcalibration away from my car again....

You did your full calibration at your car?

The intructions say to keep your GPS away from anything metal when calibrating.

[Pittfan]

Yeah, I know that the instructions said something along the lines of keeping away from metal, but I was doing it on top of the center arm rest. Not too much metal there, unless you want to get picky. I know that it wasn't the greatest place to do it, but it was kind of freezing outside at the time. Like I said, I'm going to do it outside when I get a chance . Just need some time to do it right now.

 

Thanks.

[+Warriorrider]

Yeah, I know that the instructions said something along the lines of keeping away from metal, but I was doing it on top of the center arm rest. Not too much metal there, unless you want to get picky. I know that it wasn't the greatest place to do it, but it was kind of freezing outside at the time. Like I said, I'm going to do it outside when I get a chance . Just need some time to do it right now.

 

Thanks.

Yeah, I never know how far is far enough. I wish the book was a little more detailed about this. I don't know if they are talking inches or feet.

[+BillyBean]

I read somewhere that sometimes the compass in the Platinum may be off while enclosed inside the sheet metal of a car. Has anyone else heard that?

[+Warriorrider]

I read somewhere that sometimes the compass in the Platinum may be off while enclosed inside the sheet metal of a car. Has anyone else heard that?

Right. I cannot use the compass screen while in the car. But that is ok. I only need that screen when on foot.

[Pittfan]

Just wanted to give you guys an update on this topic.

 

I seem to basically have this one won. I went out yesterday to do a little more geocaching, and the gps worked great. 3 of 5 found. 2 of those 3 I walked right up to, and they were the first thing that I touched. The 3rd cache I had to look a bit more for, but after I got a WAAS bird locked onto my gps, it got me to within 16' of the cache spot. And, thats not to say that their coordinates could be a little off.

 

1 of the 2 that I couldn't find is well hidden. It's one of those mini caches (which I am finding out that I'm not that big of a fan of;). The post that other geocachers have logged say that it is well hidden around some rocks. I plan on going back out to it. The 2nd geocache that I couldn't find yesterday, is that one that has been bothering me all the long since I have been posting regarding this topic. I didn't get a good chance to attack it yesterday though. It's in a local park, and the park patrol was out there yesterday, so I didn't want to make them wonder about what I was up to. I will try that one again sometime. Bottom line, the two that I didn't find yesterday, I know are a result of the gps user.

 

But, it sounds like I have this problem licked. Thanks for everyone that posted. I think it turned out that I needed to do that code trick that Thales told me about, AND doing it outside of a car in a clear opening (away from metal objects).

 

Thanks again, and happy posting!

Jason

(Pittfan)

[Z15]

Geometric Dilution of Precision (GDOP)

 

 

They more birds you have the tighter the accuracy, check out the link above. It may help understand why GPS seems so far off at times. Its all in the geometry..

 

 

Edited February 9, 2005 by elcamino