Groundspeak Forums: Common coordinate format for geocaching? - Groundspeak Forums

Jump to content

Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic

Common coordinate format for geocaching?

#1 Guest_Rubbertoe_*

  • Group: Guests

Posted 22 June 2001 - 12:49 PM

Okay, I've set my GPS unit to "DM" - and it allows me to enter the coordinates exactly as I've found them on a couple of geocache pages.



But, I noticed on the form to log a cache that I've hidden myself - that it asks for coordinates in the format of:



Latitude (N/S HDDD MM.MM)*

(WGS84 datum)



Now, what exactly do those letters stand for? I'd assume that a "DM" type coordinate would be more like ( N/S DDD MM.MMM ) instead... and what is that "H" in there for?



Is "DM" the most commonly used format when doing GPS activities? Maybe someone can point me to a good page about this so I'm not so confused.

I certainly don't wanna screw up when I hide my first cache here in a few days. Posted Image

#2 Guest_Rubbertoe_*

  • Group: Guests

Posted 22 June 2001 - 12:50 PM

Heh... don't mind my errant HTML codes - I'm obviously a moron. *laugh*

Anyway - any info on the preferred formats of coordinates is appreciated. ;)

#3 Guest_Rubbertoe_*

  • Group: Guests

Posted 22 June 2001 - 12:57 PM

Answering my own question a bit... it seems that most online map servers and software choose to use either the decimal degree format, or the DD MM SS.S format.

Just wondering if there was a reason why Geocaching has determined the format that it uses to be the best of them all.

Posted Image Full of questions...

Toe.

#4 Guest_Cybeq_*

  • Group: Guests

Posted 22 June 2001 - 03:15 PM

I don't know why they use the format they do. I prefer UTM. It works well with USGS maps and the grid squares are always the same size (unlike lat/long). Every GPS I know can easily convert to any format you want to use.

------------------
"You know you've reached middle age when all you exercise is caution."

#5 Guest_RXQ_*

  • Group: Guests

Posted 22 June 2001 - 09:01 PM

The "H" stands for Hemisphere as in North, South, East, West

#6 Guest_RXQ_*

  • Group: Guests

Posted 22 June 2001 - 09:40 PM

here isn't one standard but several for coordinates. There are also over a hundred different Datum's depending on what map you use. Most hiker type, like myself, like using USGS topo maps that uses NAD27 and UTM. The real reason for Geocaching for using the HDDD MM.MMM (WGS-84)is found somewhere in this forum, because people bring up this topic every 3-4 weeks. The goods news is that your GPS unit allows you to change coordinates and Dataum with a push of a button. Actually it several pushes, but no math is required. Also trust your GPS for Coordinate Translation and not JeEep.com. A lot of people have found them to be in error.

#7 Guest_JasonW_*

  • Group: Guests

Posted 22 June 2001 - 09:56 PM

quote:
Originally posted by Cybeq:
I don't know why they use the format they do. I prefer UTM. It works well with USGS maps and the grid squares are always the same size (unlike lat/long). Every GPS I know can easily convert to any format you want to use.




Perhaps the format was chosen because this is a global pastime, not like the "World" Series Posted Image - USGS maps tend not to be available for areas outside the US.... personally I'd prefer it if everyone were to use OSGB/GRB36 format because that works with my local topographic (Ordnance Survey) maps (1:25000/1:50000 scale) however like USGS maps they are only available for a small area of the planet, so I'll live with a universal co-ordinate system for the globe - it works for everyone!!!

As you say the GPS will make the conversion and it does mean that you rarely know exactly where you're headed to - so that adds a little to the 'mystery' of seeking the cache.

#8 Guest_jeremy_*

  • Group: Guests

Posted 23 June 2001 - 01:56 AM

We use this format for three reasons -

1. It's the most accurate of all the versions, including decimal.
2. Most GPS units use this format out of the box. It reduced confusion for new geocachers.
3. Using WGS84 instead of, say NAD27 is because the satellites broadcast in WGS84. Keeping this datum makes the coordinates more precise.

The good news is there are many ways to make the translation. If you want UTM, you can go to the jeeep web site. If you want to convert back and forth between different datums and formats you can use the functions on your GPS unit.

I suppose there are many different datums that would be optimal for different people, but we had to pick some standard and stick with it Posted Image Kind of like how elevator music isn't liked by anyone really but it does the trick.

Jeremy

[This message has been edited by jeremy (edited 23 June 2001).]

#9 Guest_MJ12_*

  • Group: Guests

Posted 23 June 2001 - 07:34 AM

So how would the following coordinates:

Latitude

Degree Minute Second
39°19'18.5" N

Degree Decimal
39.321805

Longitude

Degree Minute Second
84°34'32" W

Degree Decimal
-84.575555

be expressed in the format:

Latitude (HDDD MM.MM)

Longitude (HDDD MM.MM)

I appreciate the help!

#10 Guest_Rubbertoe_*

  • Group: Guests

Posted 23 June 2001 - 11:16 AM

quote:
Originally posted by RXQ:
The real reason for Geocaching for using the HDDD MM.MMM (WGS-84)is found somewhere in this forum, because people bring up this topic every 3-4 weeks.



Sorry. I looked around a bit - evidently not enough. Posted Image

But yeah - once the waypoint is saved, in whatever format, it seems to convert pretty faithfully to any other format - in the GPS unit itself, that is. Thanks

#11 Guest_JasonW_*

  • Group: Guests

Posted 23 June 2001 - 08:33 PM

quote:
Originally posted by MJ12:
So how would the following coordinates:



First things first, there's no mathematical difference in the degrees of latitude or longitude - the conversion method is the same:

DD.ddddd° - the DD will stay the same. Now multiply the 0.ddddd by 60 to give you minutes (and decimals of a minute) as MM.mmmmm if you want seconds do the same again and you'll have SS.sssss - only the first place of decimal is reasonable to use of course - if you use the rest, you're falling into the false precision argument.

To convert from DD°MM'SS.s" simply take the SS.s divide by 60, add the MM - divide the result by 60 and add the DD will give you DD.ddddd°

The ± is simple - North & East are positive, South & West are negative.

So now your examples:

Degree Minute Second
39°19'18.5" N


DD=39
MM=19
SS=18.5

You want DD°MM.mmmm' so all you need is to divide the SS by 60 and add it to the minutes - in this case 19.3083 so your final answer is 39°19.308' N

Degree Decimal
39.321805


DD = 39
dddddd = 0.321805

Multiply the dddddd by 60 -> 19.3083 (hmmmm familiar)

So the DD°MM.mmm' is 39°19.308' (it's positive so it's either North or East, it's latitide so it must be North)

39°19.308'N

Longitude

I'll leave those as an exercise for the interested reader Posted Image

#12 Guest_RXQ_*

  • Group: Guests

Posted 23 June 2001 - 11:03 PM

Rubbertoe, what can I say, 3-4 weeks ago or yesterday, I knew the answer was there somewhere. You see,I have trouble with this time space continuum, that why I got a GPS. Now I know where and when I am, at all times.(Cue in Twilight Zone theme music).

#13 Guest_MJ12_*

  • Group: Guests

Posted 24 June 2001 - 08:19 AM

Thanks for the info - I still don't know what the "H" stands for, but I guess I don't need to know. I was able to enter the coordinates with the explanation you gave me. The main thing I needed to know was that I had to convert seconds to minutes.

#14 Guest_davis513_*

  • Group: Guests

Posted 24 June 2001 - 09:07 AM

t>
Originally posted by RXQ:
The "H" stands for Hemisphere as in North, South, East, West



#15 Guest_rebobbitt_*

  • Group: Guests

Posted 24 June 2001 - 11:27 PM

Jeremy is wrong about the accuracy of the different formats of lat/lon.

Here are the accuracies (in Latitude) for each of the lat/lon formats used:

ddd.ddddd = 3.6 ft
ddd mm.mmm = 6.0 ft
ddd mm ss.s = 10.1 ft

So decimal degrees with 5 decimal places is most accurate. Of course, you can make each one more accurate by adding decimal places, but the Garmin's only use these three, so they are the only choices for Garmin owners.

This is easy to calculate because 1 minute of latitude = 1 nautical mile = 6076 ft. So 0.001 minutes is 6.076 ft. Longitude distances vary by latitude, so the Longitude difference is cosine(latitude) * 6ft (for DD MM.MMM). 0.00001 degrees = 0.0006 minutes = 3.6ft. 0.1 seconds = 0.0016 minutes = 10.1 ft.

quote:
Originally posted by jeremy:
We use this format for three reasons -

1. It's the most accurate of all the versions, including decimal.
2. Most GPS units use this format out of the box. It reduced confusion for new geocachers.
3. Using WGS84 instead of, say NAD27 is because the satellites broadcast in WGS84. Keeping this datum makes the coordinates more precise.

Jeremy




#16 Guest_jeremy_*

  • Group: Guests

Posted 25 June 2001 - 01:35 AM

quote:
Originally posted by rebobbitt:
Jeremy is wrong about the accuracy of the different formats of lat/lon.

Here are the accuracies (in Latitude) for each of the lat/lon formats used:

ddd.ddddd = 3.6 ft
ddd mm.mmm = 6.0 ft
ddd mm ss.s = 10.1 ft



Ouch. I'm wrong? Taken into context of GPS usage (like the Garmin), the version we use on the site is the most exact, correct?

To make a fair comparison, bring this to 5 decimal places.

ddd mm.mmmmm

Obviously the more decimal places out I go, the more exact the coordinates. Because of the restrictions on Garmin (same as Magellan?), sticking to ddd mm.mmm makes more sense.

Jeremy

#17 Guest_rebobbitt_*

  • Group: Guests

Posted 25 June 2001 - 02:27 AM

No, I WAS specifically taking into context the GPS usage, using the 3 formats supported by Garmin, and in that context the "ddd.ddddd" IS the most accurate.

I am not doing a "fair" comparison, I am doing a comparison of the formats supported by Garmin. To take your example furthur, if you take "dd mm ss.s" to 5 decimal places "dd mm ss.sssss" that would be more accurate. The minutes format taken to 4 decimal places "dd mm.mmmm" would be more accurate that "dd.ddddd", but that isn't supported by Garmin, so I wasn't comparing them.

Now, I wasn't talking about which format would be the BEST to use for caching, only which one is more accurate. Taking into account Garmin's default and what most people use, "dd mm.mmm" might be the best one to use. Maybe you could change the header of the caches to display in all three different formats, so people can keep their GPS in their favorite format.

In a similar vain, WGS84 is not any more accurate that NAD27, just different, but since most GPS receivers default to WGS84, that's the one that makes sense.

Rick

quote:
Originally posted by jeremy:
Ouch. I'm wrong? Taken into context of GPS usage (like the Garmin), the version we use on the site is the most exact, correct?

To make a fair comparison, bring this to 5 decimal places.

ddd mm.mmmmm

Obviously the more decimal places out I go, the more exact the coordinates. Because of the restrictions on Garmin (same as Magellan?), sticking to ddd mm.mmm makes more sense.

Jeremy



#18 Guest_jeremy_*

  • Group: Guests

Posted 25 June 2001 - 03:40 AM

Ok. I haven't personally done the math. I was basing the accuracy on a previous post in the forums. When it comes to numbers I'm normally wrong anyway.

I asked previously about if people were interested in going with decimal only. The consensus was no due to the factory settings of the GPS units and the confusion with positive and negative for different hemispheres. So we've stuck with consistency as not to confuse anyone.

The Geodiscovery Geode uses decimal format, but otherwise I haven't had too many requests for alternate formats (other than the occasional UTM request or specific to a country). WGS84 being what the satellites use (and correct me if I'm wrong here), makes sense to use that for the site.

Jeremy

#19 Guest_Mike_Teague_*

  • Group: Guests

Posted 25 June 2001 - 11:28 AM

I'd also add that everyone check their GPS receiver's manual...

If I'm not mistaken, the accuracy of the position solution (for every GPS receiver I've owned) is _15 METERS RMS_ (without SA)

I will also add that running to more decimal places increases "precision" of the number, not "accuracy"... It has nothing to do with the solution that the GPS brain itself is calculating.. Garbage in garbage out, and all that...

Everyone already realizes that in reality, thousandths of minutes is already too much "precision" in a coordinate... We all know that we need to wander around about a 50 foot radius to find a cache once one's GPS says their "on top of it"..

A thousandth of a minute is more than good enough for any consumer GPS unit...

#20 Guest_rebobbitt_*

  • Group: Guests

Posted 26 June 2001 - 01:23 AM

use the format with most precision. Others may use the format that their GPS was set to by the factory. To each his own.

P.S. I have posted this reply several times already, but each time it has disappeared within an hour or so. I even sent a query to Groundspeak's webmaster, so far with no reply. If anyone sees this, can you reply to it to let me know I'm not dreaming? Thanks.


[This message has been edited by rebobbitt (edited 26 June 2001).]

#21 Guest_jeremy_*

  • Group: Guests

Posted 26 June 2001 - 01:45 AM

My opinion is that multiple formats are too confusing. Better to have a chosen format, stick with it, and provide links to places where that coordinate can be converted into other versions.

I get enough confusion over a single coordinate forumat. I'd hate to open up a can of worms because some cache pages have one format and some have another.

Most GPS units do conversions anyway. Why reinvent the wheel?

Jeremy

#22 Guest_Rubbertoe_*

  • Group: Guests

Posted 26 June 2001 - 09:33 AM

quote:
Originally posted by jeremy:
My opinion is that multiple formats are too confusing. Better to have a chosen format, stick with it, and provide links to places where that coordinate can be converted into other versions.



I have to agree.... and having started this whole thread (probably not the first time it's been mentioned, though) - I thought I should at least comment one more time. Posted Image

The only reason I wondered about the format, was that it seemed to be different than what most other online maps use - but, like it has been said, the GPS unit itself can do translations on the fly, so I don't think it's hurting anything by keeping a 'standard' way of doing it here.

Toe.

#23 Guest_rediguana_*

  • Group: Guests

Posted 30 June 2001 - 07:43 PM

I'm just guessing here, but won't all GPS units keep coordinates in a floating format ie 12.34567 to a certain number of decimal places. Only when the coord is formated for display will it be converted to the readable preference. This would make calculations much easier as little or no preprocessing would be required.

I don't really think it's important that we get hung up arguing over storage format vs display format. Who cares how it is stored in the GPS unit, as long as it is displayed in an appropriate format. I believe the EasyGPS also uses a floating point value, ie 12.34567.

All formats provide the same precision - within standard errors - they just have different stepping units, most of which are about the most accurate possible without using WAAS or DGPS. But they are all using the same numbers for the processing, its only the format that they are changed to for display on the unit that differs.

Of course it may be a problem if you enter coord data manually, but if you are uploading floating point waypoints using EasyGPS for example, you should have an accurate enough value.

Cheers Gavin

PS There are a couple of good arguments for the use of WGS84 here and here

The European Air Traffic Control Harmonisation and Integration Programme
http://www.wgs84.com/wgs84/wgs84.htm

US National Geodetic Survey FAQ
http://www.ngs.noaa.gov/faq.shtml#WGS84


[This message has been edited by rediguana (edited 01 July 2001).]

Share this topic:


Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic