Datum Used In Geocaching.com

[Bumpa]

This has probably been asked before, but what datum is used for the cache waypoints listed on the web site? I had assumed it was WGS864, but after trying my first find attempt, I'm not so sure!

 

Bumpa

[+Renegade Knight]

It's WGS84 However finding a cache isn't as simple as walking up to the waypoint reading a zero distance and looking down. People can be quite good at hiding caches. My overall skunk ratio is about 11% meaning at least one in 10 caches gets me the first time out and sometimes a few more times before I find it. Some I never find.

 

When you start, start for the 1/1 caches to get a feel for it. Then start in on the harder ones.

Edited December 12, 2004 by Renegade Knight

[+ABJ]

Funny, I had the same question. Today, I tried to find my first cache and could not. I, too, questioned the datum because the cache description gave coordinates for parking. I knew I was in the right spot yet my GPS was not reading the coordinates given. I came home, read my GPS manual, etc. Finally, after poking around the geocaching website I realized my error, the coordinate system I was using.

I have my GPS set to hours. minutes. seconds. The cache descriptions use hours.minutes.(decimal)minutes. I was looking for .556 minutes on my GPS when I should have looking for 33 seconds. That's quite a differnce in distance.

Good luck, I hope you find your cache.

[+Damenace]

It also depends on the type of cache you are hunting for. I have a puzzle cache called Australian Coin Cache and it does not involve WGS84. Part of the challenge is determing what datum to use.

 

Damenace

[+Jamie Z]

I was looking for .556 minutes on my GPS when I should have looking for 33 seconds.

Far easier than reading the raw coordinate data off your GPS and trying to match the cache coords is to upload the waypoints to your GPS.

 

Not only will this give you an easy-to-use arrow right to the cache (location), but you can even leave your GPS in HHDDD°MM'SS". On upload, the GPS will simply convert from the format which gc.com uses to whichever format you have your GPS set on.

 

The arrow thingy is the main reason though. It makes looking for the cache much easier.

 

Jamie

[Jimbo810]

Hi All,

 

Just getting back to the origional question though. I haven't heard any explanation of what the different datums mean. I know the difference between the MGRS and CGRS. I use the MGRS regularily. Its the other datum sets that leave me scratching my head. NAD27 for a myriad of countries, WGS84 and the list goes on.

 

Jimbo810

[+Sputnik 57]

All datum sets use a mathematical ellipsoid as a "guess" of the shape of the earth. They are all a little off, but the North American Datum of 1927 (NAD27) has become pretty widely used. Most USGS Topo maps still use this datum, I believe. Later, an international standard called the World Geodetic System of 1984 (WGS84), based upon the North American Datum of 1983, was adopted. Most GPS receives come set on WGS84 from the factory, and unless otherwise noted, all cache pages on geocaching.com use the WGS84 datum.

 

The NAD27 location can be 200 yards away from the WGS84 datum spot with the same coordinates .

 

Edit: To correct the name and elaborate.

Edited December 16, 2004 by Sputnik 57

[+Rakusan]

Pretty close ...

 

The geodetic datum is not a "guess," but a mathematical approximation, based on a web of highly-accurate measurements of the relative positions of chosen points across a region, continent, or the entire world. These measurements are fitted together by painstaking computation, to determine the exact curvature that will be a best fit to the measured positions. Most of these datums (incidentally, "data" means raw material for computation whereas "datums" specifically means more than one of these mathematical models) were created for individual nations, and due to the way they're devised they don't fit together very well - it requires some fancy computation to "shift" a position determined on one datum to the coordinate system of another datum.

 

After WWII, many international coalitions worked to tie together their geodetic networks and to devise a datum that covered continents ... for instance, the NATO nations, or South America. Part of the work of the International Geophysical Year (1957) was to find a way to tie the whole world into one geodetic whole.

 

Then came the satellite ... and astronomers, geodesists, earth and space scientists learned to use their orbits to determine the actual mass center of the Earth. From that, they were able to fit the whole puzzle of these differing datums into a workable "average fit" that could be used worldwide ... first the World Geodetic System of 1966, then a refinement in 1972, and the present WGS of 1984.

 

WGS 84 is the geodetic basis of the GPS system, because it is a good average fit ... but many older maps and charts have been made to fit the old local datum (such as the NAD 27, or North American Datum of 1927) and must be "datum-shifted" to fit the WGS 84. An extra headache for the cartographic sciences.

 

Sorry to get pedantic on you ... geodesy is part of my profession.