Map Datum

[Oaxaca Man]

Can someone explain to me why my gps has so many map datum choices? Mine is set for WGS84. Under what circumstances would I want to change it to ... say NAD83, or any other?

[+Klemmer]

Short [and VERY basic] answer: A map (or GPS receiver's) "datum" is the "zero point" for the map. It all has to start somewhere.....

 

For all geoaching purposes, just leave it on WGS84.

 

An exception you MIGHT want to think about is SOMETIMES if you are finding Benchmarks, you might want to switch to NAD83, but the differences are very small, considering the accuracy of consumer grade handheld GPS receivers. If you REALLY want to get into it, here is a Datum Discussion on the Benchmark forum from not too long ago.

[+PDOP's]

If your using paper maps in conjunction with your GPSr then it should be set to match the datum of the map. I don't know about the US but in Canada we still have NAD27 datum topo maps for many parts of the country. The difference between WGS84 and NAD27 can be hundreds of feet.

[+PDOP's]

Opps Forum hiccup - pls ignore.

Edited December 10, 2005 by PDOP's

[+Sputnik 57]

I agree. The ONLY time I have changed the datum is if I am using a paper map (USGS 24,000:1 topo in this case), and I want to match the GPSr position to the paper map. Most older USGS maps (and they are almost all old it seems) use NAD27. I guess if I go caching in England or Qataar, and have a paper map I am trying to match, one of the other datums may come in handy.

 

Apparently at one time, there was a push among the advertising folks at Garmin and Magellan to boast about how many (largely useles) datums their units had.

 

BTW, for you English majors, from what I understand, when datum is used in this map sense, the plural is datums, not data.

[+Alan2]

If your explorer friend calls from his satellite phone while traveling down some remote river in the Phillipines, and tells you he just got bit by a poisonous snake and is trembling from the poison and needs immediate help, and reports his position using a local map, you would want to input his poistion into your GPS after setting the Datum to Luzon Mindanao or possibly Luzon Phillipines. If he still can talk, ask him what island he's on just to make sure. After entering the coordinates, you would switch your GPS back to the more commonly used WGS84, and report the translated coordinates to the Phillipine search and rescue. Don't forget to tell them that your coordinates are in the WGS84 datum.

 

Other than that, a more common use would be when you're using US Geodetic Service quad maps. Most of them are NAD27. So when you take the coordinates, you have to enter into the GPS with the Datuum set at NAD27. Also, some topo maps on the web might give the old NAD27 or accept data from you the same way. Bottom line is to find out what Datum the map or web is using and match your GPS to it. Once the data is in the GPS, it will automatically change the coordinates based on any other Datum.

Edited December 10, 2005 by Alan2

[+Labrador Wild Man]

Can some one tell me, all my waypoints on my gps60cs are WGS84.So if i get my map out to put pins in the map were i have been, i change the Datum to read NAD83 so they will be right on the map or is it all when i am in the woods and change it to Nad83 then mark it so i can read it right on the map, can some one help me on this, not to clear on this all our maps are NAD83 i still like to see them on my map on the wall .

Edited December 11, 2005 by labrador wild man

[stevesisti]

Can some one tell me, all my waypoints on my gps60cs are WGS84.So if i get my map out to put pins in the map were i have been, i change the Datum to read NAD83 so they will be right on the map or is it all when i am in the woods and change it to Nad83 then mark it so i can read it right on the map, can some one help me on this, not to clean on this all our maps are NAD83 i still like to see them on my map on the walk.

My understanding is that NAD 83 and wgs84 are so close that it's not necessary to change the datum. If you did convert and "plotted" both results on the map, I doubt you would see the difference on your map.

 

Since it's not a big deal to change the datum to NAd83, why not just change the datum and then you won't have to worry about any differential.

[+Sputnik 57]

Can some one tell me, all my waypoints on my gps60cs are WGS84.So if i get my map out to put pins in the map were i have been, i change the Datum to read NAD83 so they will be right on the map or is it all when i am in the woods and change it to Nad83 then mark it so i can read it right on the map, can some one help me on this, not to clean on this all our maps are NAD83 i still like to see them on my map on the walk.

Your GPSr will store the coordinates accurately no matter what you have your datum set to. It will know "you are here." The only time the datum matters is if you write down the coordiates to share them with others (or plot them on a map). Then, the datum (sort of like a trasparent overlay on a map) has to be the same (to make sure that your overlay and the friend's or map's overlay line up).

 

BTW, from what I hear, NAD83 and WGS84 are virtually identical in the western hemisphere.

[+Alan2]

One note; GPS conversion between WGS84 and NAD27 are wrong depending what part of the country you're in. The Mfr's use a sinple formula. But the difference vary all over the palce and could be of 100 feet or more. USGS NADCOM web page use 10000's of samples to actual compute the real differences. You can get more info by visiting their web page.

[+EraSeek]

[+NomadVW]

Yes, see - if you're looking at maps on a Japanese map, you'll most definitely want to switch to Tokyo Mean. Otherwise you'll find yourself sometimes almost a mile off.

 

Like everyone else said though, datums only really matter to the geocacher when he is comparing positions to paper (or online electronic without their own datum conversion) maps. I do Tokyo/WGS84 conversions all the time - so it is something some people do often.