[NLRS] GPS Map Datum

[Scott Blixt]

Just curious. To those that use GPS. Which map datum are you all using? 
I believe mine came defaulted to WGS 84.

Ive been using WGS84 because that is supposed to be the most recent map 
datum. The numbers relating to the year when they were done. 27 as in 
1927 and 84 1984. I have found though that there is some differences in 
actual locations between the 2.






Here is something I found during a search:

Map Datums

A datum describes the model that was used to match the location of 
features on the ground to coordinates and locations on the map. Maps 
all start with some form of survey. Early maps and surveys were carried 
out by teams of surveyors on the ground using transits and distance 
measuring "chains". Surveyors start with a handful of locations in 
"known" positions and use them to locate other features. These methods 
did not span continents well. Frequently they also did not cross 
political borders either. The "known points" and their positions are 
the information that the map datum is based. As space based surveying 
came into use, a standardized datum based on the center of the earth 
was developed.

Every map that shows a geographic coordinate system such as UTM or 
Latitude and Longitude with any precision will also list the datum used 
on the map.

The Global Positioning System uses an earth centered datum called the 
World Geodetic System 1984 or WGS 84. WGS 84 was adopted as a world 
standard from a datum called the North American Datum of 1983 or NAD 
83. For all practical purposes there is no difference between WGS 84 
and NAD 83.

Most USGS topographic maps are based on an earlier datum called the 
North American Datum of 1927 or NAD 27. (Some GPS units subdivide this 
datum into several datums spread over the continent. In the Continental 
United States use NAD27 CONUS.)

In the Continental United States the difference between WGS 84 and NAD 
27 can be as much as 200 meters.

You should always set your GPS unit's datum to match the datum of the 
map you are using.

On a USGS topographic map the datum information is in the fine print at 
the bottom left of the map. The datum will always be NAD 27. There may 
be information on how many meters to shift a position to convert it to 
NAD 83. Think of this as the error that will be introduced if you leave 
your GPS unit set to WGS 84. A dashed cross in the SW and NE corners of 
the map gives a visual indication of the difference between the two 
datums.

If you have somehow set your GPS to use the Borneo Datum of 1818, it's 
hard to say how far off you position may be. Let's just sat that this 
"datum thing" is something you need to pay attention to.

If you are coordinating with aircraft, they will likely have their 
datum set to WGS 84, as most aviation charts now use WGS 84. Should you 
worry about the difference in datums? Typically a pilot will not have 
any difficulty locating you on the ground if you can get them within 
several hundred meters of your location. If you are engaged in a 
mission that requires more precision, then your datums should match.




Scott Blixt
ka0jwc
sblixt@myclearwave.net