They differ when used with the NAD 27 Datum. MGRS and USNG 100,000-meter Square Identifications are Different when Used with the NAD 27 Datumīoth the MGRS and USNG systems default to the WGS84 datum. Then your datums should match or you should convert coordinates to the other datum. If you are engaged in a mission that requires more precision, This may not be the case for a ship in rough seas. Should you worry about the difference in datums? Typically a pilot will not have anyĭifficulty locating you on the ground if you can get them within several hundred meters of your location.

If you are coordinating with ships or aircraft, they will likely have their datum set to WGS 84, as most nautical and aviationĬharts now use WGS 84. Using maps and charts with different datums on the same mission Let's just sat that this 'datum thing' is something you need to pay attention to. If you have somehow set your GPS to use the Borneo Datum of 1818, it's hard to say how far off you Most GPS receivers designed for use with land navigation allow you to change the Map Datum in the Set Up menu. The SW and NE corners of the map gives a visual indication of the difference between the two datums.

Think of this as the error that will be introduced if you leave your GPS unit set to WGS 84. There likely will be information on how many meters to shift a position to convert it from NAD 27 to NADĨ3. During the time period after 1983, but before shifting to NAD 83, Older USGS mapsĭatum will almost always be NAD 27. On a USGS topographic map the datum information is in the fine print at the bottom left of the map. You should always set your GPS unit's datum to match the datum of the map you are using. Precision will also list the datum used on the map. Finding the datum used by your mapĮvery map that shows a geographic coordinate system such as UTM or Latitude and Longitude with any There is a good article in Wikipedia about the various North American datums and their history. NAD 23 CONUS datum is one of these terrestrial based datums established from a triangulation station in Meades Ranch, Kansas.Īs space based surveying came into use, a standardizedĭatum based on the center of the earth and an ellipsoid the was a good fit to the entire surface of the earth was developed. Positions and the ellipsoid used are the information that the map datum is based. Frequently they also did not cross political borders. These methods did not spanĬontinents well. Of locations in 'known' positions and use them to locate other features. Of surveyors on the ground using transits and distance measuring 'chains'. Prior to advent of satellites, most surveying was done on the ground or by using photos taken from an airplane.Įarly maps and surveys were carried out by teams Various regions of the world selected an ellipsoid that best approximated their portion of the earth. The earth is a lumpy bumpy three dimensional thing, that can be approximated with a nice clean mathematical ellipsoid. When you survey large areas, you need to take the curvature of the earth into account in your calculations. Cartographers that include a coordinate grid on their maps, must also specify the datum used.The datum should be written with individual coordinates or included with explanatory notes when many coordinates are used.A coordinate with an unknown datum is an approximate location at best.The datum is an important component of a coordinate.A failure to use the correct datum can introduce hundreds of meters of position error.The difference between WGS 84 and NAD 27 can be as much as 200 meters. There is typically only a meter or two difference between WGS 84 and NAD 83 in the Continental United States. WGS 84 was adopted as a world standard from a datum called the North American Datum of 1983 or NAD 83. The Global Positioning System uses an earth centered datum called the World Geodetic System 1984 or WGSĨ4. (Some GPS units subdivide this datum into several datums spread over the continent.

Most USGS topographic maps are based on an earlier datum called the North American Datum of 1927 or NADĢ7.

WGS 84 – World Geodetic System of 1984 (The default datum used by the GPS system).

NAD 83 – North American Datum of 1983 (Used on most newer USGS maps).

NAD 27 CONUS - North American Datum of 1927 for the Continental United States (Common on older USGS maps).

The three common datums in use in the Continental United States are: The datum you have setup in your GPS receiver must match the datum used to create the map you are using. Map Datums An incorrect datum, can put you hundreds of meters from your actual position

Datum example