Map Datums

A map datum is a mathematical model that describes the shape of an ellipsoid — in this case, the earth. Because the shape of the earth isn’t uniform, over 100 datums for different parts of the earth are based on different measurements.
Some serious math is involved here for getting into the nuts and bolts of map datums. If you’re the scholarly type, these Web sites provide lots of details on projections and datums:

• Datums and Projections: A Brief Guide http://biology.usgs.gov/geotech/documents/datum.html
• Peter Dana’s excellent Geographer’s Craft site www.colorado.edu/geography/gcraft/notes/notes.html

Datums all have names, but they aren’t stuffy sounding. Datums often have exotic, Indiana Jones-style names such as the Kerguelen Island, Djakarta, Hu-Tzu-Shan, or Qornoq datums. (The United States uses such boring datums as NAD 27 and WGS 84.)
You only need to be concerned with datums under a few circumstances, such as these:

• A location is plotted on two different maps.
• A map and a Global Positioning System (GPS) receiver are being used.
• Two different GPS receivers are being used.

In these instances, all the maps and GPS receivers must use the same datum. If the datums are different, the location ends up in two different physical places even though the map coordinates are exactly the same. This is a common mistake: GPS receivers use the WGS 84 datum by default, and USGS topographic maps use the NAD 27 datum. If you mix the datums, your location can be off by up to 200 meters (roughly 200 yards, if you’re metrically challenged).
Utilities can convert coordinates from one datum to another but it’s easier just to get all the datums on the same map.