Understanding datums

Now you know that a geographic coordinate system uses a spheroid (or less accurately a sphere) to model the earth. You also know that a spheroid doesn't describe the earth's shape exactly—a perfectly smooth spheroid does not reflect the undulations and other variations on the earth's surface. Because no single spheroid can model the bumpiness all over the earth's surface, there is more than one spheroid in use.

A geographic coordinate system needs a way to align the spheroid being used to the surface of the earth for the region being studied. For this purpose, a geographic coordinate system uses a datum. A datum specifies which spheroid you are using as your earth model and at which exact location (a single point) you are aligning that spheroid to the earth's surface.

The red spheroid is aligned to the earth to preserve accurate measurements for North America. The blue spheroid is aligned to the earth to preserve accurate measurements for Europe.

A datum defines the origin of the geographic coordinate system. The origin is the point where the spheroid matches up perfectly with the surface of the earth and where the latitude-longitude coordinates on the spheroid are true and accurate. All other points in the system are referenced to the origin. In this way, a datum determines how your geographic coordinate system assigns latitude-longitude values to feature locations.

Just as there are different spheroids for different parts of the world, there are different datums to help align the spheroid to the surface of the earth in different regions.

Does changing datums affect your data?

If you change the datum of the geographic coordinate system, you should know that the coordinate values of your data will also change. For example, consider a location in Redlands, California, that is based on the North American Datum of 1983 (also known as NAD 1983 or NAD83). The coordinate values of this location measured in degrees, minutes, and seconds (DMS) are:

–117° 12' 57.75961" (longitude)
34° 01' 43.77884" (latitude)

Now consider the same point on the North American Datum of 1927 (NAD 1927 or NAD27).

–117° 12' 54.61539" (longitude)
34° 01' 43.72995" (latitude)

The longitude value differs by about three seconds, while the latitude value differs by about 0.05 seconds.

In both the NAD 1927 and the NAD 1983 datums, the spheroid matches the earth closely in one part of the world (North America) and is quite a bit off in others. Notice that the datums use different spheroids and different origins. For NAD 1927, the origin aligns the Clark 1866 spheroid with a point in North America. For NAD 1983, the origin (the center of the earth) aligns the center of the spheroid with the center of the earth.

The most recently developed and widely used datum for locational measurement worldwide is the World Geodetic System of 1984 (WGS 1984). This datum is identical to NAD 1983 for most applications. The coordinates for the same location (Redlands, California) using WGS 1984 are:

–117° 12' 57.75961" (longitude)
34° 01' 43.778837" (latitude)