Simply put, datum is a model used for calculating the location of the Earth's center. This center calculation is used as a reference point in establishing coordinate locations, such as latitude and longitude, on the Earth's surface.
There are many different datums. Three that you will see most often are:
NAD27: North American Datum of 1927
NAD83: North American Datum of 1983
WGS84: World Geodetic System of 1984
The MapServer displays all coordinates using NAD27.
Why does it matter?
Coordinates calculated using one datum may represent a geographic location several hundred feet away from the same coordinate numbers calculated from another datum. This is because the two sets of coordinates are being measured from different reference points.
Therefore, before relying on coordinate location information, it is important to know what datum was used in establishing the coordinates. And if you're using a GPS unit, make sure to set your GPS to that datum before you type in those coordinates. Otherwise, you might find yourself looking for that campsite in the middle of a pond!
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