What's the Deal with Datum?

Here at Fishing Hot Spots we frequently receive phone calls and emails from anglers who are trying to use one of our maps with their GPS unit or a mapping software package and are experiencing difficulties with the GPS coordinates provided on our map. The vast majority of the time the issue is the same - the datum of the map is different than their GPS unit or software is set to. This situation accounts for the majority of the customer service calls we receive so we thought we'd address it, in the simplest of terms, in order to assist you in your use of maps and electronics together.

All quality maps are created in a specific datum and projection. These two terms, datum and projection, are intimately tied together and refer to a method that is used to draw a map of the curved earth on a flat piece of paper. In cartography, there are many projections and datums that can be used, depending on the final application of the map. A projection is the actual method, a complex mathematical formula, used to flatten the curved earth for drawing a map. Datum refers to a fixed reference point on the face of the earth that remains constant.

The most common datums encountered in use in North America are NAD27, NAD83 and WGS84. NAD stands for North American Datum and the number refers to a year, eg: 1927 or 1983. WGS stands for World Geographic System and the number refers to 1984. In the three cases mentioned, the year refers to the year in which the standard reference point was established. Each datum mentioned has a different reference point and, unfortunately, they do not always have a direct correlation to one another. Although close in proximity, the slight difference at the source of the reference point grows larger as you move further away from it on the face of the earth. This can result in points taken from a map with a differing datum than what your electronics are set to appear in the wrong position.

No datum is any better than another, they are simply used for different applications. When using any map with your GPS unit or mapping software be sure to check what datum the map is drawn in and be aware the vast majority of these electronic products come with their default datum set to WGS84. Fishing Hot Spots maps can utilize either a NAD27 or NAD83 datum. In most cases, simply changing the default datum of your electronic device to that of the map will correct any issue with coordinates not showing up where they should.

Cartography is a complex and sophisticated process with countless variables possible. When a map is made it is made for a specific purpose and employs the most appropriate datum for its purpose. For information on datums, map projections and cartography we suggest visiting the following web sites: