A. A datum is a mathematical model of the Earth which approximates the shape of the Earth, and enables calculations such as position and area to be carried out in a consistent and accurate manner. Lines of latitude and longitude on a map or chart are referenced to a specific map datum.
Every chart has a map datum reference.
The map datum is usually listed in the title block of the chart. If you are comparing the GPS coordinates to a chart or other reference, the map datum in the GPS unit should be selected to the same map datum that is listed as the datum used to generate the chart or other reference.