A. A datum is a mathematically defined reference surface 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, you should change the map datum in the GPS unit to the same map datum that is used on chart or other reference. The datum used is often marded at the bottom of the chart, so you can easily match up the datums between your GPS and the chart.