When the unit detects that external power is available, the unit will automatically switch to the external source and not use the batteries in the unit. It is recommended that the batteries remain in the unit in the event that you have a loss of battery power. If the batteries remain in the unit, when external power is lost the unit will automatically switch to batteries and continue to function normally. 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, 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. They are usually broadcast on existing marine beacon frequencies around 300 kHz. They are operational in many coastal areas and on the river systems. We no longer include Warranty Registration cards with our handheld units.