What is a Datum?
The latitude and longitude of places on a chart or map depend on what mathematical shape is used to represent the Earth when the chart or map is drawn. Different shapes get used for mapping different areas of the world and these are known as ellipsoids. A datum references a particular ellipsoid known as the reference ellipsoid. Different datums can also have different origins and rotations. Both these factors affect the numerical representation of a position.
Datums affect the positions of Latitude/Longitude and local grid systems (UTM, British National Grid etc).
So how does this affect me?
If you have two charts of the same area and they have been drawn on different datums, then the latitude and longitude for the same place is likely be different on each chart. In Australia the difference between an AGD66 and a WGS 84 charted position can be up to 200 meter's (0.1 miles). This could be the difference between one side of a reef and the other!
If you plot a GPS latitude and longitude on a chart which uses a different datum, you must apply some corrections otherwise you will plot your position in the wrong place. Mapping software such as OziExplorer will usually do these corrections for you.
CHECK YOUR DATUMS CAREFULLY all latitudes and longitudes which you use for navigation must relate to the SAME datum.
How does OziExplorer Handle Datums?
OziExplorer can handle many different datums and knows how to translate positions from one datum to another. Provided you have entered your datums correctly OziExplorer will ensure that all positions are translated to the correct datum as required.
How to set Datums in OziExplorer
Setting Datums incorrectly will make the positions used in OziExplorer incorrect.
The Map Datum - (set in the Map Calibration screen) must always be set to be the same as the Datum the map was drawn in. (Actually to be more correct the Map Datum must be set to be the same as the Datum used to obtain the positions used to calibrate the map). Changing the Map Datum to anything else will definitely cause positions to be calculated and displayed on the map incorrectly.
The Data File Datum can be set to any datum you like. It only affects the positions stored in the file. It is only provided as a configurable option in case you want to work directly with the data file.
The GPS Upload/Download Datum - must always be set to match the datum your GPS expects its position data to be transmitted in.
The GPS NMEA Output Datum - must always be set to the same datum that the GPS uses for the positions it sends in the NMEA sentences.