Datums
(Adapted from the OziExplorer Help file)
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 do mapping programs Handle Datums?
Good mapping programs like OziExplorer can handle many different datums and knows how to translate positions from one datum to another. Provided you have entered your datums correctly the program will ensure that all positions are translated to the correct datum as required.
How to set Datums in your GPS and in mapping software like OziExplorer
Note: Setting Datums incorrectly will render the positions
displayed in your mapping program incorrect.
- The GPS Datum. For Bhutan, set to Indian Bangladesh.
- The GPS Upload/Download Datum - must always be set to match the datum
your GPS expects its position data to be transmitted in.
For Garmins this Datum should always be set to WGS 84, even in Bhutan!
For Magellans this Datum should always be set to WGS 84.
For Lowrance/Eagle - Most should have this Datum set to WGS 84 but care should be taken if using a GlobalNav 200 or Eagle Explorer as certainly some of these units require the Upload/Download Datum to be set to the same datum as the GPS is set to. - 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.
For Bhutan this is Indian Bangladesh.
- For Garmins the Datum must be set to the same as the Datum setting in the GPS (Indian Bangladesh for Bhutan). If using moving map mode and you change the GPS datum then you should always change this datum also.
- For Magellans you need to check this, some models output in WGS84 and others such as the 315/320 output in the Datum the GPS is set to.
- For Lowrance/Eagle - Most should have this Datum set to WGS 84
but care should be taken if using a GlobalNav 200 or Eagle Explorer
as certainly some of these units require the NMEA Output Datum
to be set to the same datum as the GPS is set to.
- The Map Datum - (in OziExplorer it is 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. For Bhutanese Topo maps this is Indian Bangladesh.
- The Data File Datum can be set to any datum you like, but for Bhutan it should be set to Indian Bangladesh. 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.
