What is a map datum ?
A map datum is a reference surface which is defined mathematically and approximates the shape of the earth in particular areas. At different areas across the world, different map datums were (and some still are) used due to the differences in the earth's general surface shape at different places. Specific map datums are more applicable to particular areas or regions than others.
A particular map datum is physically represented by a group of surface based trigonometric stations, whose positions have been accurately measured and is used to determine the reference surface (or map datum)
A map datum enables us to calculate the position of a specific location accurately and consistently. However, there are significant deviations, when a waypoint (a longitude and latitude set) located with a particular map datum, is used with a different map datum as reference. So it might appear that specific points on the earth's surface, have different latitudes and longitudes, depending on the map datum that was used, or alternatively, one waypoint (a longitude and latitude set) may seems to point to different physical positions !
Currently there are many, many map datums in use. The Garmin products support more than 100 different map datums (see our list and applicable countries) to reduce the inaccuracy caused by the deviations of different map datums. Theoretically this implies that one waypoint can locate more than 100 different places, all depending on the map datum that was used as reference.
All GPS and map users should be aware of this peculiarity and take note of the most applicable map datum to be used. If you use waypoints from any source, or use a map with longitudes and latitudes, make sure that your GPS is using the same map datum as used originally to determine the waypoint.
In South Africa, most older maps use the CAPE datum (based on the modified Clarke 1880 ellipsoid), (not to be confused with the CAPE CANAVERAL map datum). So if you are working with older South African maps and would like to use waypoints indicated on them, SET YOUR GPS to the ",CAPE", map datum.
The World Geodetic System 1984 ellipsoid, or WGS84 in short, is the basis for the Hartebeeshoek94 datum, suitable for all GPS users. All waypoints references are moving towards this internationally accepted datum. So if you use data with this map datum, SET YOUR GPS to the ",WGS84", map datum.
Seth Tyler writes for GPSYES.com where he presents articles and information on gps equipment sales, gps for trucks, and related topics...
