What is geodetic datum?
Datum is like frame of reference. Each point at earth surface is defined as combination of latitude, longitude and elevation. But all the three values depend on origing of coordinates, direction of axes, and accuracy of measured Earth radius and polar flattening. Different datums are used in different countries. As a result, coordinates of the same point may differ in two datums. For instance, Russian maps are based on “Pulkovo-1942” datum. The one used in GPS is WGS84. In Moscow region, they are shifted approximately in 150 m. To avoid loss of GPS precision (wich is about 15 m) you should always make sure that all coordinates are defined in or converted to the same datum.
