Datums

A datum is a known as a constant surface which is used to describe the location of phenomena on the earth. Earth-based mapping coordinate systems are always developed on a specific datum. There are many different coordinate systems each developed on a specific datum (earth reference shape). The British National Grid for example is a coordinate system developed on the OSGB36 Datum. This coordinate system is described as follows:

Starting from an origin which has been defined by the Ordnance Survey Great Britain there is a 100km grid overlaid on the map. Each of these squares covers an area of 10,000 square kilometres and is described by a digit representing the Easting and another one representing the Northing or alternatively by 2 characters. It can then be further split into squares with a size of 100 square km and so on. In the example above we achieve a precision of only 100m but you can surely imagine how we can describe more accurate positions.

Positions in the WGS84 coordinate system have to consider an ellipsoidal shape of the earth and they can be described as either Cartesian coordinates or through latitude and longitude. Latitudes can have values between +90 and -90 degrees where 0 is at the equator and longitudes can have values between +180 and -180 degrees where 0 has been defined as a place in the Royal Observatory in Greenwich / United Kingdom. As mentioned earlier Virtual Earth in 2D-mode can only work between +85 and -85 degrees because of the Mercator projection.

Units

Within a coordinate system there may be different ways to describe a position. We have seen in the example of the OSGB36 coordinate system that we can start either with a pair of characters or with digits. In WGS84 you can describe a position either in degrees, minutes and seconds or in decimal degrees. Most GPS return the data in the standard 0183 version 2 or higher of the National Marine Electronic Agency (NMEA) which would use degrees, minutes and decimal seconds as units. Virtual Earth, on the other side, would require decimal units but of course the conversion is extremely simple. The OSGB36 datum mentioned above would translate into WGS84 coordinates as follows:

OSGB36
Easting 5623
Northing: 2327

WGS84
Latitude: 51.969 (decimal degrees) = 51 deg 58 min 08.400 sec
Longitude: 0.361 (decimal degrees) = 00 deg 21 min 39.600 sec