This tool converts coordinates from one datum to another datum. For example from WGS 1984 to Dutch grid or from UTM to Ordnance Survey Great Britain 1936.
Enter the coordinates and choose the current datum of the coordinates and to which datum they need to be converted.
Example of Dutch grid (RD / AME-7): 192309 325406 (x y)
Example of UTM coordinates (UTM): 31U 705083 5644673 (zone easting northing)
Example of British grid coordinates (OGB-7): TG 51409 13177 or 651409 313177 ([zone] easting northing)

This was not necessary if the world was a perfect mathematical sphere, but unfortunately this is not the case. The shape of the earth is more or less the shape of an ellipsoid. There are a number of ellipsoids describing the earth and every ellipsoid fits nicely at one location and has some error at another location. This is the reason why there is more then one way to determine your position on the earth. Some countries developed their own method to determine the position. These coordinate systems are usually 'normal' rectangular and expressed in meters from a predefined position, examples are the Dutch grid (Rijksdriehoek systeem) or the British grid system. The method used to express coordinates is called a datum. The datum that is used by Geocaching.com is called World Geodetic System 1984 (WGS 1984) and uses the WGS 84 ellipsoid. The error margin of this ellipsoid is estimated at 2cm. Some examples of datums are shown in the table below. The conversion tool has more datums available.