The datum is the common term for the idealized model of the earth that underlies a geographic coordinate system.
Some common datums are:
World Geodetic System of 1984 (WGS84)
North American Datum of 1983 (NAD83)
North American Datum of 1927 (NAD27)
Ordnance Survey of Great Britain 1936 (OSGB36)
The main component of the datum is the spheroid which describes the size and shape of the earth.
Because the earth is not a perfect ellipsoid, the datum often is only valid for a specific portion of the earth. OSGB36 is a better description of the surface of the earth for the island of Great Britain than NAD83 is. Conversely, NAD83 is a better model of the surface of the earth for North America than OSGB36 is.
Because of these model differences, a geodetic location (longitude, latitude) in one datum (such as WGS84) is not necessarily the same location in a different datum (such as OSGB36). This is known as datum shift. A translation process is necessary to convert a geodetic location from one datum to a geodetic location in another datum to ensure that the two geodetic locations describe the same physical location on earth. VSP currently offers translation between the following datums:
NAD83 -> WGS84
WGS84 -> NAD83
WGS84 -> OSGB36
OSGB36 -> WGS84
NAD27 -> NAD83
NAD27 -> WGS84
NAD83 -> NAD27
WGS84 -> NAD27
Translating from one datum to another is necessary when connecting to the Map Tile Servers. The Map Tile Servers use WGS84 and must be translated to your chosen coordinate system for display. So, currently the Tile Servers only work with projected coordinate systems that use datums WGS84, NAD83, NAD27 or OSGB36. VSP may support more translations in the future as demand and resources warrant.