The datum conversion method that Views Maps implements by default is based on the assumption that the datum used is one that describes latitudes and longitudes read from an ellipsoid surface whose center is slightly shifted from the center of the Earth so that it is tangent to the geoid surface in the area where the datum is used.

This datum is implemented with the class IlvHorizontalShiftDatum, a subclass of IlvHorizontalDatum. The datum of reference for calculating shift parameters is the WGS84 datum. The National Imagery and Mapping Agency (NIMA) publishes the shift parameters relative to this reference datum for a large number of data.

We use the WGS84 datum to convert coordinates from datum D1 to datum D2. The latitude and the longitude of a point on the D1 ellipsoid surface are converted to the X,Y, and Z Cartesian coordinates. These coordinates are then reprojected on the datum WGS84 ellipsoid surface. Repeat the same operation to obtain the latitude and the longitude of the point on the D2 ellipsoid surface. The Molodensky’s formula is based on this conversion principle and is implemented by the IlvMolodenskyConverter class. With this technique of conversion, the precision of data is of ten meters or so.

You can specify the datum of a projection system with IlvProjection::setDatum() method and retrieve it with IlvProjection::getDatum().

If the source and target projection for an imported map have different geodetic data, datum conversion is carried out by the method toViews().