Users of geoinformation often need transformations from a particular 2D coordinate system to another system. This includes the transformation of polar coordinates into Cartesian map coordinates, or the change of map projection - transformation from one 2D Cartesian (x, y) system of a specific map projection into another 2D Cartesian (x′, y′) system of a defined map projection. This transformation is based on relating the two systems on the basis of a set of selected points whose coordinates are known in both systems, such as ground control points or common points such as corners of houses or road intersections. Image and scanned data are usually transformed by this method. The transformations may be conformal, affine, polynomial or of another type, depending on the geometric errors in the data set. A datum transformation involves the change of the horizontal datum which is often accompanied with a change of map projection.