Changing the coordinates of a point set referring to one datum, so to make them refer to another datum, is called a "datum transformation". In the case of vertical data, this consists of simply adding a constant shift to all height values. In the case of plane or spatial coordinates, datum transformation takes the form of a similarity or "Helmert transformation", consisting of a rotation and scaling operation in addition to a simple translation. In the plane, a Helmert transformation has four parameters; in space, seven.