Conversion of the measured length of a base line to the length of the vertical projection of the base line onto a specified surface defined by reference to mean sea level. Theoretically, the specified surface should be an equipotential surface. Because the shape of the surface must be known for the definition to be practical and because the shape of an equipotential surface is known only approximately, if at all, a sphere or rotational ellipsoid is usually specified instead. This works because a base line is so short that a sphere or rotational ellipsoid is a very good fit to an equipotential surface over the length of the base line. Base lines are reduced to sea level when the location of the geoid and the location of the reference ellipsoid are not known.