Classification of the elevations (or differences of elevation) in a leveling network according to their accuracy or other characteristic. Class-ification of an elevation (vertical control) according to its accuracy is inseparable from the classification of the leveling network in which the elevation occurs, because the values of all the differences of elevation in a particular network are solved for simultaneously - usually by the method of least squares. The U. S. National Geodetic Survey uses the same categories for classifying vertical control as it uses for classifying leveling and leveling networks networks. The criterion used for each category is derived in a complicated fashion but is roughly as follows: