� Although geopotential numbers are useful for the adjustment of vertical networks, for many purposes “true” orthometric heights above a physically defined reference surface are still necessary � A geopotential number can be converted to a “true” orthometric height   by dividing the geopotential number by the mean value of gravity along the plumb line between the point and the reference surface › H = C/gm › Since the mean value of gravity cannot be directly measured (because the reference surface lies within the Earth beneath the point), a model must be used to derive the value as a point, and other variables