16
HEIGHT
SYSTEMS
GEOPOTENTIAL NUMBERS
GEOPOTENTIAL NUMBERS
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
