An outstanding example of a conservative field of force is
that of gravity. If we let 
represent the gravity
potential then
The gravity potential is commonly referred to as the geopotential. It has the dimensions of energy per unit mass.
If we take the z axis directed along the plumb line at a given position but opposite to the direction of gravity then we can write for the magnitude of gravity
or
Taking arbitrarily zero at the level 
(say, mean
sea level) gives
Thus is interpreted simply as the potential energy per
unit mass by virtue of its relative position in the field
of gravity.
Since 
is everywhere normal to the
equiscalar surfaces of (as implied by Eq. 63) it is obvious
that the latter surfaces are level surfaces in the
sense that there is no component of gravity tangential to
these surfaces.
A plane which is tangential to a level surface at a particular
location on the earth will be referred to as a horizontal plane.