From Eq. (94) we see that for small changes of z
If we take a representative value of g, say 980 cm/sec 
,
we see that the change of 
corresponding to one meter
change of elevation is equivalent to
This relation suggests a practical unit of geopotential as
ergs/gm. This unit is referred to as a
dynamic meter, i.e.
Thus, one meter change in elevation corresponds to a change in
geopotential of about 0.98 dynamic meter. Or, a change of
geopotential of one dynamic meter infers a change of elevation
of about 1.02 meters. It must be emphasized, however, that
the dynamic meter is merely a synonym for the more cumbersome
expression ergs/gm which represents gravitational
potential energy per unit mass.
Frequently, goepotential in the atmosphere is referred to as dynamic height, as suggested by the use of the practical unit above. In the sea, the geopotential is negative in reference to that of the sea level geoid. The term dynamic depth is therefore commonly employed as the negative of the geopotential in the sea. Again, it is to be emphasized that dynamic height and dynamic depth are merely other names for geopotential.
It is of interest to compute the variation in spacing of two
equigeopotential surfaces near
the Earth's surface. This can be done by use of
Eq. (92). Consider the level surface represented by a dynamic
height of 10,000 dynamic meters relative to the sea level
geoid. The actual elevation of this surface is 10,240 meters
at the equator and 10,187 meters at the poles, the difference
being 53 meters. On the other hand, the depth of the
-10,000 dynamic meter geopotential surface in the sea (if the sea
were actually this deep) would be 10,236 meters at the equator and
10,183 meters at the poles, giving the same anomaly of spacing
between equator and pole as above. The mean difference in the spacings
of the 0 to 10,000 dynamic meter surface and the -10,000 to 0
dynamic meter surface is only 4 meters.
It is apparent that a surface of constant positive elevation above sea level actually slopes downward towards the equatoar relative to the level surfaces, i.e. there is a component of gravity along a surface of constant positive elevation towards the equator (see Fig. 2.09-1). On the other hand, a surface of constant depth in the sea has a component of gravity along the surface towads the pole, and hence slopes downward toward the pole relative to the level surfaces.
It should be apparent that in ascertaining slopes of isobaric surfaces in the atmosphere or the ocean, in order to evaluate horizontal components of pressure forces, it is the equi-geopotential surfaces which should be used as a reference rather than surfaces of constant geometrical height or depth. The difference is actually quite small, so so are the magnitudes of the isobaric slopes which are to be reckoned with. Consequently, the net effect is significant.