Global Positioning System Reference
In-Depth Information
at the location of the ore, for example. A less dense area will show up as a lo-
cal dip. In fact, the geoid is a very complicated shape, di√ering from the
simple ellipsoid of earlier geographers by as much as 110 m in places (i.e.,
up to 110 m higher or lower than the ellipsoid). The idea of the geoid is illus-
trated in figure 1.9; here, you can also see a geometrical explanation of why
it is close to being an ellipsoid. Figure 1.10 shows an example of local vari-
ations in the gravitational field—in this case, over the state of New Jersey.
Modern instruments can measure the force of gravity—and therefore
can measure gravitational equipotential surfaces—very accurately indeed.
The ''breathing'' of the earth, as it bulges and contracts due to the changing
direction of lunar gravity, can be detected. To define a fixed geoid, this tidal
FIGURE 1.10.
Local variation in
gravitational field intensity.
This gravitational anomaly
map of New Jersey shows
small variations of gravita-
tional acceleration, from
-
0.05 cm s
-
2
to
+
0.03 cm s
-
2
.
U.S. Geological Survey.
