Global Positioning System Reference
In-Depth Information
(b)
FIGURE 1.9.
The geoid. (a) Here, the deviation of the earth from an ellipsoid is greatly
exaggerated. The geoid is an equipotential surface, so it largely overlaps with sea
level, with slight variations due to underlying mass irregularities (gravitational anom-
alies). (b) For an exaggerated ellipsoid, we see why the plumb lines are perpendicular.
To the force of gravity (black arrows, acting toward the ellipsoid center) we add cen-
trifugal force (white arrows) resulting in the total force (broad arrows) that deter-
mines the plumb line direction. The geoid is the surface that is perpendicular to all
possible plumb lines. Note that anomalies (not shown) can cause the plumb line di-
rection to veer; therefore, the geoid is slightly different from the ellipsoid.
