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the surrounding terrain, and the assumed structure of the Earth's crust
(Figure 13.3)

Figure 13.3 Factors considered when reducing observed gravity

Undulation and Deflections by the Gravimetric Method

The method providing the basis from which the undulations of the
geoid may be determined from gravity data was published in 1849 by
a British scientist, Sir George Gabriel Stokes. However, the lack of

observed gravity data prevented its application until recent years. In
1928, the Dutch scientist, Vening Meinesz, developed the formulas by
which the gravimetric deflection of the vertical (Figure 13.4) can be

computed.
The computation of the undulations of the geoid and the
deflections of the vertical require extensive gravity observations. The

areas immediately surrounding the computation point require a dense
coverage of gravity observations and detailed data must be obtained
out to distances of about 500 miles. A less dense network is required

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