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although the Worden gravimeters have been used extensively for such
work in the past.
Since relative gravity surveys can determine only differences in
gravity from point to point, every relative gravity survey must include

measurements at one or more reoccupiable points where acceleration
of gravity is known. Such points are called base stations. Then all
gravity difference measurements are computed with respect to the

known gravity value at the base station. Hence, tying a relative gravity
survey to a base station establishes the "gravity datum" of that survey.
The earliest "gravity datum" was the so-called Potsdam System. In
1906, reversible pendulums were used to measure absolute gravity at

Potsdam, Germany. The value measured at this time was later adopted
as the initial point for the Potsdam gravity reference system. Using
relative measurements, several points on each continent were

connected to Potsdam, and these served as the fundamental base
stations for many relative gravity surveys.

Task 2. Put up 5 general and 5 special questions to the text

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