thousand actual swings of the pendulum. This type of measurement requires several hours of
observation, and the equipment involved is complex and cumbersome. Consequently, absolute gravity
is usually measured at a limited number of reference stations called gravity base stations.
p. Gravity measuring instruments, which use base stations for referencing, are generally called
gravimeters. Since these instruments do not measure absolute gravity, the results are known as relative
measurements. With a gravimeter, gravity at different places is compared with the value of gravity at a
pendulum base station. In recent years, mostly geophysical-prospecting companies have developed
several different types of gravimeters. Some of the instruments are compact and light and permit rapid
movement from place to place. Relative gravity measurements are usually sufficient for geophysical
prospecting, but these measurements must be properly connected to a gravity base station before the
results can also be used for geodetic purposes.
(1) Gravimeters are so sensitive that they respond to and register the gravitational attraction of
the sun and the moon. Since the waters on earth have no rigidity, they are raised and lowered by forces
of attraction known as tides. These same tidal forces exert a pull on the land surface. Evaluations on
tidal effects shows that the moon's attraction is more than twice that of the sun. Although a point on the
surface of the earth does not rise or fall as much as would a ship on the surface of the ocean due to tidal
force, the slight movement of a point on the land surface causes small variations of gravity. The amount
of variation depends upon the latitude, the time of the month, and the year. These variations are a source
of error and must be computed and eliminated when obtaining readings with the gravimeter. This
computation is known as the earth tide correction.
(2) Gravity observations over ocean areas can be made from submarines. Such measurements
still require a considerable length of time, and special consideration must be given to account for the
motion of the measuring vessel. Several different types of underwater gravimeters have been built by
oil-prospecting companies to perform gravity surveys in shallow coastal areas.
(3) Gravity surveys have been made on many areas of the earth; however, there are still vast
areas with only a few or no gravity observations. An airborne gravimeter and other new developments
in gravity measuring devices should help to extend our knowledge of the earth's gravity field.
1-8. Determining the Earth's Size and Shape. In order to establish a WGS, it is essential that the size
and shape of the selected ellipsoid of revolution closely approximate the true figure of the earth. While
Eratosthenes' measurement method has not been changed in principle to the present day, certain very
important refinements have been made. A long arc is measured between two points on the surface of the
earth. The angle, which subtends this measured arc, is determined from the difference in the latitude or
the longitude of the two astronomically-observed points. The value of the ellipsoid's semimajor axis and
the flattening is then computed using the appropriate mathematical formulas.
1-23
EN0593
Previous Page
