Figure 1-4. The Geoid and the Ellipsoid Rarely Coincide
PART B - GEODETIC SURVEYING
1-4. General. One of the purposes of geodesy is to determine the precise position of points on the
earth's surface. The techniques used to do this can be grouped into an astronomic observation,
horizontal control, or vertical control by gravimetric. These techniques are described here so that you
can examine the methods to join the continents and insure their proper placement on earth; establish
correct boundary lines, ending old disputes; make flying safer by identifying the correct placement and
elevation of mountain peaks; ensure the correct missile launch information and target locations in this
country and in foreign countries; establish exact control-point locations for accurate maps; and aid in the
establishment of a WGS. These techniques should also help to visualize how these measurements are
used to determine the earth's size and shape.
1-5. Astronomic Observation. The position of a point can be obtained directly by observing the stars.
Astronomic positioning is the oldest positioning technique. It has been used for many years by mariners
and, more recently, by airmen for navigational purposes. Explorers often used the astronomic technique
to locate themselves in uncharted areas. Geodesists must use astronomic positions along with other
geodetic survey data to establish precise positions. As the name implies, astronomic positions are
obtained directly by measuring the angles between the plumb line at the point and a star, or series of
stars, and recording the precise time at which the measurements are
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