e. Astronomic observations provide angular relationships only. They provide information on the
shape of the earth, not its size. In order to determine distances between astronomic stations, the size of
the earth must be determined by horizontal surveying techniques.
1-6. Horizontal Control Technique. Geodesy not only involves determining the shape of the earth; it
also involves surveying--measurements on the surface of the earth. Surveys may be plane or geodetic.
While plane surveys are restricted to small geographic areas and are computed as though the surface is
level or plane, geodetic surveys are used over large geographic areas and require consideration not only
of the curvature of the earth but also of the geoid. This is not the only difference between the two types
of surveys. Instruments used in geodetic surveying are more accurate. Instrument errors are either
removed or predetermined, and the procedures are more rigorous in order to minimize observation
errors. The extension of horizontal control can be carried out in various ways. Basically, the procedure
starts from some point or points of a known position, the distance and angles to new points are
measured, and the positions of these points are computed by means of the measured data. The most
common methods of establishing horizontal control are traverse, triangulation, and trilateration. Other
methods include celestial techniques. Horizontal angles measured on the earth's surface can be used on
the ellipsoid without any additional correction. There may be a very small difference between
horizontal angles on the earth's surface and the corresponding angle on the ellipsoid, but the correction is
usually smaller than your ability to measure the angle and should be ignored, except in the case of first-
order surveys.
a. Traverse. A traverse begins at a position with a known azimuth to some other point, and the
angles and distances along the line of survey points are measured (Figure 1-7).
(1) Closed Traverse. A closed traverse starts and ends at the same point or at points which have
known relative horizontal positions. The measurements can be adjusted by computations to minimize
the effect of accidental errors made in the measurements. Mistakes (blunders) can be detected by
rechecking the computations.
(a) Loop Traverse. A loop traverse forms a continuous loop enclosing an area. The
computed circuit closure, or error of closure, for a loop traverse normally indicates whether a large
mistake was made in the measurements. When mistakes are eliminated, the error of closure will indicate
the size of the accidental errors. However, systematic errors will seldom show in the error of closure.
For example, when the tape used for distance measurements is longer than its nominal length, all of the
recorded lengths will be proportionally too small and will cause little or no change in the computed error
of closure of the traverse.
(b) Connecting Traverse. A connecting traverse starts and ends at separate points whose
relative positions have been determined by a survey of an equal or higher order accuracy. A connecting
traverse of third-order accuracy, for example, may be run and adjusted between two stations whose
relative positions were determined by a first-, second-, or third-order traverse or triangulation.
EN0593
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