PART D - CONNECTING HORIZONTAL DATUMS
datums, differences occur because of the different ellipsoids used, as well as the relative deflections of
the vertical at their initial points. The latitude and longitude components of the absolute deflection at the
initial points result in a parallel shift between two systems. This shift is caused because the minor axes
of the reference ellipsoids do not coincide with the axis of the earth. Furthermore, deflection errors in
azimuth cause a relative rotation between the two systems. Since using different ellipsoids causes a
difference in the scale of horizontal control, a stretch occurs in the corresponding lines of the various
geodetic nets. These discrepancies are generally larger for a datum oriented by a single astronomic
point than they are for a datum oriented by the astro-geodetic method. However, even in the astro-
geodetic nets, the deflections of the vertical are only relative and the system cannot be considered
absolute. Consequently, it is impossible to determine the discrepancies between various systems unless
direct observations can be made.
1-11. Unconnected Geodetic Systems. Without more information, the computation of geodetic
information from one datum to an unconnected datum is impossible. Regardless of how accurate the
individual datum may be for computations within themselves, there is no accurate way to perform
computations for distances or azimuth between unconnected geodetic systems. Since the modern
military requires geodetic computations between previously unconnected datums, the major geodetic
datums of the earth must be unified. The methods used to accomplish this task are the datum
transformation method, the arc measuring method, and the gravimetric method.
a. Datum Transformation Method. The datum transformation method is restricted to surveys of a
limited scope. It consists of a systematic elimination of discrepancies between two overlapping
triangulation networks. This is done through mathematical processes involving moving the origin,
rotating and stretching networks to fit another. While this method is usually used to connect small local
surveys to a national network, it can also be applied when extending control for detailed mapping
purposes. However, the datum transformation method can only be used where control exists for
common points in the different systems.
b. Arc Measuring Method. The arc measuring method can establish survey ties between
unconnected systems. Arcs across relatively narrow waters and land areas inconvenient for ground
surveys can sometimes be obtained by electronic distance measurements. Thus, high-precision super-
range navigation (HIRAN) radar trilateration provides a method for measuring distances over areas
where ground stations can be established within 400 or 500 miles of each other. This offers an
operational method of connecting separate geodetic datums. The HIRAN trilateration has become a
standard tool, within its capacity and range, in the coordination of geodetic systems. As mentioned
earlier, celestial triangulation methods also permit the establishment of arc distances over oceans and
inaccessible terrain.
c. Gravimetric Method. Components of the gravimetric method were previously discussed in this
chapter. By using a single reference ellipsoid and determining the
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