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.

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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