absolute deflections of the vertical and geoid separations for each initial point of all datums, the datums
can be joined more accurately to a single unified absolute system.
PART E - GEODETIC SYSTEMS
1-12. General. The amount of work and time involved in establishing a single unified absolute system
of geodetic datums is tremendous. For military purposes, we must know the distance and directions
between widely separated points. Therefore, we must undertake this task. In this lesson, we discussed
how geodetic datums are established and some methods by which the various datums can be connected.
We also discussed the tools and techniques of geodesy. Now let's examine some of the problems.
1-13. Major Datums Before World War II. Every technically advanced nation has developed its own
national geodetic system or systems, depending on its technical, industrial, or military requirements.
The systems were developed by the expansion and unification of smaller local systems or by new
nationwide surveys to replace the outdated local ones. Neighboring countries normally did not use the
same geodetic datum, since the cost of establishing a separate datum was small, and the nation's military
interest was against the use of a common datum. International surveys on the same datum were
restricted to the measurements of long arcs across international boundaries in order to determine the size
and shape of the earth's ellipsoid. The result of this policy was that many systems developed which
differed from one another remarkably, as did the map series based upon them. (An example of the many
datums in Southeast Asia is illustrated in Figure 1-22.) Due to the relatively small areas covered by
these national datums, military operations requiring geodetic information were restricted to fairly short
distances. The development of long-range rockets and guided missiles during the war illustrated the
need for more extensive geodetic information.
EN0593
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