surveying iii (topographic and geodetic surveys) - Modern Prepper

1-17. The World Geodetic System. Because local or even preferred systems fail to provideintercontinental geodetic information, a unified world system has become essential. To establish a worldsystem, all of the available observed data must be considered. This will determine the absolute referencesystem which best fits the entire earth. Once such a system is established, computed geodeticcoordinates will be comparable worldwide. To develop such a system for our military purposes, allbranches of the military have actively participated in a program to develop a Department of Defense(DOD) WGS.a. Because of its intercontinental-range missile capability, the United State Air Force (USAF) hasplayed an active role in the development of a WGS. Due to the wide scope of the project, USAFparticipation has extended over a 20-year period in order to obtain sufficient data upon which to base aworld system.b. Active participation by various USAF agencies has permitted the gathering of extensive dataneeded to formulate a unified WGS. An important phase of this preparatory work is the HIRANtrilateration net which spans the North Atlantic Ocean from the eastern coast of Canada to Norway. Thistie permits connection of the North American and European Datums. However, since both of thesedatums are based on ellipsoids which provide a good fit only in the areas of their origins, connecting thetwo datums through the HIRAN tie overextends the usefulness of either ellipsoid. We have seen howlarge, systematic geoid separations can occur when a geodetic datum is extended far from its origin.Nevertheless, ties made with HIRAN will provide effective checks when the two major datums areoriented to a common absolute ellipsoid. Another significant example of HIRAN trilateration is theWest Indies loop.c. A second major area in which preparatory work has been done in anticipation of a WGS is in thecollection and analysis of gravity observations. Through an extensive program, several thousand gravityreference stations have been interconnected throughout the world. The connection of these base stationsby airplane and gravimeter has permitted the reduction of numerous observations to a common usablesystem and, in effect, has established a world gravity system. This has permitted application to geodeticproblems of data collected for oil prospecting and other geophysical purposes.(1) The collection of extensive gravity data permitted preparation of mean gravity anomalies for1 x 1 and 5 x 5 degree geographical sectors that are needed for further gravimetric computations. It alsopermitted preparing the gravimetric geoid used in the orientation of the ellipsoid for the NIMA WGS.Much of the work in the collection and the analysis of gravity data was done by contracts with civilianuniversities.(2) An additional phase of gravity work includes computing gravity anomalies for places inwhich there are no observations. As stated previously, gravimetric computations require knowledge ofgravity anomalies over the entire earth. Some regions of the earth are completely void of observationpoints, and the best substitute must be found. A theoretical anomaly can be found by considering thetopography near the point and making certain applications of our knowledge of the earth's crust.Although the computations for each point are tedious and involve consideration of theEN0593 1-38