surveying iii (topographic and geodetic surveys) - Modern Prepper

a. If the shape of the earth was exactly that of an ellipsoid of revolution, the measurement of onearc on the surface of the earth should give its dimensions. In practice, numerous arcs from differentareas of the earth are used to obtain a truer ellipsoid figure. As previously pointed out, astronomicobservations are referred to the geoid. This causes errors in arc measurements, which must beeliminated by using gravity anomaly corrections and gravimetric computations. By mathematicallyremoving the effects of the deflection of the vertical at the ends of the measured arcs, a more refined andtruer ellipsoid can be determined.b. Measurements can be solved to find both the size and the shape of the earth. Reductions areoften accomplished by the use of the flattening value obtained by some other method. This simplifiesthe arc problem to one in which only the size of the earth is sought.(1) The shape of the earth (not its size) can be obtained from gravity anomalies. Since thetheoretical gravity formula depends on the assumed flattening of the earth, gravity anomalies are used tofind the gravity formula which best satisfies certain assumptions about the structure of the earth's crust.The flattening that corresponds to this corrected gravity formula can be found by working backwards.(2) Artificial earth satellites can also provide a good means of measuring the flattening of theearth. This method will assume greater importance as tracking techniques with special geodeticsatellites improve.c. To determine the size and the shape of an ellipsoid for use in a WGS, many methods should beused based on as large a group of observations as possible. Failure to do this would result in an ellipsoidappropriate for a small area of the earth but not necessarily an adequate reference for the entire earth.PART C - GEODETIC DATUMS1-9. General. The geodetic positions of points lying on the earth's surface are determined with respectto a group of specific initial quantities that form a geodetic system or datum. Since the relationshipbetween geodetic positions remains true only so long as they are on the same geodetic datum. Positionsderived from different datums are not directly comparable in computations. Consequently, the desireddata, such as distance and direction, will be different. The difference will depend on the errors in theinitial quantities of the datums.a. Since a datum can be defined as any numerical or geometrical quantity or set of such quantities, adatum is a starting point. In geodesy, the following two datums must be considered: a horizontal datumthat forms the basis for the computations or horizontal control surveys in which curvature is consideredand a vertical datum to reference heights.EN0593 1-24