surveying iii (topographic and geodetic surveys) - Modern Prepper

Table 1-2. Deviation of the Laplace Equation(b) Refer to Table 1-2, and note that equations (1), (2), and (3) use the gravimetric method toobtain the components of the deflection of the vertical (ξ and η). If the astronomical coordinates andazimuth have been observed, the geodetic coordinates can be computed. The largest computeddifference in position between astronomic and geodetic observations is approximately 1,000 feet. Theaverage difference between the computed positions is somewhere in the neighborhood of 300 feet.These differences in position look very small. However, we are looking for a precise position;consequently, the geodesist must indicate the exact pinpoint position, not an approximation. This isespecially critical at the initial point of any weapons system since any deviation will influence the finallocation of the impact area.(c) The Laplace equation (5) in Table 1-2 is important because it gives the relationshipbetween the azimuth and longitude differences without knowing the components of the deflection of thevertical. In survey networks this fundamental relationship is used as a check to determine the accuracyof the observed geodetic and astronomic data.(d) The astro-geodetic deflections of the vertical are only relative since the deflections of thevertical are computed with respect to a specific ellipsoid. If the ellipsoid is changed, the deflections ofthe vertical also change (Figure 1-17). It is necessary to assume a specific orientation of the referenceellipsoid with respect to the geoid before computing the astro-geodetic deflections. This orientation isfixed by the initial values of the datum point from which the geodetic coordinates were computed. Anychange in these initial values, therefore, changes the deflection of the vertical at each point.Consequently, the astro-geodetic deflections of the vertical have the same restrictions as the geodeticpositions. They are related to the geodetic datum.1-27 EN0593