surveying iii (topographic and geodetic surveys) - Modern Prepper

Figure 1-15. Products of the Gravimetric Method(2) Computing the undulations of the geoid and deflections of the vertical requires extensivegravity observations. The areas immediately surrounding the computing point require a dense coverageof gravity observations. Detailed data must be obtained from distances out to 500 miles. A less densenetwork is required for the rest of the earth. The deflections and undulations computed frominformation obtained from these extensive observations are considered absolute values when referred toan earth-centered, referenced ellipsoid. In other words, the axis of rotation for the ellipsoid coincideswith the rotation axis of the earth, and the geometrical center of the ellipsoid coincides with the earth'scenter of gravity (Figure 1-15).n. The collection of gravity observations requires measuring the gravity at or near the earth'ssurface. The ultimate goal is to obtain a good distribution of gravity observations over the entire surfaceof the earth. Since 70 percent of the earth's surface is water, this is not an easy task. Consequently,geodesists are forced to use Stokes's theorem method to determine geoid separation in areas wheregravity observations have not been made.o. The absolute acceleration of gravity is usually measured with a pendulum apparatus. Theswinging period of a pendulum is accurately timed, and from this time interval the value of gravity iscomputed. Pendulum measurements require lengthy observations, and the average period of swing mustbe determined from severalEN0593 1-22