surveying iii (topographic and geodetic surveys) - Modern Prepper

Figure 1-10. Simple Triangulation Net - Example 2d. Celestial Techniques. Celestial triangulation methods permit the extension of long arcs acrossoceans and inaccessible terrain. All of these methods--the star occultation, the moon-position camera,and the observation and tracking of artificial earth satellites possess one common characteristic--observed data is not affected by the direction of the vertical at the observation point. The solar eclipsemethod has been discontinued due to the infrequency of total solar eclipses and the inconvenience andfrequent inaccessibility of suitable observation sites located along the narrow path of the moon's shadow.(1) The star occultation method provides information similar to that obtained through a solareclipse from which the distance between observation points can be computed. This method is usedprimarily for connecting isolated islands in the ocean (Figure 1-11). Occultation may be defined as thepassing of a larger celestial body in front of a star. The moon, which is used as the occulting body forgeodesy, revolves around the earth about once a month and in its orbit blocks out certain stars. Usingtelescopes fitted with electronic observing and recording devices and a recording chronometer forprecise time determination, it is possible to record the exact instant of the occultation of a star. Fordetermining a position of a point on the earth, at least two pairs of observations are needed. A pairconsists of occultation measurements at one known station and at another station whose position shouldbe determined on the same datum as the known station. The same star must be observed on about thesame point of the moon's limb at both stations. An approximate geographic position of the point, whosegeodetic position has to be determined, is known for the prediction of the occultation. The observationsyield a correction to the assumed position of the unknown point.EN0593 1-14