surveying iii (topographic and geodetic surveys) - Modern Prepper

(1) Horizontal geodetic datum consists of a starting point and an ellipsoid on which tocompute. Use the following five conditions or parameters in determining the computations: latitude,longitude, azimuth, equatorial radius, and flattening. Computations are made from the latitude andlongitude of the initial point. The azimuth of a line from the initial point gives a direction with whichthe computations are made. A change in any one of these five quantities will change the datum; hence,the coordinates of all points on the datum would also change.(2) The vertical datum, to which the elevations of the points are referenced, is usually the MSLsurface. However, it may be any arbitrary level surface defined by an assumed elevation for some point.b. Geodetic-datum orientation may be established using a single astronomical point and oneastronomical azimuth, an astronomical-geodetic method, or a gravimetric method.(1) Single Astronomical Point and One Astronomical Azimuth. After a reference ellipsoid isdetermined or established, the other three parameters or factors must be defined. The simplest means isto select a first-order triangulation station located at the center of the surveyed area. At this point, theastronomic coordinates are observed and the astronomic azimuth from this point to another controlstation in the net is determined. In this method, it is assumed that the normal (perpendicular) to theellipsoid coincides with the plumb line or the normal (perpendicular) to the geoid (Figure 1-16).Therefore, the observed astronomical coordinates and azimuth are adopted, without any corrections, ascoordinates and azimuth on the ellipsoid. The two components are defined as zero--the deflection of thevertical and the undulation of the geoid.(a) Reference Figure 1-16 and note that a datum oriented by a single astronomical pointproduces large geoid separations. Also, it is not earth-centered, and the rotational axis of the ellipsoiddoes not coincide with that of the earth. The inconvenience of such an orientation is that the positionsderived from different datums are not directly comparable in any geodetic computation.(b) This type of orientation could be used locally for positions in the net, although largeerrors would probably be introduced as the survey is expanded because the entire net would be shiftedrelative to the axis of the earth.1-25 EN0593