ABSTRACT
A datum is a reference to which other values are related. In surveying, a vertical datum could be as simple as an arbitrary benchmark assigned an orthometric height of 100.000 meters (or feet, etc.). A horizontal datum could be defined by a stake pounded in the ground for a point of beginning (P.O.B.) and assigned arbitrary coordinates such as east = 5,000.000 meters, north = 10,000.000 meters. If both horizontal and vertical values are assigned to the same point, the result could be called a 3-D datum. The definition of units of measurement, orientation of azimuth, and coordinate system are all implicit in such a definition. Another important implicit assumption is that horizontal is perpendicular to the plumb line, or, as is the case with the GSDM, horizontal is taken to be perpendicular to the ellipsoid normal through the standpoint or through the P.O.B. as defined in chapter 1. Presumably rectangular Cartesian coordinates, either 2-D or 3-D, are used to describe the location of all points with respect to the datum origin and with respect to each other. The permanence and value of such an assumed datum depend upon the stability of monumented points, the quality of coordinates on those points, and the extent to which assumptions associated with establishment of the origin are documented, followed, and made available to others. When using the GSDM, the user selects the P.O.B. and works with local flat-Earth components in a well-documented system.
