ABSTRACT

Modern geodetic practice includes very detailed datum definitions, and the location of points anywhere within the birdcage of orbiting Global Navigation Satellite System satellites can be determined within very small tolerances—either relative or absolute. A brief history of horizontal datums in the United States would be incomplete without considering the transition from separate horizontal and vertical geodetic datums to an integrated three-dimensional (3-D) datum that includes both horizontal and vertical. Datums lacking global extent are called regional geodetic datums. With the advent of the Space Age, the tools of satellite geodesy made it possible to survey the world as a whole and regional datums were no longer able to accommodate “big picture” observations adequately. With due consideration and use of a local Point of Beginning, the Global Spatial Data Model accommodates both spatial and geospatial data and provides an efficient bridge between a flat-Earth 3-D datum and a 3-D geodetic datum.