Elevations and the Global Spatial Data Model (GSDM)

Earl F. Burkholder

Abstract:	Spatial data are three-dimensional (3-D). Modern measurement systems collect spatial data in a 3-D environment and computer data bases store digital spatial data in 3-D. The WGS84 earth-centered earth-fixed (ECEF) geocentric coordinate system defined by the Defense Mapping Agency (now NIMA) can be viewed as a coherent 3-D geodetic datum which has a single origin at the earth’s center of mass. With the advent of GPS, and other technologies, many spatial data computations can be performed more efficiently in the ECEF environment. The global spatial data model (GSDM) is a collection of geometrical relationships and error propagation procedures (functional & stochastic models) that incorporates the ECEF system and describes a comprehensive environment for working with 3-D spatial data. Previously, horizontal and vertical data were considered separately, in part, because they are assigned different origins. Horizontal data are referenced by latitude/longitude on a mathematical ellipsoid while elevation is taken to be the distance from sea level (the geoid). Geoid height is the difference between those two origins, the ellipsoid and the geoid. But, globally, the geoid defies precise location and absolute elevation suffers accordingly. The approach here is to define the terms concisely, put the pieces together carefully, and to suggest that elevation (along with horizontal) should also be referenced to the earth’s center of mass via the mathematical ellipsoid. That will provide a single origin for 3-D data and the need for incidental geoid modeling will be enormously reduced – especially for routine use of GPS technology. Making that change will not diminish the value of previous geoid modeling and on-going efforts to locate the elusive geoid should continue because there are identifiable scientific and engineering applications for which knowledge of the equipotential surfaces is still required.
Published in:	Proceedings of the 58th Annual Meeting of The Institute of Navigation and CIGTF 21st Guidance Test Symposium (2002) June 24 - 26, 2002 Hyatt Regency Hotel Albuquerque, NM
Pages:	217 - 222
Cite this article:	Burkholder, Earl F., "Elevations and the Global Spatial Data Model (GSDM)," Proceedings of the 58th Annual Meeting of The Institute of Navigation and CIGTF 21st Guidance Test Symposium (2002), Albuquerque, NM, June 2002, pp. 217-222.
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