Comparison of Geodetic, State Plane, and Geocentric Computational Models
Surveyors make measurements and process those data to compute positions. Plane surveying uses two-dimensional (2D) coordinates based on assumptions of a flat earth. Plats, maps, and land boundary descriptions are prepared consistent with those assumptions and many “local practice” clients are well-served. With these descriptions, the client is well served, and, over the years, many “local practice” businesses have functioned successfully. But, as technology advances and as the scope of a project or service area gets larger, those flat earth assumptions become limiting, and plane surveyors are exposed to new challenges. Modern measurement systems, evolved during the digital revolution, now routinely collect 3D digital geospatial data. Similarly, computational processes now used in data reduction go well beyond the flat earth assumptions, and models for processing 3D digital geospatial data have evolved from flat earth models to various ellipsoidal models to a plethora of map projections to 3D models that support computations in 3D space worldwide, for example, the global spatial data model. This article includes a comparison of three models used to determine a 3D geodetic position based on a simple total-station side shot from a known station. The three methods are geodetic computation on the ellipsoid, state plane computation on a mapping grid, and geocentric computation in 3D space.
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