Using GPS results in true 3-D coordinate system

GPS computations are performed in the geocentric coordinate system and transformed to geographic (latitude/longitude) or state plane coordinates for extended use in data-base applications or engineering projects. State plane coordinates are very useful, but for some applications the difference between grid and ground distance becomes particularly bothersome. State department of transportation (DOT) offices are confronted with the grid/ground distance dilemma on highway projects, for which center-line stationing must be ground based and state plane coordinates are used for project control. A questionnaire was sent to all 50 state DOTs asking for input as to how the problem is currently being addressed and asking for suggestions for solutions. Responses received from 46 of the 50 state DOTs are summarized in an appendix. The proposal included in this paper is to use the geocentric coordinate system as the basis for all three-dimensional control computations, including conventional terrestrial measurements. To eliminate the distance distortion problem, results can be outputted in a local coordinate system. Equations for transforming between geographic, geocentric, and local coordinate systems are included in an appendix.