Oblate Spheroidal Coordinates, Confocal Ellipsoidal Coordinates and Geodesy What is the name of the orthogonal coordinate system that is most commonly used in modern geodesy\geomatics engineering to model the reference ellipsoid? I suspect it is either _oblate spheroidal coordinates_ or _confocal ellipsoidal coordinates_. Furthermore, please indicate one or more distinguishing features of the two aforementioned coordinate systems. Thank you.

Map Projections- A Working Manual by John P. Snyder, U. S. Geological Survey Professional Paper 1835, chapter 3 THE DATUM AND THE EARTH AS AN ELLIPSOID (available through Google Books and at Federal Repository libraries) says the earth is modeled as an oblate ellipsoid.

Oblate spheroidal points are identified by the intersection of an ellipsoid (the reference ellipsoid of the earth), a one-sheet hyperbola (roughly corresponding to latitude) and a plane (longitude). Confocal ellipsoidal points are identified by the intersection of an ellipsoid, a one-sheet hyperbaloid and a two-sheet hyperbaloid. You can find more complete descriptions and drawings at many websites, including Wolfram. There are numerous books: Schaum's Outline Mathematical Hanbook, the CRC Standard Mathematical Tables and Formulae are two pretty common references.