[Proj] datum matters

Clifford J Mugnier cjmce at lsu.edu
Sun Mar 29 20:36:29 EST 2009

In regard to this thread, I shall offer my two cents' worth of opinions:
For Large scale mapping generally at 1:50,000 and larger, the classical geodetic Datum is the primary consideration before delving into cartographic projections of various flavors.  However, with such considerations taken into account there are a very finite number of mathematical models appropriate for use with any specific Datum.  Those models are generally specifically legislated by some political body that has venue over that specific geographic region.  Since PROJ does not "connect" specific Datums with specific mathematical models, e.g., Rome 1940 with either a Gauss-Boaga Transverse Mercator projection (East or West Zone), for a national system or a Cassini-Soldner projection for a local cadastral system (some several hundred currently exist), ... both being the ONLY appropriate projections to be used with Rome 1940 ... the user needs to be educated enough to know that a Krovak Oblique Conic is entirely inappropriate to use with that Datum for that country.
What does that?  EPSG attempts to provide the guidance for the appropriate mathematical models for the specific classical geodetic Datums they list in their data base, e.g. the French Army Truncated Cubic Lambert for some areas in North Africa.
I do not see PROJ doing that ... essentially it would be an attempt to duplicate the EPSG contrivance.
Separation of true geodetic concerns from classical cartographic applications seems to be a reasonable approach.  Remember, the late John P. Snyder never attempted to write about the connections between projections and Datums with respect to large scale topographic mapping.  John did not care for such intricacies, and he mostly left such to me except for his private consulting work when he actually used my software.  The various versions of Transverse Mercator are indeed an excellent resource when the user needs to couple specific math models with specific Datums.
For large-scale stuff, I think separating the two is the way to go.  Let the user figure it out at the user's peril.  If the user is apprehensive about such decisions, there indeed are people in private practice that will recommend such decisions for a fee - most are associated with the "Oil Patch" since it does make a difference for them and their clinetle.
Cliff Mugnier
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