[Proj] datum matters
j.l.h.hartmann at uva.nl
Mon Mar 30 16:57:59 EST 2009
Agree wholeheartedly! Even Gerald Evenden would agree, I guess: separate
projections from datums and you get a much clearer viewof the process.
Pity he isn't on this list any more. Projections are about getting data
from an ellipsoid on to a flat surface, datums are about translating and
rotating the ellipsoid itself. I completely agree with Cliff that the
user should know what he/she is doing.
Dr. J. Hartmann
Department of Geography
University of Amsterdam
Clifford J Mugnier wrote:
> 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
> Proj mailing list
> Proj at lists.maptools.org
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