[Proj] datum matters

[Jan Hartmann]

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.

Jan

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
> LSU
> ------------------------------------------------------------------------
>
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