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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.<br>
<br>
Jan<br>
<br>
Dr. J. Hartmann<br>
Department of Geography<br>
University of Amsterdam<br>
<br>
Clifford J Mugnier wrote:
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<div id="idOWAReplyText4582" dir="ltr">In regard to this thread,
I shall offer my two cents' worth of opinions:</div>
<div dir="ltr"> </div>
<div dir="ltr">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, <em>e.g</em>., 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.</div>
<div dir="ltr"> </div>
<div dir="ltr">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, <em>e.g.</em> the French
Army Truncated Cubic Lambert for some areas in North Africa.</div>
<div dir="ltr"> </div>
<div dir="ltr">I do not see PROJ doing that ... essentially it would
be an attempt to duplicate the EPSG contrivance.</div>
<div dir="ltr"> </div>
<div dir="ltr">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.</div>
<div dir="ltr"> </div>
<div dir="ltr">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.</div>
<div dir="ltr"> </div>
<div dir="ltr">Cliff Mugnier</div>
<div dir="ltr">LSU</div>
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