[OSRS-PROJ] Orthographic Projections and MapServer

[Ron Russell]

This discussion is surely about the limiting case of a more
general problem of how to define the limits of a particular
projection. 

For example it would be good to be able to
define the limits of a Transverse Mercator projection to
avoid the sillyness we have had of people trying to display
New Zealand in the British National Grid! ("The projected
space was available, so why not?"  was the response I
got!).

More seriously, we currently have a project that uses
world data as a background. There are two huge polygons
in this data (The American continent and Europe, Asia and
Africa). When this is displayed in British National Grid parts
of what I assume to be Siberia/Alaska appear in Europe
(presumably an artifact of the clipping of coordinates
approaching infinity).

I assume that these limits would have to be stored in terms of
Geographical Coordinates which leads to problems of deciding
their granularity. (I am assuming that the projection is independent
of the data; it certainly is in our systems).

Clipping to the limits would be done before projection using
spherical/ellipsoidal geometry. This seems
horrific, particularly for polygons, but then computing power is
becoming cheaper!

Finally, in my experience, clipping of polygons cannot be done
on the fly. You have to bite the bullet and do it all at once. Our
polygons contain holes (like the Great Lakes and the Caspian
Sea).

---------------------------------------------------------------------
Ron Russell
Senior Software Engineer, Laser-Scan
+44(0)1223 420414

http://www.laser-scan.com

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