[OSRS-PROJ] orthographic projection with proj4 and mapserver.

Gerald I. Evenden gerald.evenden at verizon.net
Wed Aug 13 20:58:27 EDT 2003

On Wed, 2003-08-13 at 18:32, Craig Bruce wrote:
> "Gerald I. Evenden" <gerald.evenden at verizon.net> wrote:
> > Whatever "on-the-fly" means?
> >
> > Presumably "mapserver" is some kind of graphic program.
> "On-the-fly" presumably means that the source data is reprojected in real
> time as requested when the map is drawn.  The "map server" is likely an
> OpenGIS® Consortium Web Map Server (OGC WMS):

Hmmm, "on-the-fly", I didn't know there was any other way.
> Proper graphical clipping is a tough problem.  It looks like the specific
> problem in this case is with the polygons that are half visible and half
> invisible.  The invisible vertices appear to be all substituted with (0,0)
> coordinates, though I thought that the PROJ.4 library used -infinity as
> its 'invalid coordinate' value.  To plot such a polygon mathematically
> correctly, you'd need to cut it in two (or more) pieces along the line of
> the horizon of the projection, which would be a smooth curve in this case.
> Other short-cut cheats are also possible.

Indeed, not a trivial problem.  In all the map graphics that I use in
the PROJ.4 manuals, only vector plotting was performed.  The clipping
problem for vectors is much simpler than polygons---that's why I didn't
fill the continental outlines.  ;-)

BTW, libproj4 forward routine returns HUGE_VAL when it cannot project a
point.  Program [l]proj returns an "*".  The HUGE_VAL method has been
in PROJ.4 use for a *long* time so I suspect that the problem lay in
the calling software.

Gerald I. Evenden <gerald.evenden at verizon.net>

PROJ.4 Discussion List
See http://www.remotesensing.org/proj for subscription, unsubscription
and other information.

More information about the Proj mailing list