[OSRS-PROJ] Projection problems at horizon

Gerald I. Evenden gevenden at capecod.net
Wed Feb 21 20:23:48 EST 2001

You are involved with a graphic problem, not a projection problem.
PROJ does not take care of projectable region boundaries other than
to indicate that the point is not projectable.  In other cases, the
point may be "wrapped" to the opposite side of the map when exceeding
180 from the central meridian.

Many of the illustrations in the PROJ.4 manuals involve use of special
programs that determine interuption points and perform necessary
graphic control in drafting the vectors.  Most of the illustrations that
only needed wrapping control were drafted with MAPGEN---
cylindrical, pseudocylindrical are examples.   Esoteric projections like
the obliques required special, unpublished procedures.

Further special graphic methods are required in generating
interrupted projections.

Graphic control software is not part of the PROJ.4 package
as distributed by the author.  Also, please do not request copies
of them as they are *very* "imaginative," with totally undocumented
methods and usage.  Using them requires experimentation and
twisting and tweeking for each particular projection case.

Jerry Evenden and the Low Riders, Katie and  Daisy May
gevenden at capecod.net  http://www.capecod.net/~gevenden
----- Original Message ----- 
From: Bart Adriaanse <BartA at DEMIS.nl>
To: <osrs-proj at remotesensing.org>
Sent: Wednesday, February 21, 2001 11:50 AM
Subject: [OSRS-PROJ] Projection problems at horizon

> Dear proj users,
> I am experiencing a problem at the horizon while doing worldmap
> projections.
> For example, i take worldwide coastline data in Lat-Lon and i project
> them using orthographic projection to create a view of the globe similar
> to what one would see from space.
> Instead of ending up with a graphic like the one in the proj
> documentation, i end up with several lines that run from the horizon to
> the center of the map.
> No doubt what is happening is that if a coordinate is beyond the
> horizon, it will be returned as zero by the projection formulae,
> indicating that it is not visible. What i was hoping for is that these
> coordinates would be "snapped" to the nearest visible point on the
> horizon instead of the projection origin, thus avoiding these undesired
> lines from the horizon to the center.
> No need to say, the ortho projection is not the only one with this
> behaviour and the results are even worse when i project a bunch of
> country polygons...
> I would imagine that i am not the first one trying to adress this
> problem and would greatly appreciate suggestions/comments form other
> proj.4 users.
> Best Regards,
> Bart Adriaanse
> Demis bv
> www.demis.nl
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