[OSRS-PROJ] Projection problems at horizon
BartA at DEMIS.nl
Thu Feb 22 10:51:47 EST 2001
Thanks for your feedback, nice to see that you are still listening....
> 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.
Basically you are right, it is a graphic problem and not a projection
It would be very difficult however to move the rejected points to the
horizon in a generic way without taking some care of this in the
projection algorithm itself. Then again, if during projection one
decided to move these points to the horizonm, doing an inverse
projection on the output would make these points end up at incorrect
lat-lon positions, which of course is exactly your point.
I would appreciate any information on ways to solve this graphic
problem, it is however not a major problem because making these nice
"globes" is less important to us than mapping some countries national
coordinate system properly ;-)
Thanks for the feedback,
> 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 the obliques required special, unpublished procedures.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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