| Adding Geographic Overlays to GeoTIFF Files [message #58188] |
Sat, 19 January 2008 11:38 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Folks,
I have been preparing a set of lectures on map projections,
and I've had occasion to revisit a topic you may remember
me ranting about a year ago Christmas. Namely: how come
when you draw outlines around an image that is in a map
projection, the outlines are *slightly* off?
I have reported the problem to ITTVIS again (I first reported
it in IDL 6.3) but I haven't heard word of any bug fix and
it persists in IDL 7.
In a nutshell, if you want to do this, don't get your
UV_BOX, which is what you need to set up a UV data
coordinate system you can draw into, from the map structure
returned my MAP_PROJ_INIT. Get it instead via the UVRANGE
keyword in MAP_PROJ_IMAGE.
I mention this because the instructions I gave for
georegistering the GeoTIFF image contained a lot of nonsense
about moving image boundaries by half a pixel, etc. that I have
subsequently learned is not necessary. (Although it is
entertaining reading if you like adventure stories where
the hero blunders about and saves the princess by
acting correctly for all the wrong reasons.)
I have written an article on this topic which
incorporates a better (I hope!) understanding of the
entire situation. You can find it here:
http://www.dfanning.com/map_tips/tiffoverlay.html
As always, I appreciate feedback on any nonsense that Coyote
slipped in after I edited the article.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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