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Re: Confusions with map_image! [message #66858 is a reply to message #66857]

Fri, 12 June 2009 08:05

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

hethomas writes:

> I am not surprised by the results that are in my original grid (the
> one that I made as an amalgamation of my data). The thing that is
> surprising me is that when I use map_image on the image, the values
> change significantly, depending on the window size (and more
> obviously, whether the bilinear keyword is set) etc. It is this that
> has made me wonder how valid the results of Map_image are and what
> variables I should be using to get a "correct" result!

Well, yes, if you are using BILINEAR your results
*will* change with the size of the window! Those
extra pixel values have to come from somewhere.
BILINEAR interpolation will make them up for you.
(Often in the way you *wish* they were, rather than
the way they are.) Better in your case to use
nearest-neighbor interpolation so pixels are
replicated, but original values are not changed.

MAP_IMAGE is really a *display* technique (and a
poor one at that). It would probably be much better
to fit a map coordinate system to your image, rather
than fit the image to your coordinate system. There
are lots of articles on my web page and in the archives
of this newsgroup that address this issue. (Including
one this morning that points out how impossible this
is to do!)

But I would steer well clear of MAP_SET and any of
its associated routines if you really care about map
projected images. :-)

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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[Message index]

		Re: Confusions with map_image! By: hethomas on Fri, 12 June 2009 08:20
		Re: Confusions with map_image! By: David Fanning on Fri, 12 June 2009 08:05
		Re: Confusions with map_image! By: hethomas on Fri, 12 June 2009 07:48
		Re: Confusions with map_image! By: David Fanning on Fri, 12 June 2009 07:32
		Re: Confusions with map_image! By: hethomas on Fri, 12 June 2009 07:02
		Re: Confusions with map_image! By: David Fanning on Fri, 12 June 2009 06:39
		Re: Confusions with map_image! By: hethomas on Fri, 12 June 2009 06:24
		Re: Confusions with map_image! By: David Fanning on Fri, 12 June 2009 05:37
		Re: Confusions with map_image! By: jameskuyper on Fri, 12 June 2009 17:49

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