| Re: AVHRR Image Mapping Problem [message #52005 is a reply to message #51852] |
Sat, 23 December 2006 17:31   |
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Chris[2]
Messages: 39
Registered: August 2003
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Member |
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Hi all,
Just another data point: The PDF document on the GIMMS website states that
the images are projected onto the Clarke 1866 Ellipsoid, while the GeoTIFF
tags in the file indicate that it is WGS-84. I wonder which one is correct.
Now, switching to the Clarke 1866 ellipsoid didn't actually help (it
probably only changes it by a few hundred meters or so), but it makes you
wonder whether the rest of the GeoTIFF information is accurate given that
the ellipsoid is incorrect...
-Chris
ITTVIS
"David Fanning" <news@dfanning.com> wrote in message
news:MPG.1fec703a235800ae989e46@news.frii.com...
> Folks,
>
> Does anyone have any experience working with AVHRR NDVI
> image data or Albers map projection? I have obtained
> the data, which is of the African continent from here:
>
> ftp://ftp.glcf.umiacs.umd.edu/glcf/GIMMS/Regional/Albers/Afr ica
>
> The image is in an Albers Conical equal area projection
> and the centers of the four corner pixels are known from
> the documentation:
>
> ; YX coordinates of the four corners (LL, UL, UR, LR)
> longitude = [-23.49, -24.6, 64.523, 63.414]
> latitude = [-42.243, 43.711, 43.712, -42.242]
>
> This is a GeoTiff file, so I also pull the Standard
> Parallels out of the geotiff information stored in
> the file (they are -19 and 21).
>
> I follow the method outlined on this page (which has
> worked perfectly for a polar stereo map projection),
> using instead of a Stereo projection, an Albers
> projection with standard parallels:
>
> http://www.dfanning.com/map_tips/precipmap.html
>
> The method *ALMOST* works! :-)
>
> But the continental outlines do not QUITE line up properly.
> You can see my result here:
>
> http://www.dfanning.com/misc/africa.jpg
>
> Do you think this might be an Albers projection problem?
> A difference between MAP_PROJ_INIT and MAP_SET? (I have
> tried different DATUMS with no change in effect.)
>
> Or, do you think this might just be right? :-(
>
> Cheers,
>
> David
>
> P.S. Don't worry about the colors. They are still screwed up.
> --
> 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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