| Re: trouble with map projections [message #84992 is a reply to message #84988] |
Thu, 20 June 2013 13:50   |
chris.orphanides
Messages: 3
Registered: June 2013
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Junior Member |
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On Thursday, June 20, 2013 3:25:20 PM UTC-4, David Fanning wrote:
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>> I am trying to take a satellite image of sea surface temperature
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> (SST), subset it, and then project it, and I can't seem to get it right.
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> I am able to read the image and subset the area I am interested in
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> without a problem. Getting it projected and having things line up is
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> another story.
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>> What makes sense to me is first using the map_proj_init function to create the map projection I want to put the image into (Lambert Conformal Conic), then use the map_proj_image function to warp the image to the proper projection. However, when I do this, the resulting array has lost its SST values and is all 0.0s. Can anyone tell me what I am doing wrong? I have experimented with many of the mapping capabilities in IDL, but I just can't get it right. The code I
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> described is below Thanks in advance for your help.
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>> range = [-78.1900, 34.0300, -61.8100, 45.4900]
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>> nec_prj = MAP_PROJ_INIT('Lambert Conformal Conic', /GCTP, $
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>> ELLIPSOID='WGS 84', $
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>> LIMIT=range, $
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>> CENTER_LATITUDE=40.00, $
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>> CENTER_LONGITUDE=-70.00, $
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>> STANDARD_PAR1 = 36.1667, $
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>> STANDARD_PAR2 = 43.8333 )
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>> necprj_sst = MAP_PROJ_IMAGE(nec_region, range, MAP_STRUCTURE = nec_prj)
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>> ; nec_region is the SST data for the region I am interested in,
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>> ; subset to fit the range in the lines above
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>> A little additional information: The main input image before I subset it is described as being in a Cylindrical Lat-Lon projection with a regular 0.01 degree grid and a WGS 84 Ellipsoid. Since Cylindrical is the default IDL projection I didn't set a map projection for this image. I would prefer to set the ellipsoid to WGS 84 for the Cylindrical projection, though it doesn't appear possible in IDL (I would love it if I was wrong about this). Also, I am working with the
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> type of images described here: ( http://podaac.jpl.nasa.gov/dataset/JPL_OUROCEAN-L4UHfnd-GLOB -G1SST)
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> As the Stooges would say, "No, no, no. You're doing it all wrong!"
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> You need to create a map projection that describes your image as you
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> downloaded it. I'm not sure why you think IDL can't do a Cylindrical map
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> projection with a WGS-84 ellipsoid, but this is a VERY common projection
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> for satellite images and IDL handles it perfectly. Then, you create a
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> map projection for what you want the image to end up as. Finally, you
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> use Map_Image to warp the image from one map projection to the other.
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> Here is an article that describes the process:
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> http://www.idlcoyote.com/map_tips/warpimage.html
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> Cheers,
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> David
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> --
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> David Fanning, Ph.D.
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> Fanning Software Consulting, Inc.
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> Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
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> Sepore ma de ni thue. ("Perhaps thou speakest truth.")
David, thank you for your quick response. I didn't think that I could do a Cylindrical map projection with a WGS 84 Ellipsoid because in the map_proj_init() help page it lists Sphere as the only available ellipsoid when using IDL's own map projections. In the GCTP map projections it says that Equirectangular only takes a sphere as well and doesn't say you can specify the semimajor or semiminor axes. What am I missing here? Does the below work even though it doesn't seem like it should?
g1_prj = MAP_PROJ_INIT('Equirectangular', ELLIPSOID='WGS 84', /GCTP, LIMIT=[-80, -180, 80, 180])
It runs successfully, and when peeking at the result some of it looks right, but I am hesitant.
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