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Re: trouble with map projections [message #84993 is a reply to message #84992]

Thu, 20 June 2013 14:20

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

chris.orphanides@noaa.gov writes:

> 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.

Ah, yes, I guess I was thinking of a Cylindrical Equal Area projection,
which was introduced in IDL 8.0.

Yeah, you're probably screwed. :-)

You probably have to use ENVI to get your map projections right. I have
NO idea my MAP_PROJ_INIT allows that ellipsoid, although in the back on
my mind I seem to remember a change that allowed any ellipsoid with map
projections. But, I can't find any mention of it anywhere. Sorry!

I guess your only solace is that on a map with those limits, the
difference between a sphere and a WGS84 ellipsoid are going to be very
small. I've seen a hell of a lot worse in scientific papers. :-)

Cheers,

David

--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thue. ("Perhaps thou speakest truth.")

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[Message index]

		trouble with map projections By: chris.orphanides on Thu, 20 June 2013 11:37
		Re: trouble with map projections By: David Fanning on Thu, 20 June 2013 12:25
		Re: trouble with map projections By: chris.orphanides on Thu, 20 June 2013 13:50
		Re: trouble with map projections By: David Fanning on Thu, 20 June 2013 14:20
		Re: trouble with map projections By: chris.orphanides on Thu, 20 June 2013 17:05

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