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Re: More Map Projection Madness [message #78241 is a reply to message #78215]

Tue, 01 November 2011 13:20

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

David Fanning writes:

> It seems the name "Albers Equal Area" is ambiguous.
> In fact, it seems to choose the old MAP_SET routine
> "Albers Equal Area Conic" as the map projection.
> To make it choose the same projection as projection
> 103, I have to set the CGTP keyword.
>
> alberMap = MAP_PROJ_INIT('albers equal area', /GCTP, $
> DATUM='WGS 84', $
> CENTER_LATITUDE=geotag.PROJNATORIGINLATGEOKEY, $
> CENTER_LONGITUDE=geotag.PROJNATORIGINLONGGEOKEY, $
> STANDARD_PAR1=geotag.PROJSTDPARALLEL1GEOKEY, $
> STANDARD_PAR2=geotag.PROJSTDPARALLEL2GEOKEY)
>
> Then, this behaves identically to the the projection when
> I use the projection index 103.

This silent substitution of command thing is more
pernicious than I first expected. Not only is the
"albers equal area conic" map projection substituted
for the "albers equal area" projection, but the
sphere radius that is used for the conic map projection
is set to the length of the semi-major axis of the
WGS84 ellipse!

In other words, this command:

mapStruct = Map_Proj_Init('Albers Equal Area', $
ELLIPSOID='WGS 84', $
STANDARD_PAR1=21.0, STANDARD_PAR2=-19.0)

is silently substituted with this command:

mapStruct = Map_Proj_Init('Albers Equal Area Conic', $
SPHERE_RADIUS=6378137.0, $
STANDARD_PAR1=21.0, STANDARD_PAR2=-19.0)

Naturally, you get the wrong results if you don't
realize this. You can find out more about this in
this article:

http://www.idlcoyote.com/map_tips/choosemap.php

Cheers,

David

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

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[Message index]

		Re: More Map Projection Madness By: David Fanning on Thu, 03 November 2011 11:43
		Re: More Map Projection Madness By: David Fanning on Tue, 01 November 2011 13:20
		Re: More Map Projection Madness By: David Fanning on Tue, 01 November 2011 11:02
		Re: More Map Projection Madness By: chris_torrence@NOSPAM on Tue, 01 November 2011 10:30
		Re: More Map Projection Madness By: David Fanning on Tue, 01 November 2011 10:32
		Re: More Map Projection Madness By: David Fanning on Tue, 01 November 2011 09:35
		Re: More Map Projection Madness By: Paul Van Delst[1] on Mon, 07 November 2011 08:19
		Re: More Map Projection Madness By: David Fanning on Fri, 04 November 2011 10:34

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