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Re: Map Projection [message #86279 is a reply to message #86276]

Tue, 22 October 2013 11:15

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Morgan Silverman writes:

> Thank you for the explanation. I'm still not sure how to determine what the ellipsoid should be though. I used 19 in my code based on one of your gallery examples but I don't know if that would be correct. I'm plotting model back-trajectories over the map.
>
> mapCoord = Obj_New('cgmap', 'Equirectangular', Ellipsoid=19, Limit=limit, $
> xrange=xrange, yrange=yrange, /latlon_ranges, center_lon=centerlon, position=pp)
> mapCoord -> Draw
> cgMap_Grid, map=mapCoord, /box
> cgMap_Continents, map=mapCoord, /continents, /countries, /usa
> cgplots, lon_1500(index), lat_1500(index), map=mapCoord

The ellipsoid you choose should be the same ellipsoid that was used to
determine the latitude and longitude values data you are working with.
If you don't know and/or can't find out, then you have to default to
something "sensible". This is typically WGS84 for any data collected by
satellites in the past, say, 20 years, and Spherical if you really don't
have the foggiest idea. Some map projections only allow spherical
datums. Others allow whatever datum you choose to use.

I've never seen the ellipsoid set by anyone using Map_Set, which makes
it extremely easy to use. The downside, of course, (aside from using
extremely old map software) is that the results aren't all that
accurate. If you are working with global maps, or even maps the size of
the United States, clamoring for accurate results can be used as an
accurate prediction of an anal personality. Some people just like
scientific work to be RIGHT. Other people ascribe to the "close enough"
rule. It's really whatever works for you. :-)

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]

		Map Projection By: morganlsilverman on Tue, 22 October 2013 07:47
		Re: Map Projection By: David Fanning on Tue, 22 October 2013 08:08
		Re: Map Projection By: morganlsilverman on Tue, 22 October 2013 10:25
		Re: Map Projection By: Andy Sayer on Tue, 22 October 2013 10:45
		Re: Map Projection By: David Fanning on Tue, 22 October 2013 11:15
		Re: Map Projection By: suicidaleggroll on Tue, 22 October 2013 08:12
		Re: Map Projection By: Phillip Bitzer on Tue, 29 October 2013 17:23

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