| Re: Map Projection [message #86276 is a reply to message #86269] |
Tue, 22 October 2013 10:25   |
morganlsilverman
Messages: 46
Registered: February 2013
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Member |
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On Tuesday, October 22, 2013 11:08:04 AM UTC-4, David Fanning wrote:
> Morgan Silverman writes:
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>> I'm trying to create a basic plot with a map of the US and trajectory data plotting on top. I'm trying to follow the map projection method using mapCoord = Obj_New('cgmap', 'Lambert Azimuthal', Limit=limit).
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>>
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>> I keep coming across an ellipsoid statement in most of the examples I've found but I can't find any explanation as to what it is. Different examples have use ellipsoid=24, ellipsoid=19, ellipsoid=WGS84, etc...I don't know if I need this or how to set it if I do.
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>>
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>> Can someone please explain what the ellipsoid statement is?
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> When most people come to maps for the first time they believe what they
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> learned in grade school: that every point on the Earth can be described
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> with a latitude and longitude value. Which is true. But what they
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> *don't* tell you is that no point on the Earth has a *unique* latitude
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> and longitude value. What you are calling *this* latitude and *this*
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> longitude depend on what reference standard you are using. This is
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> called a "datum", or in your case, the "ellipsoid".
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> If you use a GPS device to find your location on the Earth, it is
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> probably being calculated with a WGS84 ellipsoid, the standard ellipsoid
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> for most satellite data. If you plot that point on a map projection
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> using a spherical ellipsoid (the default ellipsoid for many map
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> projections) then the point you place on the map projection to
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> illustrate your position will NOT be the point on the Earth where you
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> are standing! You can be many, many meters off, simply because you are
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> using different reference ellipsoids to calculate latitude and
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> longitude.
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>> And, is mapCoord=Obj_New('cgmap', ....) the best way to go about
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> plotting a map of the United States?
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> It has pretty much always worked for me. :-)
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> The nice thing about cgMap is that is *doesn't* work in lat/lon space,
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> where people coming to map projections for the first time think you are
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> suppose to be working. It works in projected meter space, which is a
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> MUCH better place to be in if you are working with rectangular map
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> projected images.
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> If you are trying to put data on top of a coordinate system set up with
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> cgMap, you are going to have to pass the coordinate system object to
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> whatever routine (cgContour, cgPlotS, etc.) you are using, so it knows
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> how to convert the lat/lon values you are trying to plot into the
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> projected meter values of the coordinate system.
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> If this seems beyond your abilities, then I would simply use cgMap_Set
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> to set up the map and keep working in lat/lon. It's not ideal. But, it
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> often works well enough for the purpose.
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>
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> Cheers,
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>
>
> David
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>
>
> --
>
> 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.")
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
Thanks.
-Morgan
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