| Another Map Projection Problem with Map_Proj_Init [message #78416] |
Mon, 28 November 2011 07:50  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Folks,
I have discovered another map projection problem this morning.
It seems Map_Proj_Init is calculating incorrect UV ranges
(they are wrong by 10 degrees) when the center latitude is
not 0.0. Here is a short test program:
FOR j=0,10 DO BEGIN
center_lon = 0 > j*36.0d < 360.0
mapStruct = Map_Proj_Init(117, CENTER_LONGITUDE=center_lon)
print, center_lon, mapStruct.uv_box[[0,2]]
ll = Map_Proj_Inverse(mapStruct.uv_box[[0,2]], $
mapStruct.uv_box[[1,3]], MAP_STRUCTURE=mapStruct)
print, center_lon, ll[0,0], ll[0,1]
print, ""
ENDFOR
END
Here are the results. The numbers should be symmetric,
but you can see they are not.
0.000000 -20015077. 20015077.
0.000000 180.00000 -180.00000
36.0000 -19570298. 19347908.
36.0000 -140.00000 -150.00000
72.0000 -19125518. 19792688.
72.0000 -100.00000 -110.00000
108.000 -19792688. 19125518.
108.000 -70.000000 -80.000000
144.000 -19347908. 19570298.
144.000 -30.000000 -40.000000
180.000 -20015077. 18903129.
180.000 0.00000000 -10.000000
216.000 -19570298. 19347908.
216.000 40.000000 30.000000
252.000 -19125518. 19792688.
252.000 80.000000 70.000000
288.000 -19792688. 19125518.
288.000 110.00000 100.00000
324.000 -19347908. 19570298.
324.000 150.00000 140.00000
360.000 -20015077. 20015066.
360.000 180.00000 179.99990
I couldn't figure out why my grid lines were always getting
screwed up when I calculated them using the UV range! :-(
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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| Re: Another Map Projection Problem with Map_Proj_Init [message #78614 is a reply to message #78416] |
Tue, 06 December 2011 05:35  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Fabzou writes:
> Just to be sure: if I never used UV_box values but a LOT of the
> projections I am using have a non-zero center longitude, this bug won't
> affect me in any sense, will it?
Well, I don't know. Probably not. I was using the information
to label my maps, and it was certainly affecting me there.
These kinds of errors make me a little nervous, because
I have the impression map projections are not really
understood at Excelis. This impression is nearly always
reinforced by the explanations I get for the error.
Although, to be fair, the explanation is almost always
passing through a third party, who I don't really expect
to understand map projections, and that may be adding
noise to the explanation.
In any case, I tend to use my own version of these map
routines, which fix the numerous problems I have encountered
while I've been looking into it. (Not to say my own programs
are completely bug free. But I do work hard to make sure they
are right.)
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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| Re: Another Map Projection Problem with Map_Proj_Init [message #78615 is a reply to message #78416] |
Tue, 06 December 2011 02:32 |
Fabzou
Messages: 76
Registered: November 2010
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Member |
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Thanks David.
Just to be sure: if I never used UV_box values but a LOT of the
projections I am using have a non-zero center longitude, this bug won't
affect me in any sense, will it?
Fab
On 12/06/2011 01:43 AM, David Fanning wrote:
> David Fanning writes:
>
>> I have discovered another map projection problem this morning.
>> It seems Map_Proj_Init is calculating incorrect UV ranges
>> (they are wrong by 10 degrees) when the center latitude is
>> not 0.0. Here is a short test program:
>>
>> FOR j=0,10 DO BEGIN
>> center_lon = 0> j*36.0d< 360.0
>> mapStruct = Map_Proj_Init(117, CENTER_LONGITUDE=center_lon)
>> print, center_lon, mapStruct.uv_box[[0,2]]
>>
>> ll = Map_Proj_Inverse(mapStruct.uv_box[[0,2]], $
>> mapStruct.uv_box[[1,3]], MAP_STRUCTURE=mapStruct)
>> print, center_lon, ll[0,0], ll[0,1]
>> print, ""
>> ENDFOR
>> END
>>
>> Here are the results. The numbers should be symmetric,
>> but you can see they are not.
>>
>> 0.000000 -20015077. 20015077.
>> 0.000000 180.00000 -180.00000
>>
>> 36.0000 -19570298. 19347908.
>> 36.0000 -140.00000 -150.00000
>>
>> 72.0000 -19125518. 19792688.
>> 72.0000 -100.00000 -110.00000
>>
>> 108.000 -19792688. 19125518.
>> 108.000 -70.000000 -80.000000
>>
>> 144.000 -19347908. 19570298.
>> 144.000 -30.000000 -40.000000
>>
>> 180.000 -20015077. 18903129.
>> 180.000 0.00000000 -10.000000
>>
>> 216.000 -19570298. 19347908.
>> 216.000 40.000000 30.000000
>>
>> 252.000 -19125518. 19792688.
>> 252.000 80.000000 70.000000
>>
>> 288.000 -19792688. 19125518.
>> 288.000 110.00000 100.00000
>>
>> 324.000 -19347908. 19570298.
>> 324.000 150.00000 140.00000
>>
>> 360.000 -20015077. 20015066.
>> 360.000 180.00000 179.99990
>>
>> I couldn't figure out why my grid lines were always getting
>> screwed up when I calculated them using the UV range! :-(
>
> It turns out, according to the folks in technical support,
> that to get correct UV_BOX values from Map_Proj_Init you
> MUST use the LIMIT keyword, even (well, *especially*) if
> you don't plan to limit the map projection in any way.
>
> Another way to say this is that if you plan to use
> Map_Proj_Init and set the CENTER_LONGITUDE to anything
> other than 0, you MUST also use the LIMIT keyword to
> produce correct results. This is, as you might expect,
> not documented. :-)
>
> I've updated my cgMap object to handle the problem
> correctly.
>
> Cheers,
>
> David
>
>
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| Re: Another Map Projection Problem with Map_Proj_Init [message #78617 is a reply to message #78416] |
Mon, 05 December 2011 16:43 |
David Fanning
Messages: 11724
Registered: August 2001
|
Senior Member |
|
|
David Fanning writes:
> I have discovered another map projection problem this morning.
> It seems Map_Proj_Init is calculating incorrect UV ranges
> (they are wrong by 10 degrees) when the center latitude is
> not 0.0. Here is a short test program:
>
> FOR j=0,10 DO BEGIN
> center_lon = 0 > j*36.0d < 360.0
> mapStruct = Map_Proj_Init(117, CENTER_LONGITUDE=center_lon)
> print, center_lon, mapStruct.uv_box[[0,2]]
>
> ll = Map_Proj_Inverse(mapStruct.uv_box[[0,2]], $
> mapStruct.uv_box[[1,3]], MAP_STRUCTURE=mapStruct)
> print, center_lon, ll[0,0], ll[0,1]
> print, ""
> ENDFOR
> END
>
> Here are the results. The numbers should be symmetric,
> but you can see they are not.
>
> 0.000000 -20015077. 20015077.
> 0.000000 180.00000 -180.00000
>
> 36.0000 -19570298. 19347908.
> 36.0000 -140.00000 -150.00000
>
> 72.0000 -19125518. 19792688.
> 72.0000 -100.00000 -110.00000
>
> 108.000 -19792688. 19125518.
> 108.000 -70.000000 -80.000000
>
> 144.000 -19347908. 19570298.
> 144.000 -30.000000 -40.000000
>
> 180.000 -20015077. 18903129.
> 180.000 0.00000000 -10.000000
>
> 216.000 -19570298. 19347908.
> 216.000 40.000000 30.000000
>
> 252.000 -19125518. 19792688.
> 252.000 80.000000 70.000000
>
> 288.000 -19792688. 19125518.
> 288.000 110.00000 100.00000
>
> 324.000 -19347908. 19570298.
> 324.000 150.00000 140.00000
>
> 360.000 -20015077. 20015066.
> 360.000 180.00000 179.99990
>
> I couldn't figure out why my grid lines were always getting
> screwed up when I calculated them using the UV range! :-(
It turns out, according to the folks in technical support,
that to get correct UV_BOX values from Map_Proj_Init you
MUST use the LIMIT keyword, even (well, *especially*) if
you don't plan to limit the map projection in any way.
Another way to say this is that if you plan to use
Map_Proj_Init and set the CENTER_LONGITUDE to anything
other than 0, you MUST also use the LIMIT keyword to
produce correct results. This is, as you might expect,
not documented. :-)
I've updated my cgMap object to handle the problem
correctly.
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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