| 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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