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Re: map_set stereographic projection [message #48589 is a reply to message #48494] Fri, 28 April 2006 09:58 Go to previous message
mattie is currently offline  mattie
Messages: 4
Registered: January 2006
Junior Member
kuyper@wizard.net writes:

> dvila wrote:
>> Hi all,
>>
>> I'm trying to deal with a pre-projected polar stereographic image with
>> this geometrical characteristics:
>>
>> # It is a 1121x881 polar stereographic grid.
>> # Point (1,1) is at 23.117N 119.023W.
>> # Point (1,881) is at 53.509N 134.039W.
>> # Point (1121,1) is at 19.805N 80.750W.
>> # Point (1121,881) is at 45.619N 59.959W.
>> # The y-axis is parallel to 105W.
>> # The resolution is 4.7625km at 60N.
>> # The pole point is (I,J) = (400.5,1600.5)
>>
> ; These are the u-v values corresponding to pixel centers along each
> edge.
> topv = (uv[1,1]+uv[1,2])*0.5
> botv = (uv[1,0]+uv[1,3])*0.5
> leftu = (uv[0,0]+uv[0,1])*0.5
> rightu = (uv[0,2]+uv[0,3])*0.5


Aren't these the u-v values corresponding to the _center_ of the
gridcell along each edge? (assuming the initial data gave the
centerpoints of the gridcell.)

>
> xscale = (rightu-leftu)/(1121-1)
> yscale = (topv-botv)/(881-1)
>
> ; U-V coordinates of midpoints of outer edges
> u = [leftu-0.5*xscale, 0.5*(leftu+rightu), rightu+0.5*xscale, $
> 0.5*(leftu+rightu)]
> v = [0.5*(botv+topv), topv+0.5*yscale, 0.5*(botv+topv),
> botv-0.5*yscale]
> lonlat = MAP_PROJ_INVERSE(u, v, MAP_STRUCTURE=stereo)
> limit = [lonlat[1,*],lonlat[0,*]]

Again, I'm not sure, but don't you have to add half a gridcell to each
direction to get the outter limit of each grid cell?

I'm asking these questions honestly because I've never quite convinced
myself which is the actual limit of the map_set command. I think it's
the outside edge of the grid cell that you want to set. So for this
example, it will be quite close (half a pixel over 800. But if you're
working with zoomed regions. 10x10 grid cells or whatever, it becomes
noticable.


But this was a very informative tutorial on how to register images when
corner points are known.

Thanks
Matt

btw. Does anyone know of a free news server that will let me post
(besides google)? The university just turned ours off. Thanks.


--
Matthew Savoie - Scientific Programmer
National Snow and Ice Data Center
(303) 735-0785 http://nsidc.org

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