| Re: four corners for lat & long pixels [message #54743] |
Thu, 05 July 2007 09:26  |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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titan wrote:
> thank you for your answer but unfortunately i'm trying this procedure
> for the first time!! :o(
> could you please be more clear?
>
> thank you very much
Let's assume that 'latitude' is an array with dimensions (cols, rows).
Most of the work that needs to be done can be done with the following
commands:
ix = DINDGEN(cols+1)-0.5
iy = DINDGEN(rows+1)-0.5
corner_lat = BILINEAR(latitude, ix, iy)
Note: ix and iy have now been replaced by two-dimensional arrays.
Unfortunately, when it goes outside of the bounds of the input array,
BILINEAR switches over to a nearest-neighbor rather than using
bilinear extrapolation. IDL has many routines for performing various
kinds of interpolation, but a quick survey didn't turn up a single one
that switched over to extrapolation beyond the boundaries of the
original data. Therefore, you'll have to fix up all four edges. I'll
show you how to handle the first row; the technique for the other
three edges are similar:
top = latitude[0:cols-2,0:1]+latitude[1:*,0:1]
corner_lat[1:cols-1,0] = (2.0*top[*,0] - top[*,1])/6.0
Finally, special handling is also needed for the four outermost
corners. For the first row and column, bilinear extrapolation to the
corner point is equivalent to:
corner_lat[0,0] = 0.5625*latitude[0,0]-.
1875*(latitude[1,0]+latitude[0,1])+0.1875*latitude[1,1]
Similar calculations apply at the other three corners.
You shouldn't worry about the more sophisticated approaches I
mentioned earlier, until you understand this simple approach well
enough.
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