[gdal-dev] GDAL Cubic Interpolation

[Even Rouault]

Harvey,

> 
> I subsampled a GLOBE image of elevation data by a subsampling factor of 12
> to a resolution of 0.1 deg x 0.1 deg on integer multiples of 0.1 deg. The
> original GLOBE image is: Latitude from +90-1/240 deg to -90+1/240 deg in
> 1/120 deg steps, and Longitude from -180+1/240 deg to +180-1/240 deg in
> 1/120 deg steps.
> 
> I understand that the cubic interpolation method in gdalwarp accounts for
> the subsampling factor, so 48x48 adjacent pixels influence the target pixel
> for a subsampling factor of 12.
> 
> Does gdalwarp assume the original image is a sphere, where 48x48 adjacent
> pixels influence all target pixels, or a rectangle, where a limited number
> of pixels influence the target pixels at the top, bottom, left, and right
> edges of the subsampled image?

A rectangle, with fallback to bilinear resampling at edges

> If gdalwarp assumes the original image is a
> rectangle and uses a modified interpolation method at the four edges of the
> subsampled image, the modified method affects two rows and two columns at
> the top, bottom, left and right edges of the subsampled image. If gdalwarp
> assumes the original image is a rectangle, is there a command option so
> gdalwarp considers spherical wrapping when interpolating? 

Nothing directly straightforward. Your below plan is the a reasonable solution.

> Alternatively, I
> assume I could augment the original GLOBE image with the appropriate
> additional rows and columns on all four edges. If I did that, how do I tell
> gdalwarp the extent of the augmented input image?

You'll have to recreate a properly referenced input image, for example with

 gdal_translate -ullr upperleftx upperlefty lowerleftx lowerlefty

Even

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