[gdal-dev] understanding gdalwarp default output resolution

[Kyle Shannon]

On Mon, Oct 28, 2013 at 8:55 AM, Ivan Marchesini
<ivan.marchesini at gmail.com> wrote:
> Dear all,
> I'm using gdalwarp to reproject a wgs84 srtm (3 arcseconds) tile to a
> projected utm zone. I do not set "-tr" or "-ts" options, hence the final
> output resolution is estimated by the gdalwarp module itself.
>
> For my tile I obtained an output resolution of 81.19 meters and I'm
> wondering if one could explain which is the rationale behind this value.
>
> Many thanks
>
> Ivan
>
>
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Forgot the list...

Ivan,
gdalwarp uses GDALSuggestedWarpOutput:
http://gdal.org/gdal__alg_8h.html#a816819e7495bfce06dbd110f7c57af65

The docs state:

"""
...
Then a resolution is computed with the intent that the length of the
distance from the top left corner of the output imagery to the bottom
right corner would represent the same number of pixels as in the
source image. Note that if the image is somewhat rotated the diagonal
taken isn't of the whole output bounding rectangle, but instead of the
locations where the top/left and bottom/right corners transform. The
output pixel size is always square. This is intended to approximately
preserve the resolution of the input data in the output file.
...
"""

Essentially it tries to match the size of the source image in x size and y size.

kss

-- 
Kyle