[Gdal-dev] geotiff "pixel is point" issue?

[Tim Beckmann]

Hi,

I've been looking at using gdal_translate to convert an HDF4 image to 
geotiff.  During my investigation, I happened to notice that the gdal 
geotiff driver might not be handling the "pixel is point" verses "pixel is 
area" distinction properly.

If I understand things properly, the gdal default for pixel coordinates is 
the same as geotiff "pixel is area".  The upper-left image corner 
specifies the upper-left corner of the upper-left pixel, and the 
bottom-right image corner specifies the lower-right corner of the 
lower-right pixel of the image.

According to 
http://www.remotesensing.org/geotiff/spec/geotiff2.5.html#2.5.2, the 
"pixel is point" option changes how the coordinates in the geotiff file 
are interpretted so that the image corner points are relative to the 
center of the pixels instead of the edges.

I don't see anything in the geotiff driver that is handling the "pixel is 
point" case to properly fix up the affine transformation coefficients.  Am 
I missing something?

Also, it would be nice if a "pixel is point" option could be passed to the 
geotiff driver when generating a geotiff output. (Assuming my 
interpretation of pixel is point/area is correct).  That way the corners 
passed to the process could be relative to the center of the pixel if that 
is what is currently available.

Thanks
Tim

--------------------------------------------------------------------------
Tim Beckmann
Software Project Lead
Science Applications International Corporation (SAIC)
Contractor to the USGS/EROS
Sioux Falls, SD 57198
605-594-2521    Phone
605-594-6940    Fax
tbeckman at usgs.gov
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