[gdal-dev] I'm conflicted: GeoTIFF's PixelIsPoint, myth or reality?

[Garrett Potts]

Hello All:

Hmmm,  Not sure why this would be hard to add.  It would be nice to  
have in the metadata of the file opened an indication of what the tie  
point represents.  Center of pixel or upper left corner of pixel 
( AREA).  Even if GDAL doesn't do auto shifting to a common  
representation it would be nice to have in the API a way to query  
this information.

Take care

Garrett

On Nov 26, 2007, at 3:01 PM, Brent Fraser wrote:

> Frank (and anyone else interested in 1/2 pixel accuracy in
> GeoTIFFs),
>
>   Years ago I read about how the authors of the GeoTIFF spec
> never  intended the PixelIsPoint/PixelIsArea tag to indicate
> the image origin (center or top-left of top left pixel) of
> the SRS tie point.  Regardless of the intent, some (most?)
> s/w developers used it that way; perhaps the actual origin
> was not obvious in the spec.
>
>   My problem is that GDAL follows the spec (or conversely
> other popular apps don't!), ignores the tag for
> geo-referencing purposes and uses the tie point as the
> top-left of the top left pixel.  No problem when creating
> GeoTIFFs with GDAL as it sets PixelIsArea.  And all is good
> when reading PixelIsArea-style GeoTIFFs with GDAL.
>
>   But when reading PixelIsPoint-style GeoTIFFs (and
> translating them), the reported tie point is shifted 1/2
> pixel in X and Y (well, shifted from my expectation).  I've
> read past GDAL emails explaining changing GDAL would be
> large effort (to say nothing of buying into the PixelIsPoint
> myth) so perhaps a simple addition would keep me (and
> others) within 1/2 a pixel:
>
>  Have the GDAL utilties issue a warning message when a
> PixelIsPoint-style GeoTIFF is read.  And maybe have an
> option ( "-w" ?) to ignore the embedded tie point values and
> instead try to use a world file (tfw), if it exists?
>
>
>
> Brent Fraser
> GeoAnalytic Inc.
> Calgary, Alberta
>
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