Re: [gdal-dev] Frank Eats Crow - or Fixing PixelIsPoint Interpretation

Ivan Lucena ivan.lucena at
Tue Nov 16 10:04:46 EST 2010


Although I agree with your answer to Joaquim's question I would like to extend the subject a little bit.

If I understood the RFC33 correctly the CreateCopy()'s of others driver will not going to fell the effect of that change. I mean, the GeoTransform will still be based on the center of the pixel. That is good.

But for the sake of data quality, for driver/formats that support both reference modes, I believe it will be important to carry away that information to the output. If you take the outputted raster and use it in surface analysis or 3D visualization packages it will be better to know where the pixel reference really is. Right?

IMHO, for those driver, including GTIFF itself, would be nice to know what was the pixel reference of the source dataset in order to make a decision to write reference in a PixelIsPoint or PixelIsArea fashion.

Should we establish some automatic and standard way of dealing with that?

Best regards,


>  -------Original Message-------
>  From: Frank Warmerdam <warmerdam at>
>  To: Joaquim Luis <jluis at>
>  Cc: gdal-dev <gdal-dev at>
>  Subject: Re: [gdal-dev] Frank Eats Crow - or Fixing PixelIsPoint Interpretation
>  Sent: Nov 16 '10 09:40
>  Joaquim Luis wrote:
>  > Frank,
>  >
>  > Wouldn't this apply to the netCDF grids as well? GMT has always made the
>  > distinction on the area vs point representation. The grids created by
>  > GMT hold that information on the global attribute "node_offset". As for
>  > other netCDF CF grids created elsewhere it is also possible to find
>  > which model is used by checking the grid limits vs array size because
>  > for the same limits the PixelIsArea model has one less row and column
>  > than the PixelIsPoint.
>  Joaquim,
>  The gdal/frmts/netcdf/gmtdataset.cpp code already examines the node_offset
>  and adjusts the geotransform accordingly.  It does not currently attempt
>  to expose the node_offset via the AREA_OR_POINT metadata item though it
>  could.
>  I do not see the GMT format driver being pertinent to the RFC.
>  Best regards,
>  --
>  ---------------------------------------+--------------------------------------
>  I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at
>  light and sound - activate the windows |
>  and watch the world go round - Rush    | Geospatial Programmer for Rent
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