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

Frank Warmerdam warmerdam at pobox.com
Tue Nov 16 10:54:34 EST 2010

Ivan Lucena wrote:
> Frank,
> 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?


In fact, I strongly disagree with conflating the sampling technique of
the sensor (area average vs. point sample) with the georeferencing.  And
for formats that can store the georeferencing based on either a top left
corner of top left pixel vs. a center of top left pixel form, I see no
value in distinguishing them.  There is no real significance.

So, in my opinion, the only reason we might consider doing something with
the GMT driver at this point is if we decide that the node_offset is really
intended to indicate something about the sampling nature of the sensor.
If so, we might want to set it based on the AREA_OR_POINT metadata on write
and set the AREA_OR_POINT metadata on read.

I don't know enough about the GMT format to know if there is intended to
be some physical significance to the node_offset setting.

I will note that the AREA_OR_POINT metadata item is already well defined
in the GDAL Data Model document.  It says:

AREA_OR_POINT: May be either "Area" (the default) or "Point". Indicates whether 
a pixel value should be assumed to represent a sampling over the region of the 
pixel or a point sample at the center of the pixel. This is not intended to 
influence interpretation of georeferencing which remains area oriented.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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