[gdal-dev] Geotiff AREA_OR_POINT

Michael Sumner mdsumner at gmail.com
Mon Jun 28 18:21:15 EDT 2010


> Am I correct in interpreting this post to mean that the pixel-is-area and pixel-is-point tiffs >are read the exact same way by GDAL, with the only difference being in the value of the >AREA_OR_POINT metadata key? Should viewers then be looking for >the "AREA_OR_POINT" key in addition to the georeferencing information, and shift the >display to center the top-left pixel on (0,0) if it's set to Point?

That's my understanding, it's just a metadata tag there for use if you
need it. I would say that viewers should only modify their behaviour
from Area if they have the distinction for centre point vs. cell based
views.

I've seen it used to set  centre-based TIN-like renderings, vs.
cell-based image renderings - but there's nothing in the GDAL raster
storage model that prevents you taking on either interpretation AFAIK,
though the default is cell-based.

(There can be a problem interpreting more flexible array formats like
NetCDF, that can store irregularly spaced coordinate vectors for, say,
the centres of cells on the X axis - and there's rarely any metadata
(in the file) to say explicitly that it's centre- not corner-based.  )

Cheers, Mike.

On Tue, Jun 29, 2010 at 8:12 AM, Gillian WALTER
<GWALTER at mdacorporation.com> wrote:
> Hi,
>
> I'm trying to wrap my head around Geotiff's AREA_OR_POINT=Area versus
> AREA_OR_POINT=Point, and how that translates into GDAL.  I've  read the
> "PixelIsPoint Raster Space" and "PixelIsArea Raster Space" sections of the
> geotiff spec, and tried to find previous posts on the topic with very
> limited success (
> http://osgeo-org.1803224.n2.nabble.com/GRASS-Problem-importing-geotiff-with-r-in-gdal-td1879130.html
> ).  Am I correct in interpreting this post to mean that the pixel-is-area
> and pixel-is-point tiffs are read the exact same way by GDAL, with the only
> difference being in the value of the AREA_OR_POINT metadata key?  Should
> viewers then be looking for the "AREA_OR_POINT" key in addition to the
> georeferencing information, and shift the display to center the top-left
> pixel on (0,0) if it's set to Point?
>
> Thanks,
>
> Gillian Walter
> Software Developer
> Geospatial Services Inc.
> 57 Auriga Drive - Suite 201
> Ottawa, ON K2E 8B2
> Telephone:  (613) 727-1087 #245
> Facsimile:  (613) 727-5853
> Email:  gwalter at mdacorporation.com
>
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