[Geotiff] GeoTIFF geographic grid alignment

[David Burken]

Max,

I was looking at your stuff again and what I said was wrong for your 
case where you keep the tie geographic position the same you would use 
-.5, -5.

So:
     ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
     ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
     GeoKeyDirectoryTag:
         GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
         GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
         GeographicTypeGeoKey = 4326 (GCS_WGS_84)
And:
     ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
     ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
     GeoKeyDirectoryTag:
         GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
         GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
         GeographicTypeGeoKey = 4326 (GCS_WGS_84)

Are the same.

Typically we use (0, 0) with point but the geographic tie is relative to 
the center of the pixel not the edge.  So I apologize for the confusion.

Dave


On 05/10/2010 10:28 AM, tjn98 wrote:
> Dear All,
>
>   I'm looking for some guidance on the correct interpretation of
> GeoTIFF raster spaces, in particular their correct alignments at the
> half-pixel level. Judging by the number of mutually contradictory
> examples I've found, this is a common source of confusion.
>
>   My interpretation of the rather terse text in the GeoTIFF Format
> Specification (Revision 1.0) is that, for the example of a global
> map measuring 129,600 longitude pixels by 64,800 latitude pixels,
> a "PixelIsArea" raster described by:
>
>     ModelTiepointTag = (0.0, 0.0, 0.0, -180.0, 90.0, 0.0)
>     ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
>     GeoKeyDirectoryTag:
>         GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
>         GTRasterTypeGeoKey   = 1    (RasterPixelIsArea)
>         GeographicTypeGeoKey = 4326 (GCS_WGS_84)
>
> exactly fits into a --180 -> 180 by --90 -> 90 degree box that touches
> the edges of the extreme cell boundaries, and that a "PixelIsPoint"
> raster, whose entries fall exactly in the centre of the "PixelIsArea"
> cells in the first example is described by:
>
>     ModelTiepointTag = (-0.5, -0.5, 0.0, -180.0, 90.0, 0.0)
>     ModelPixelScaleTag = (0.002777777778, 0.002777777778, 0.0)
>     GeoKeyDirectoryTag:
>         GTModelTypeGeoKey    = 2    (ModelTypeGeographic)
>         GTRasterTypeGeoKey   = 2    (RasterPixelIsPoint)
>         GeographicTypeGeoKey = 4326 (GCS_WGS_84)
>
>   Variants I have seen include both +0.5 and --0.5 pixel offsets for
> the PixelIsArea case, negative ModelPixelScaleTag values for the
> "J" direction and 0.0 pixel offsets for the PixelIsPoint case. QGIS,
> for example, appears to assume the last.
>
>   Can anyone help?
>
>     Many thanks,
>
>       Tim.
>
> ------------------------------------------------------------------------
>  Dr T.J. Nightingale
>   Space Science and Technology Department
>    Rutherford Appleton Laboratory
>     Chilton, Didcot                   Phone: +44/0 1235 445914
>      Oxon OX11 0QX                    Fax:   +44/0 1235 445848
>       United Kingdom                  Email: tim.nightingale at stfc.ac.uk
> ------------------------------------------------------------------------
>
>
>
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