[Geotiff] GeoTIFF geographic grid alignment

[tjn98]

Dear All,

  I¹m 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 I¹ve 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 ³PixelIsArea² 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 ³PixelIsPoint²
raster, whose entries fall exactly in the centre of the ³PixelIsArea²
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
³J² 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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