[Geotiff] GeoTIFF geographic grid alignment

[Max Martinez]

Tim,

 

I think the GeoTIFF spec tries to show through its examples that
although the orientation of the positive J raster space axis is usually
in the opposite direction of the positive Y model space axis, it is
expected that both pixel scales will be positive under these conditions.
(They probably could have been clearer if they described full map
extents of the images in the examples). Negation should be used
appropriately as the current condition differs from that usual
condition. This is also supported by the example on page 27 where the
ModelTransformationTag content equivalents of an image with tiepoint
(I,J,K,X,Y,Z) and scale (Sx, Sy, Sz) is provided. So I think you have
that right in your example.

 

We use a -0.5, -0.5 translation to translate pixel is area to pixel is
point just has you have done below.

 

Max

________________________________

From: geotiff-bounces at lists.maptools.org
[mailto:geotiff-bounces at lists.maptools.org] On Behalf Of
dburken at comcast.net
Sent: Monday, May 10, 2010 1:48 PM
To: tjn98
Cc: Geotiff at lists.maptools.org
Subject: Re: [Geotiff] GeoTIFF geographic grid alignment

 

Hi,

If RasterPixelIsPoint I think it would be (0.5, -0.5).

Dave

----- Original Message -----
From: "tjn98" <tim.nightingale at stfc.ac.uk>
To: Geotiff at lists.maptools.org
Sent: Monday, May 10, 2010 10:28:07 AM GMT -05:00 US/Canada Eastern
Subject: [Geotiff] GeoTIFF geographic grid alignment

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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