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