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Max,<br>
<br>
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.<br>
<br>
So:<br>
<font face="Consolas, Courier New, Courier"><span
style="font-size: 10pt;"> 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>
And:<br>
</span></font><font face="Consolas, Courier New, Courier"><span
style="font-size: 10pt;"> 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>
Are the same.<br>
<br>
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.<br>
<br>
Dave<br>
<br>
</span></font><br>
On 05/10/2010 10:28 AM, tjn98 wrote:
<blockquote cite="mid:C80DD687.742E%25tim.nightingale@stfc.ac.uk"
type="cite">
<title>GeoTIFF geographic grid alignment</title>
<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>
------------------------------------------------------------------------<br>
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 moz-do-not-send="true"
href="tim.nightingale@stfc.ac.uk">tim.nightingale@stfc.ac.uk</a><br>
------------------------------------------------------------------------
<br>
<br>
<br>
</span></font>
<br>
<p>-- <br>
Scanned by iCritical.
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<br>
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