<html><head><style type='text/css'>p { margin: 0; }</style></head><body><div style='font-family: Arial; font-size: 12pt; color: #000000'>Hi,<br><br>If <font face="Consolas, Courier New, Courier"><span style="font-size: 10pt;">RasterPixelIsPoint I think it would be (0.5, -0.5).<br><br>Dave<br><br></span></font>----- Original Message -----<br>From: "tjn98" <tim.nightingale@stfc.ac.uk><br>To: Geotiff@lists.maptools.org<br>Sent: Monday, May 10, 2010 10:28:07 AM GMT -05:00 US/Canada Eastern<br>Subject: [Geotiff] GeoTIFF geographic grid alignment<br><br>
<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 href="tim.nightingale@stfc.ac.uk" target="_blank">tim.nightingale@stfc.ac.uk</a><br>
------------------------------------------------------------------------ <br>
<br>
<br>
</span></font>
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