[Geotiff] GeoTIFF geographic grid alignment

[dburken at comcast.net]

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 

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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