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<title>GeoTIFF geographic grid alignment</title>
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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Tim,<o:p></o:p></span></font></p>
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10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>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.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>We use a -0.5, -0.5 translation to
translate pixel is area to pixel is point just has you have done below.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Max<o:p></o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
geotiff-bounces@lists.maptools.org [mailto:geotiff-bounces@lists.maptools.org] <b><span
style='font-weight:bold'>On Behalf Of </span></b>dburken@comcast.net<br>
<b><span style='font-weight:bold'>Sent:</span></b> Monday, May 10, 2010 1:48 PM<br>
<b><span style='font-weight:bold'>To:</span></b> tjn98<br>
<b><span style='font-weight:bold'>Cc:</span></b> Geotiff@lists.maptools.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [Geotiff] GeoTIFF
geographic grid alignment</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3 color=black
face=Arial><span style='font-size:12.0pt;font-family:Arial;color:black'>Hi,<br>
<br>
If </span></font><font size=2 color=black face=Consolas><span style='font-size:
10.0pt;font-family:Consolas;color:black'>RasterPixelIsPoint I think it would be
(0.5, -0.5).<br>
<br>
Dave<br>
<br>
</span></font><font color=black face=Arial><span style='font-family:Arial;
color:black'>----- 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>
</span></font><font size=2 color=black face=Consolas><span style='font-size:
10.0pt;font-family:Consolas;color:black'>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>
<st1:country-region w:st="on"><st1:place
w:st="on">United Kingdom</st1:place></st1:country-region>
Email:
<a href="tim.nightingale@stfc.ac.uk" target="_blank">tim.nightingale@stfc.ac.uk</a><br>
------------------------------------------------------------------------ <br>
<br>
<br>
</span></font><font color=black face=Arial><span style='font-family:Arial;
color:black'><o:p></o:p></span></font></p>
<p><font size=3 color=black face=Arial><span style='font-size:12.0pt;
font-family:Arial;color:black'>-- <br>
Scanned by iCritical. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face=Arial><span style='font-size:
12.0pt;font-family:Arial;color:black'><br>
<br>
_______________________________________________ Geotiff mailing list Geotiff@lists.maptools.org
http://lists.maptools.org/mailman/listinfo/geotiff<o:p></o:p></span></font></p>
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