[Geotiff] GeoTIFF geographic grid alignment
tjn98
tim.nightingale at stfc.ac.uk
Fri May 14 03:12:36 PDT 2010
Dear Max, David,
Thanks for getting back to me. I see I¹m not alone in my confusion.
I think we all agree that ModelTiePointTag 0.5 offsets (of whatever
sign) ARE required for RasterPixelIsPoint and ARE NOT required for
RasterPixelIsArea. This is a big step forward for me!
We also agree that the ModelPixelScaleTag entries should be positive
(though I think this is inconsistent with the left-handed I/J system
and right-handed lon/lat system).
I would have thought that the offset sign would have to be the same for
both the I and J directions (and K for that matter) as the handedness
of the IJK axis system doesn¹t have any effect on this. Even though J
is positive down, the edge of the cell centred on the first point still
has to start ³before² the beginning of the axis, as would be the case
for I also. Consequently, I¹d think both must be negative.
If the offsets were applied instead to the geographic tie point
information, then one adjustment would have to be positive (Sx) and
the other negative (Sy).
How does this sound?
Tim.
------------------------------------------------------------------------
Tim 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
------------------------------------------------------------------------
On 14/05/2010 09:03, "David Burken" <dburken at comcast.net> wrote:
> Max, Tim,
>
> If your tie upper left corner of the pixel (AREA) and relative to an Easting
> Northing of (10,000, 10,000) and your pixel scale is (100, 100) meters then to
> convert to point (center) it would be:
>
> Move the Easting 50 meters to the east or right ( +.5 pixel)
>
> Move the Northing 50 meters to the South or down ( -.5 pixel)
>
> So doing ( -.5, -5 ) is wrong in a map projection.
>
> We actually did a drawing of this on ossim.org to show the error when ignored
> but I cannot find it. Sorry.
>
> Hope that helps,
> Dave
>
>
> On 05/13/2010 05:15 PM, Max Martinez wrote:
>> GeoTIFF geographic grid alignment
>>
>>
>> Tim,
>>
>>
>>
>> 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.
>>
>>
>>
>> We use a -0.5, -0.5 translation to translate pixel is area to pixel is point
>> just has you have done below.
>>
>>
>>
>> Max
>>
>>
>>
>>
>>
>>
>>
>> From: geotiff-bounces at lists.maptools.org
>> [mailto:geotiff-bounces at lists.maptools.org] On Behalf Of dburken at comcast.net
>> Sent: Monday, May 10, 2010 1:48 PM
>> To: tjn98
>> Cc: Geotiff at lists.maptools.org
>> Subject: Re: [Geotiff] GeoTIFF geographic grid alignment
>>
>>
>>
>>
>>
>>
>> Hi,
>>
>> If RasterPixelIsPoint I think it would be (0.5, -0.5).
>>
>> Dave
>>
>> ----- Original Message -----
>> From: "tjn98" <tim.nightingale at stfc.ac.uk>
>> <mailto: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
>>
>> Dear All,
>>
>> I¹m 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 I¹ve 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 ³PixelIsArea² 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 ³PixelIsPoint²
>> raster, whose entries fall exactly in the centre of the ³PixelIsArea²
>> 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
>> ³J² 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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