[Geotiff] GeoTIFF geographic grid alignment
David Burken
dburken at comcast.net
Fri May 14 06:39:40 PDT 2010
Max, see below:
On 05/14/2010 06:12 AM, tjn98 wrote:
> Dear Max, David,
>
> Thanks for getting back to me. I see I'm 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!
>
You can have a tie of 0,0 or any other value and be point or area. The
point/area just tells the application that the tie is relative to the
center(point) or upper left.
So given scale of 50,50 then:
Area tie of (0, 0) (10000, 10000) is the same as:
Point tie of (0, 0) (10050, 9950)
Here are two images one point and one area. They contain the exact same
image data and should swipe perfectly.
$ ossim-info -d area.tif | grep tie
tiff.image0.model_tie_point: 0.000000000000000 0.000000000000000
0.000000000000000 537855.000000000000000 3964495.000000000000000
0.000000000000000
$ ossim-info -d point.tif | grep tie
tiff.image0.model_tie_point: 0.000000000000000 0.000000000000000
0.000000000000000 537860.000000000000000 3964490.000000000000000
0.000000000000000
$ ossim-info -d point.tif | grep scale
tiff.image0.model_pixel_scale: 10.000000000000000 10.000000000000000
0.000000000000000
$ ossim-info -d area.tif | grep scale
tiff.image0.model_pixel_scale: 10.000000000000000 10.000000000000000
0.000000000000000
Hope that helps,
Dave
> 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).
>
Yes positive
> 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 doesn't 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 "before" the beginning of the axis, as would be the case
> for I also. Consequently, I'd 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,
>
> 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
> ------------------------------------------------------------------------
>
>
>
>
>
>
> --
> Scanned by iCritical.
>
>
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