[Geotiff] GeoTIFF geographic grid alignment

[David Burken]

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:
>
> 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>
> 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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