[Gdal-dev] Indexing with non-square boundary

[Ed McNierney]

Avi -

I do not think you can do that with GDAL and I could probably make a case
that it's not really what gdaltindex is meant to do.  The modification to
allow polygonal tiles instead of rectangular ones would have serious
implications to any tileindex client software.

I have addressed a similar problem by simply loading my imagery metadata
into a PostGIS table.  I can then do a very simple point-in-polygon query
against PostGIS to determine exactly which source image is being used for a
particular location.

     - Ed
  
-- 
Ed McNierney
Chief Mapmaker
Demand Media / TopoZone.com
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: (978) 251-4242
Fax: (978) 251-1396
ed at topozone.com


> From: Avi A Blackmore <support at satshot.com>
> Organization: Agri ImaGIS Technologies
> Reply-To: <sysadmin at satshot.com>
> Date: Thu, 9 Aug 2007 13:49:42 -0500
> To: <gdal-dev at lists.maptools.org>
> Subject: [Gdal-dev] Indexing with non-square boundary
> 
> Hi there,
> 
> I've run into a thorny issue with gdaltindex, when indexing our satellite
> imagery set.  The imagery taken by the satellites is, of course, shot on
> an angle: the satellites' paths around the Earth are not vertical, but
> skewed on an angle of about 12-13 degrees.  The actual image files, of
> course, are rectangular, which results in large black "null" areas in the
> image.  We use UMN Mapserver to display the imagery, and make the black
> null areas transparent, so that they don't cause a display problem.
> 
> The problem is, these null areas, while invisible when the image is
> displayed, are still considered "part" of the image as far as gdaltindex
> is concerned.  The bounding box is calculated from the corners of the
> image file, not from the actual corners of the "non-null" parts of the
> image.  
> 
> The result is that, when our site does a query on the image index layer,
> we get a lot of spurious "hits" in our imagery list: images whose bounding
> box intersects the current extents of the map even though the actual
> "live" image data is well outside those extents.
> 
> So my question is, using GDAL/OGR, how would I create a tile index whose
> bounding boxes are polygons that reflect the actual image path, and omit
> the black null areas?  I haven't found a way to do this using gdaltindex.
> -- 
> Avi Blackmore
> System Administrator/Programmer
> Agri ImaGIS Technologies, Inc.
> 
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