[gdal-dev] Corner Coordinates for GeoTIFF/GeoPDF

[David Klaus]

Cory,

So I apologize if I misunderstood your question, but it sounds like you may
be trying to implement a geographic to grid transformation of a GeoPDF via
a matrix transformation of the four corners of the PDF?

I have written software to place imagery geolocated using geographic
coordinates into documents geolocated using grid coordinates. I spoke with
a geodesist at my company about a similar issue and he recommended that I
take a sample of points across the image, transform these points from
geographic coordinates to grid coordinates and compute a best fit affine
transformation between the two sets of points.

Obviously there is still error inherent in this process (it is best fit not
correct fit), but -- for our clients at least -- it produces results with
reasonable error and it is not terribly difficult to implement. If I
misunderstood your question I apologize. Anyway, I hope that is helpful,

David

On Wed, Sep 3, 2025 at 10:18 PM Cory Foy via gdal-dev <
gdal-dev at lists.osgeo.org> wrote:

> Hi All,
>
> My apologies if this has been asked previously. I’m relatively new to GDAL
> and recently was able to get it building with PDFium support using GDAL
> 3.11 for iOS. I’m the developer of a mapping application which allows users
> to import GeoPDFs and GeoTIFFs (among many other things). I was able to
> back port the GetExtent code that is going in to 3.12 and extract the
> extent from a GeoPDF, which was very exciting!
>
> However, what I’m ultimately trying to do is take a GeoPDF/GeoTIFF and
> place it on the map. I do this now with KMZ GroundOverlays by bringing in
> the four corner points and transforming the image to fit those points. I’m
> guessing the flow I’ll use is something like (and assuming WGS84 just to
> make things easier):
>
> - Call GDALGetExtentWGS84LongLat to get the envelope
> - Call GDALGetGeoTransform to get the geotransfmrm
> - Calculate the four corners to get the bounding box:
> - LL: (env.MinX, env.MinY)
> - UL: (env.MaxX, env.MinY)
> - LR: (env.MinX, env.MaxY)
> - UR: (env.MaxX, env.MaxY)
> - Apply the transformation matrix from the GeoTransform
>
> Does this seem roughly correct? I know there’s GDALWarp but I don’t know
> enough about it to know if it would help handle this situation.
>
> Thanks so much!
>
> Cory
>
>
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