[gdal-dev] Geotransform for non-North-up images?

[G Seiffert]

Hi all from GDLA.

Thanks for the opportunity to as a question. It's regarding the 
Geotransform 
Tutorial(https://gdal.org/tutorials/geotransforms_tut.html). Tried to 
get info in the web but since this seems a tricky one, my searches failed.

The tutorial only deals with the ideal case of 'North up' images, for 
which GT(2) and GT(4) are zero. However, my images are 'East up' and 
potentially 'any direction up' (underwater photomosaic surveys by ROV). 
The rotation works with '90' for GT(2, 4). But the scaling seems 
completely ignored. Any hint? If it would be easy, I assume your 
tutorial would give an example for how to deal with "non-N-up' images, 
but ...

My pics are 1920x1080, with pixel resolution of 0.0015 (yes, 0.15cm per 
pixel, we're flying just 3m above the bottom).

In case I rotate the images prior to geotransform (in Photoshop), 
geotransform works perfect, with GT(2, 4) = 0. Scaling spot on. I can 
live with that for our last survey but I'm also looking for a solution 
in case our survey heading cannot be 0, 90, 180, or 270, but has to be 
something like 35° (due to bottom currents etc.).

Best regards, any hint qappreciated,
Gerhard


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