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

[Javier Jimenez Shaw]

Those transformation parameters can be seen as a 3x3 transformation matrix
like this

Xg     GT1 GT2 GT0     Xp
Yg  =  GT4 GT5 GT3  *  Yp
 1      0   0   1       1

Where [Xp, Yp] is the coordinate in pixels, and [Xg, Yg] is the
"geographic" coordinate (whatever it means ;)

the 2x2 top left corner of the transformation matrix can be seen as a 2D
rotation-scaling matrix:
https://en.wikipedia.org/wiki/Transformation_matrix#Examples_in_2_dimensions
[GT0, GT3] is the "geographic" coordinate of the pixel [0, 0] (regardless
rotation or scaling)

Cheers
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
Entre dos pensamientos racionales
hay infinitos pensamientos irracionales.



On Mon, 21 Dec 2020 at 12:34, G Seiffert <hh1 at posteo.de> wrote:

> 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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