What I do to compute the (single) GSD (or pixel size) when there is a rotation is this:
GSD = sqrt(abs(determinant([2x2 top left corner]))) see that pixel size can be different in X and Y directions. If you know your raster has no such distortion, you can just take the sqrt. abs is important. Usually Y goes from top to bottom in images files, while the other way around in coordinate systems. That produces a negative determinant. Cheers .___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ... .... ._ .__ Entre dos pensamientos racionales hay infinitos pensamientos irracionales. On Mon, 21 Dec 2020 at 12:55, Javier Jimenez Shaw <[email protected]> wrote: > 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 <[email protected]> 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 >> >> >> _______________________________________________ >> gdal-dev mailing list >> [email protected] >> https://lists.osgeo.org/mailman/listinfo/gdal-dev >> >
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