Introduction to Geotransforms:
A geotransform is an affine transformation from the image coordinate space (row, column), also known as (pixel, line) to the georeferenced coordinate space (projected or geographic coordinates).
A geotransform consists in a set of 6 coefficients:
GT(0)x-coordinate of the upper-left corner of the upper-left pixel.
GT(1)w-e pixel resolution / pixel width.
GT(2)row rotation (typically zero).
GT(3)y-coordinate of the upper-left corner of the upper-left pixel.
GT(4)column rotation (typically zero).
GT(5)n-s pixel resolution / pixel height (negative value for a north-up image).
Transformation from image coordinate space to georeferenced coordinate space:
X_geo = GT(0) + X_pixel * GT(1) + Y_line * GT(2) Y_geo = GT(3) + X_pixel * GT(4) + Y_line * GT(5)
Note that the pixel/line coordinates in the above are from (0.0,0.0) at the top left corner of the top left pixel to (width_in_pixels,height_in_pixels) at the bottom right corner of the bottom right pixel. The pixel/line location of the center of the top left pixel would therefore be (0.5,0.5).
In case of north up images:
GT(4)coefficients are zero.
GT(5)is the pixel size.
GT(3)position is the top left corner of the top left pixel of the raster.