Even Rouault wrote: >> Hi, >> >> I am trying to understand the geotransform parameters by reading >> http://www.gdal.org/gdal_datamodel.html but I am confused. In the >> Affine GeoTransform section there are formulas >> >> Xgeo = GT(0) + Xpixel*GT(1) + Yline*GT(2) >> Ygeo = GT(3) + Xpixel*GT(4) + Yline*GT(5) >> >> The latter formula feels odd to me and I think that it should rather >> be >> >> Xgeo = GT(0) + Xpixel*GT(1) + Yline*GT(2) >> Ygeo = GT(3) + Ypixel*GT(5) + Xline*GT(4)
> Jukka, > That would look more odd to me ;-) since it would seem to add 2 new variables > to the equations (but perhaps that was just a typo, and you wanted only to > switch the 2 terms of the addition ?) In the original equation, my guess is > that Xpixel means "X or pixel or column" and Yline means "Y or line or row", according to the terminology people use. > The equations are just the result of the standard matrix multiplication for a > 2D affine transformation : > [ 1 ] [ 1 0 0 ] [ 1 ] > [ Xgeo ] = [ GT(0) GT(1) GT(2) ] * [ Xpixel ] > [ Ygeo ] [ GT(3) GT(4) GT(5) ] * [ Yline ] > (usually you put the 1 at the bottom and right of the matrices, but that's > only a presentation change) Ok, it was just a combination of lazy reading and lazy thinking from my side: - For some reason I was unconsciously thinking that if "Xpixel" means the location of a pixel in left to right direction then there should be another parameter "Ypixel" for defining the location in top to bottom direction. However, this parameter is called as "Yline". - Because I did not understand the meaning of "Yline" I did not bother to think about it at all. - As the formula for calculating X coordinate has an order (initial x) + (pixel size(x) * pixel count x) + (correction term of x because of rotation) my brain believed that the formula for Y should follow the same order. Actually it is using another order which keeps the parameter index numbers nicely in order 0, 1, 2, 3, 4, 5 (initial y) + (correction term of y because of rotation) + (pixel size(y) * pixel count y) I am not sure if there is anything to learn about this. Programmers may think a bit differently than end users about what is logical but I was lurking the developers document even I am just a user so I can only accuse myself. -Jukka Rahkonen- _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
