GDAL GetGeoTransform Documentation — Is there an oversight, or what am I misunderstanding?












1















The documentation for GDALDataset::GetGeoTransform says some things that are rather confusing.




Fetches the coefficients for transforming between pixel/line (P,L) raster space, and projection coordinates (Xp,Yp) space.



Xp = padfTransform[0] + P*padfTransform[1] + L*padfTransform[2];
Yp = padfTransform[3] + P*padfTransform[4] + L*padfTransform[5];


In a north up image, padfTransform[1] is the pixel width, and padfTransform[5] is the pixel height.



The upper left corner of the upper left pixel is at position (padfTransform[0],padfTransform[3]).



The default transform is (0,1,0,0,0,1) and should be returned even when a CE_Failure error is returned, such as for formats that don't support transformation to projection coordinates.




If this is a matrix, it makes sense that Pixel_X is at 1,1 and Pixel_Y is at 2,2.



Here is my question:



So should the equation for Yp be:



Yp = padfTransform[3] + L*padfTransform[4] + P*padfTransform[5];


in which the positions of L and P are swapped?



Or if I am confused about something, can someone help me get unconfused?










share|improve this question





























    1















    The documentation for GDALDataset::GetGeoTransform says some things that are rather confusing.




    Fetches the coefficients for transforming between pixel/line (P,L) raster space, and projection coordinates (Xp,Yp) space.



    Xp = padfTransform[0] + P*padfTransform[1] + L*padfTransform[2];
    Yp = padfTransform[3] + P*padfTransform[4] + L*padfTransform[5];


    In a north up image, padfTransform[1] is the pixel width, and padfTransform[5] is the pixel height.



    The upper left corner of the upper left pixel is at position (padfTransform[0],padfTransform[3]).



    The default transform is (0,1,0,0,0,1) and should be returned even when a CE_Failure error is returned, such as for formats that don't support transformation to projection coordinates.




    If this is a matrix, it makes sense that Pixel_X is at 1,1 and Pixel_Y is at 2,2.



    Here is my question:



    So should the equation for Yp be:



    Yp = padfTransform[3] + L*padfTransform[4] + P*padfTransform[5];


    in which the positions of L and P are swapped?



    Or if I am confused about something, can someone help me get unconfused?










    share|improve this question



























      1












      1








      1








      The documentation for GDALDataset::GetGeoTransform says some things that are rather confusing.




      Fetches the coefficients for transforming between pixel/line (P,L) raster space, and projection coordinates (Xp,Yp) space.



      Xp = padfTransform[0] + P*padfTransform[1] + L*padfTransform[2];
      Yp = padfTransform[3] + P*padfTransform[4] + L*padfTransform[5];


      In a north up image, padfTransform[1] is the pixel width, and padfTransform[5] is the pixel height.



      The upper left corner of the upper left pixel is at position (padfTransform[0],padfTransform[3]).



      The default transform is (0,1,0,0,0,1) and should be returned even when a CE_Failure error is returned, such as for formats that don't support transformation to projection coordinates.




      If this is a matrix, it makes sense that Pixel_X is at 1,1 and Pixel_Y is at 2,2.



      Here is my question:



      So should the equation for Yp be:



      Yp = padfTransform[3] + L*padfTransform[4] + P*padfTransform[5];


      in which the positions of L and P are swapped?



      Or if I am confused about something, can someone help me get unconfused?










      share|improve this question
















      The documentation for GDALDataset::GetGeoTransform says some things that are rather confusing.




      Fetches the coefficients for transforming between pixel/line (P,L) raster space, and projection coordinates (Xp,Yp) space.



      Xp = padfTransform[0] + P*padfTransform[1] + L*padfTransform[2];
      Yp = padfTransform[3] + P*padfTransform[4] + L*padfTransform[5];


      In a north up image, padfTransform[1] is the pixel width, and padfTransform[5] is the pixel height.



      The upper left corner of the upper left pixel is at position (padfTransform[0],padfTransform[3]).



      The default transform is (0,1,0,0,0,1) and should be returned even when a CE_Failure error is returned, such as for formats that don't support transformation to projection coordinates.




      If this is a matrix, it makes sense that Pixel_X is at 1,1 and Pixel_Y is at 2,2.



      Here is my question:



      So should the equation for Yp be:



      Yp = padfTransform[3] + L*padfTransform[4] + P*padfTransform[5];


      in which the positions of L and P are swapped?



      Or if I am confused about something, can someone help me get unconfused?







      gdal documentation






      share|improve this question















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      share|improve this question




      share|improve this question








      edited 2 hours ago









      user2856

      30.1k258105




      30.1k258105










      asked 2 hours ago









      philologonphilologon

      1205




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          3














          No.



          For a north up image with no rotation (i.e. padfTransform[4] == padfTransform[2] == 0), the formula simply becomes:



          Xp = padfTransform[0] + P*padfTransform[1];
          Yp = padfTransform[3] + L*padfTransform[5];

          # Where P = column number and L = row number


          i.e add the starting X (or Y) to the column (or row) number multiplied by the pixel width (or height).



          The rotation terms may be what's tripping you up...? When a pixel is rotated, the Y skew needs to be considered when calculating the X map coordinate and the X skew needs to be considered when calculating the Y map coordinate. There's a quite detailed explanation in the wikipedia article on world files.






          share|improve this answer


























          • That there is rotation is what was tripping me up. So can you tell me what P and L stand for? I naively supposed that P was Pixel dimension and L was Line dimension (the lines between pixels). Yes, that is as foolish as a sophomore reasoning with senior. In that vein, what do those letters stand for?

            – philologon
            2 hours ago






          • 1





            @philologon P is column number, and L is row number. These are "coordinates" on the raster image.

            – Mike T
            1 hour ago











          • Thanks. I now see it is right there in the documentation. I always find a way to embarrass myself in these kinds of things, usually by skimming too fast.

            – philologon
            1 hour ago











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          1 Answer
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          1 Answer
          1






          active

          oldest

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          active

          oldest

          votes






          active

          oldest

          votes









          3














          No.



          For a north up image with no rotation (i.e. padfTransform[4] == padfTransform[2] == 0), the formula simply becomes:



          Xp = padfTransform[0] + P*padfTransform[1];
          Yp = padfTransform[3] + L*padfTransform[5];

          # Where P = column number and L = row number


          i.e add the starting X (or Y) to the column (or row) number multiplied by the pixel width (or height).



          The rotation terms may be what's tripping you up...? When a pixel is rotated, the Y skew needs to be considered when calculating the X map coordinate and the X skew needs to be considered when calculating the Y map coordinate. There's a quite detailed explanation in the wikipedia article on world files.






          share|improve this answer


























          • That there is rotation is what was tripping me up. So can you tell me what P and L stand for? I naively supposed that P was Pixel dimension and L was Line dimension (the lines between pixels). Yes, that is as foolish as a sophomore reasoning with senior. In that vein, what do those letters stand for?

            – philologon
            2 hours ago






          • 1





            @philologon P is column number, and L is row number. These are "coordinates" on the raster image.

            – Mike T
            1 hour ago











          • Thanks. I now see it is right there in the documentation. I always find a way to embarrass myself in these kinds of things, usually by skimming too fast.

            – philologon
            1 hour ago
















          3














          No.



          For a north up image with no rotation (i.e. padfTransform[4] == padfTransform[2] == 0), the formula simply becomes:



          Xp = padfTransform[0] + P*padfTransform[1];
          Yp = padfTransform[3] + L*padfTransform[5];

          # Where P = column number and L = row number


          i.e add the starting X (or Y) to the column (or row) number multiplied by the pixel width (or height).



          The rotation terms may be what's tripping you up...? When a pixel is rotated, the Y skew needs to be considered when calculating the X map coordinate and the X skew needs to be considered when calculating the Y map coordinate. There's a quite detailed explanation in the wikipedia article on world files.






          share|improve this answer


























          • That there is rotation is what was tripping me up. So can you tell me what P and L stand for? I naively supposed that P was Pixel dimension and L was Line dimension (the lines between pixels). Yes, that is as foolish as a sophomore reasoning with senior. In that vein, what do those letters stand for?

            – philologon
            2 hours ago






          • 1





            @philologon P is column number, and L is row number. These are "coordinates" on the raster image.

            – Mike T
            1 hour ago











          • Thanks. I now see it is right there in the documentation. I always find a way to embarrass myself in these kinds of things, usually by skimming too fast.

            – philologon
            1 hour ago














          3












          3








          3







          No.



          For a north up image with no rotation (i.e. padfTransform[4] == padfTransform[2] == 0), the formula simply becomes:



          Xp = padfTransform[0] + P*padfTransform[1];
          Yp = padfTransform[3] + L*padfTransform[5];

          # Where P = column number and L = row number


          i.e add the starting X (or Y) to the column (or row) number multiplied by the pixel width (or height).



          The rotation terms may be what's tripping you up...? When a pixel is rotated, the Y skew needs to be considered when calculating the X map coordinate and the X skew needs to be considered when calculating the Y map coordinate. There's a quite detailed explanation in the wikipedia article on world files.






          share|improve this answer















          No.



          For a north up image with no rotation (i.e. padfTransform[4] == padfTransform[2] == 0), the formula simply becomes:



          Xp = padfTransform[0] + P*padfTransform[1];
          Yp = padfTransform[3] + L*padfTransform[5];

          # Where P = column number and L = row number


          i.e add the starting X (or Y) to the column (or row) number multiplied by the pixel width (or height).



          The rotation terms may be what's tripping you up...? When a pixel is rotated, the Y skew needs to be considered when calculating the X map coordinate and the X skew needs to be considered when calculating the Y map coordinate. There's a quite detailed explanation in the wikipedia article on world files.







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 56 mins ago

























          answered 2 hours ago









          user2856user2856

          30.1k258105




          30.1k258105













          • That there is rotation is what was tripping me up. So can you tell me what P and L stand for? I naively supposed that P was Pixel dimension and L was Line dimension (the lines between pixels). Yes, that is as foolish as a sophomore reasoning with senior. In that vein, what do those letters stand for?

            – philologon
            2 hours ago






          • 1





            @philologon P is column number, and L is row number. These are "coordinates" on the raster image.

            – Mike T
            1 hour ago











          • Thanks. I now see it is right there in the documentation. I always find a way to embarrass myself in these kinds of things, usually by skimming too fast.

            – philologon
            1 hour ago



















          • That there is rotation is what was tripping me up. So can you tell me what P and L stand for? I naively supposed that P was Pixel dimension and L was Line dimension (the lines between pixels). Yes, that is as foolish as a sophomore reasoning with senior. In that vein, what do those letters stand for?

            – philologon
            2 hours ago






          • 1





            @philologon P is column number, and L is row number. These are "coordinates" on the raster image.

            – Mike T
            1 hour ago











          • Thanks. I now see it is right there in the documentation. I always find a way to embarrass myself in these kinds of things, usually by skimming too fast.

            – philologon
            1 hour ago

















          That there is rotation is what was tripping me up. So can you tell me what P and L stand for? I naively supposed that P was Pixel dimension and L was Line dimension (the lines between pixels). Yes, that is as foolish as a sophomore reasoning with senior. In that vein, what do those letters stand for?

          – philologon
          2 hours ago





          That there is rotation is what was tripping me up. So can you tell me what P and L stand for? I naively supposed that P was Pixel dimension and L was Line dimension (the lines between pixels). Yes, that is as foolish as a sophomore reasoning with senior. In that vein, what do those letters stand for?

          – philologon
          2 hours ago




          1




          1





          @philologon P is column number, and L is row number. These are "coordinates" on the raster image.

          – Mike T
          1 hour ago





          @philologon P is column number, and L is row number. These are "coordinates" on the raster image.

          – Mike T
          1 hour ago













          Thanks. I now see it is right there in the documentation. I always find a way to embarrass myself in these kinds of things, usually by skimming too fast.

          – philologon
          1 hour ago





          Thanks. I now see it is right there in the documentation. I always find a way to embarrass myself in these kinds of things, usually by skimming too fast.

          – philologon
          1 hour ago


















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