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

What should I expect in this case?How can I fix badly specified geographic coordinate system with units in arc-seconds?GDAL/proj4 equirectangular projection coordinate system units assumed by OGRSpatialReference and GDALDataset::GetGeoTransform()?how to georeference my png using gdalSeeking Python GDAL documentation?Bad corner coordinates transforming SRTM *.hgt files to GRD format using gdal_translateASCII coordinates to geographic coordinates with GDAL?Georeferencing a PDF with pixels coordinates not in cornersProblem with transform in GRIB using GDALHow to calculate the image size knowing its coordinates and pixel size?

2

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

edited 16 hours ago

user2856

30.1k258105

asked 16 hours ago

philologonphilologon

1255

add a comment | 

2

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

edited 16 hours ago

user2856

30.1k258105

asked 16 hours ago

philologonphilologon

1255

add a comment | 

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

edited 16 hours ago

user2856

30.1k258105

asked 16 hours ago

philologonphilologon

1255

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

share|improve this question

edited 16 hours ago

user2856

30.1k258105

asked 16 hours ago

philologonphilologon

1255

share|improve this question

edited 16 hours ago

user2856

30.1k258105

asked 16 hours ago

philologonphilologon

1255

edited 16 hours ago

user2856

30.1k258105

edited 16 hours ago

user2856

30.1k258105

asked 16 hours ago

philologonphilologon

1255

asked 16 hours ago

philologonphilologon

1255

1 Answer
1

active

oldest

votes

4

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

edited 14 hours ago

answered 15 hours ago

user2856user2856

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
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @philologon P is column number, and L is row number. These are "coordinates" on the raster image.
  
  – Mike T
  15 hours 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
  15 hours ago
  

  
  
  
  

add a comment | 

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4

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

edited 14 hours ago

answered 15 hours ago

user2856user2856

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
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @philologon P is column number, and L is row number. These are "coordinates" on the raster image.
  
  – Mike T
  15 hours 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
  15 hours ago
  

  
  
  
  

add a comment | 

4

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

edited 14 hours ago

answered 15 hours ago

user2856user2856

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
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @philologon P is column number, and L is row number. These are "coordinates" on the raster image.
  
  – Mike T
  15 hours 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
  15 hours ago
  

  
  
  
  

add a comment | 

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

edited 14 hours ago

answered 15 hours ago

user2856user2856

30.1k258105

Xp = padfTransform[0] + P*padfTransform[1];

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



# Where P = column number and L = row number

share|improve this answer

edited 14 hours ago

answered 15 hours ago

user2856user2856

30.1k258105

answered 15 hours ago

user2856user2856

30.1k258105

answered 15 hours ago

user2856user2856

30.1k258105