I am trying to reverse the georeferencing formula for a TIFF using the world file information. Formula found at http://webhelp.esri.com/arcims/9.3/General/topics/author_world_files.htm
The raster is a UK Ordnance Survey OS Terrain 50 extract (50m resolution)
Conversion on the X coord is fine, but the y coord goes wrong putting me in a raster one cell to the north of the one it should be.
The referencing formula for y coord is:
y1 = Dx + Ey + F
Where: y1 = calculated y-coordinate of the pixel on the map x = column number of a pixel in the image y = row number of a pixel in the image D = rotation terms on the Y Axis F = translation terms; x,y map coordinates of the center of the upper-left pixel E = negative of y-scale; dimension of a pixel in map units in y direction
The D value is 0 (no rotation) in these files so I am left with:
y1 = Ey + F
To reverse this to find y form a known y1 coord I get
y = (y1-F)/E
I can't see what is wrong the above, and similar version works fine for the X coords.
Does anyone know if there is something missing from the original formula / georeferencing process or see what is going wrong?