[gdal-dev] effects of differences in .tgf and .tab files on Tiff imports

[Tim Southern]

Dear Sir/Madame,

I have recently downloaded a number of historic analogue maps which have associated .twf and .tab files.  I notice that both these text files are required by GDAL for geo-referencing according to the information given on the appropriate web page for inputing tiff/geotiff files.

In this particular case the two files are not compatible with each telling the system slightly different things.  I have copied the contents below.  How will GDAL handle this situation?

Thanks

Tim Southern

.tab file

!table
!version 300
!charset WindowsLatin1

Definition Table
  File "02SU5980.tif"
  Type "RASTER"
  (459000, 181001) (0,0) Label "Pt 1",
  (459999, 181001) (4723,0) Label "Pt 2",
  (459999, 180001) (4723,4723) Label "Pt 3",
  (459000,180001) (0,4723) Label "Pt 4"
  CoordSys Earth Projection 8, 79, "m", -2, 49, 0.9996012717, 400000, -100000


and the .twf file is

0.21168000
0.0000
0.0000
-0.21168000
459000.00000000
181000.00000000

The .tab file gives coordinates for the pixel 0, 0 as 459000, 181001 whereas the .twf file defines the same pixel as 459000, 181000.
The .tab file gives coordinates for the pixel 4723,0 as 459999, 181001 whereas the .twf gives the same pixels as 459999.76464, 181000.
The .tab file gives coordinates for the pixel 4723, 4723 as 459999, 180001 whereas .twf file gives same point as 459999.76464, 180000.23536
The .tab file gives coordinates for the pixel 0, 4723 as 459000, 180001 whereas .twf file gives same point as 459000, 180000.23536

Another obvious difference is .tab file define a rectangle 999 by 1000 metres whereas the .twf file defines a square 999.76464 by 999.76464 metres.



Tim Southern
17, Park Close,
Sonning Common,
Oxfordshire
RG4 9RY



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