Question

Extracting coordinate system from a TIF, disrupted by .tab file?

  • 30 July 2019
  • 3 replies
  • 19 views
nicholas
Rank
Badge +10

I have a geotiff file which I want to convert to an ecw file. Of course, I want the preserve the correct coordinate system during the transformation.

The geotiff file is supplied with a MapInfo .tab file, of the same name, sitting in the same folder.

I have found that the the existence of the .tab file means that FME does not read the correct coordinate system from the geotiff file. I have tested this by using a tif reader connected to a coordinateSystemExtractor transformer. The output coordinate system is "_MI_0" which results in an ecw with a datum of "RAW" which is wrong.

But, if I delete the .tab file and then run my transformation, it goes correctly. I have tested this by using the same tif reader connected to a coordinateSystemExtractor. The output coordinate system is "MGA94-56" which is correct. The resulting ecw has a datum of "GDA94" and a projection of "SUTM56".

Why does the presence of the .tab file disrupt the geotiff reader's ability to read the correct coordinate system? Does FME prioritise reading the coordinate system from auxillary file over the tif file? The .tab file does have a statement defining the coordinate system as GDA94/MGA56, but this is done correctly.

I can solve this by just making sure I delete all the .tab files before I run the process, but this does seem like a bug with the geotiff reader.

3 replies

daveatsafe
Rank
Userlevel 2
Badge +17

Hi @nicholas,

The raster readers will use a sidecar file if it is present, over the internal georeferencing. We thought this would be the easiest hierarchy of georeferencing to use, since it is much easier to remove a sidecar file than internal tags on the image.

Would you mind sending the .tab file, so we can see why it did not produce the same coordinate system as the internal GeoTIFF tags?

nicholas
Rank
Badge +10

Hi @daveatsafe

I tried to attach the .tab, but I think my workplace blocks that pop-up.

Anyway, I have posted below the contents of the tab file

!table

!version 300

!charset WindowsLatin1

Definition Table

File "SW_e466_n6971.tif"

Type "RASTER"

(466000,6972000) ( 0, 0 ) Label "Pt 1",

(467000,6971000) (10000,10000) Label "Pt 2",

(467000,6972000) (10000,0) Label "Pt 3"

CoordSys Earth Projection 8, 33, "m", 153, 0, 0.9996, 500000, 10000000 Bounds (-7745844.29605, 2035.05684549) (8745844.29605, 19997964.9432)

 

 

 

daveatsafe
Rank
Userlevel 2
Badge +17

Hi @nicholas,

The coordinate system in the Tab file has the incorrect datum: 33 is GRS80, while 116 is GDA 94. If you change it to the following, you will get the correct coordinate system in FME:

CoordSys Earth Projection 8, 116, "m", 153, 0, 0.9996, 500000, 10000000 Bounds (-7745844.29605, 2035.05684549) (8745844.29605, 19997964.9432)

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