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I have a raster file (PNG) georeferenced with PGW world file. The file was created in British National Grid EPSG 27700.

The recipient requires the file in WGS84 EPSG 4326, and has no re-projection facilities.

I thought I could easily reproject the file in QGIS - by changing both the project CRS and layer CRS to EPSG 4326, and re-exporting.

I've had a look at this post on how to export a TIFF with .tfw from QGIS, and have done this successfully. (And I know that world files can easily swap formats between PNG/TIFF (pgw/tfw) by amending the file extension, so I can translate this from tfw to pgw with no issues.)

However - when I interrogate the resulting world file, the co-ordinates still appear to be EPSG 27700 format co-ordinates. The recipient needs the file to show these in EPSG 4326.

Do I need to translate these manually in text edit? Is there an easy way to re-project world files between co-ordinate systems?

Vince
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lisagravy
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  • While the world file is one way to store geo metadata associated with an image, reprojection is not simple recalculation of the world file. The image itself needs fundamental recalculation, which should result in a new image with a new world file. You cannot clobber the world file of an unreprojected image without mangling its contents. – Vince Sep 15 '20 at 10:51
  • I did try to recalculate this and the new world file showed the same co-ordinates, that's why I asked the question. – lisagravy Sep 15 '20 at 12:33

1 Answers1

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DO NOT JUST CHANGE THE LAYER PROJECTION IN AN ATTEMPT TO REPROJECT A FILE

This does not change the coordinates in any way - it does tell QGIS that you (think you) know better than it does where to draw the image.

If you would like to reproject your file either save it from QGIS and select a new CRS so that QGIS can reproject the image and update the metadata. Or use a command line tool such as gdalwarp that is designed to handle just this job.

gdalwarp -t_srs EPSG:4326 input.tif output.tif
Ian Turton
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  • Ok, this is helpful. So I should maintain my .PNG projection as is, but select the re-projected system (EPSG 4326) on Save As > GeoTIFF. I can also use the 'Create' options to add "TFW" "Yes" to create an associated world file with the correct projection. Now the world file looks correct... but the associated TIFF is completely black. (And I do actually need a PNG with transparency!) Any further help? – lisagravy Sep 15 '20 at 12:20
  • It is possible, with out seeing the input image and the output image - and related gdalinfo output it's hard to tell - feel free to ask a new question – Ian Turton Sep 15 '20 at 12:25