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I have a North Polar Stereographic image over the UK. The top left and bottom right coordinates are: (-17.644 E, 59.683 N) (9.231 E, 45.25 N).

I want to reproject this to UKNG (aka BNG) projection.

This is my code. First I using gdal_translate to append georeferencing data to my png, output as tif.

PROJ="+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs"

gdal_translate -of GTiff -a_ullr -17.644 59.683 9.231 45.25 -a_srs "$PROJ" the_image.png test.tif

Alright, so now I have geotiff with the appropriate bounds, I want to warp it to the UKNG projection

UKNGPROJ="+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs"

gdalwarp -s_srs "$PROJ" -t_srs "$UKNGPROJ" test.tif plswork.tif

I then convert back to png so I can overlay on my OpenLayers map:

convert plswork.tif ohwhydoesntthiswork.png

And I get dodgy output. The image is diagonal and doesn't overlay onto the map correctly at all.

Both NP Stere and UKNG proj definitons come from the EPSG site, https://epsg.io/27700 , https://epsg.io/3411

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That site is actually not EPSG site. It is maintained by the MapTiler team but it tries to mirror the original data from https://epsg.org. Proj strings are probably OK even 3411 is deprecated (see 3413). But please pay attention to units in EPSG:3411 +units=m

Next have another look at your command

gdal_translate -of GTiff -a_ullr -17.644 59.683 9.231 45.25 -a_srs "$PROJ" the_image.png test.tif

You must convert the ullr coordinates from EPSG:4326 into EPSG:3411. I am not sure is georeferencing with two corners gives good result in this case but have a try.

You can safely use EPSG codes instead of proj strings, and when you have attached the projection into GeoTIFF with -a_srs you do not need to use -s_srs because GDAL will find it anyway from the image metadata.

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  • Thank you for your reply. I've converted the ullr coords from epsg:4326 -> 3411. They come out as: -a_ullr 1543965.24 -2984217.75 4138243.37 -2981192.93. Even with that, the final output (omitting the -a_srs) is even weirder!
    – dwright
    Commented Oct 30, 2020 at 15:15
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    You can use -t_srs epsg:27700 for avoiding copy-paste errors. Using OpenLayers as a viewer adds more possible sources for errors. I recommend to check the result with QGIS instead.
    – user30184
    Commented Oct 30, 2020 at 15:42
  • Hi, it does appear georeferncing with two corner only gives bad results. Do I have any other options?
    – dwright
    Commented Nov 4, 2020 at 11:38

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