Transforming Robinson to EPSG:3857 with GDAL
I’m trying to convert a full globe Robinson Projection to Mercator. For example, I use this image.
First, I apply geo-tagging:
gdal_translate -a_ullr -180 90 180 -90 -a_srs ESRI:54030 source.jpg source_tagged.tif
and finally warp it to Mercator:
gdalwarp -t_srs ESRI:54030 -s_srs EPSG:3857 source_tagged.tif target.tif
The outcome is slightly stretched vertically but nowhere near proper Mercator. What am I doing wrong?
There are a couple of issues in your commands. The first is that -180 90 isn't in the top left corner pixel of global Robinson projection GeoTIFFs. The top left corner would be something like -338.2187147689 90, and the bottom right would be 338.2187147689 -90. However, you're also specifying your srs as ESRI:54030, so those bounds need to be in projected coordinates, not lat/lons. The command to generate a GeoTIFF from your image would be:
gdal_translate -a_ullr -17005833.3305252 8625154.47184994 17005833.3305252 -8625154.47184994 -a_srs ESRI:54030 source.jpg source_tagged.tif
Your second command has the -t_srs and -s_srs switched. Given that you're projecting to EPSG:3857, you'll also need to provide bounds since Mercator goes to infinity at the poles. So the updated command will look like:
gdalwarp -s_srs ESRI:54030 -t_srs EPSG:3857 -te -180 -81 180 81 -te_srs EPSG:4326 source_tagged.tif target.tif
I made the following projected image from your example using these commands (cropping the whitespace in the image prior to running them).
