[PROJ] Transformation to rotated pole

[Even Rouault]

Ken,

Not completely clear if you want to reproject from or into rotated pole, but 
you'll fix trivially the below depending on your need.

shapefile .prj AFAIK cannot store a rotated pole CRS, so I'd just try to do it 
in one step, with the following modified & corrected CRS ("+proj=latlon" was 
wrong, it should be "+proj=ob_tran +o_proj=latlon")

ogr2ogr -s_srs "+proj=ob_tran +o_proj=latlon +lat_0=0 +lon_0=180 +o_lat_p=18.0 
+o_lon_p=-200.0 +datum=WGS84 +type=crs"  -t_srs EPSG:4326 output.shp 
source.shp

You may need a quite recent PROJ version for that to work correctly due to 
ob_tran being complicated to handle when "computing" transformation pipelines.

Actually I just did a fix in PROJ master (https://github.com/OSGeo/PROJ/pull/
2441) for ob_tran if there was a +towgs84 term (you don't need one here, as 
+a=6378137 +rf=298.257223563 +towgs84=0,0,0 really means +datum=WGS84)

Or you can also directly specify the transformation pipeline which should 
hopefully work with any GDAL >= 3 and PROJ >= 6

ogr2ogr -ct "+proj=pipeline +step +proj=unitconvert +xy_in=deg +xy_out=rad 
+step +inv +proj=ob_tran +o_proj=latlon +lat_0=0 +lon_0=180 +o_lat_p=18 
+o_lon_p=-200 +ellps=WGS84 +step +proj=unitconvert +xy_in=rad +xy_out=deg" 
output.shp source.shp

if you add -t_srs EPSG:4326, you'll also likely need to add " +step 
+proj=axisswap +order=2,1" at the end of the above pipeline to deal with the 
lat,lon axis order of EPSG:4326

(for the reverse direction remove the +inv)

Even

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