[PROJ] Transformation to rotated pole

[Ken Mankoff]

Hi Even,

I got it working with your help. Thank you for the clarification and hints.

  -k.

On 2020-11-21 at 06:32 -08, Even Rouault <even.rouault at spatialys.com>
wrote...
> 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