[Proj] Pipeline +init files, inverse

stevejking stevejking at talktalk.net
Wed Feb 21 16:18:00 EST 2018

Is it possible to use a +init pipeline file in reverse?

For example, with some explanation from Kristian, I now have two

# OSGB1936 to ETRS89(2)
<7709-3D>                  -t0 +proj=pipeline \
                      +step +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
+x_0=400000 +y_0=-100000 +ellps=airy +inv \                     
                      +step +proj=vgridshift
+grids=OSGM15_GTX_OSGBtoGRS80.gtx +inv \
                      +step +proj=hgridshift

# ETRS89 to OSGB1936
<7709-3D-Reverse>     -t0 +proj=pipeline \                    
                      +step +proj=hgridshift
+grids=OSTN15_NTv2_OSGBtoETRS.gsb +inv \ 
                      +step +proj=vgridshift
+grids=OSGM15_GTX_OSGBtoGRS80.gtx \ 
                      +step +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
+x_0=400000 +y_0=-100000 +ellps=airy

The only difference between the two is the reverse order and swapping the
+inv and 'no inv blanks' on the end of each line. I'm wondering if I need
two separate transformations?

If not is it a sensible future development? +init=<...> +reverse ?


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