cts
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orbisgis
Support new unit of measure
Reported on H2GIS repository, CTS is not able to create a CRS from local unit of measure as +units=us-ft
See https://github.com/orbisgis/h2gis/issues/980
I cannot see any reason why us-ft would not be recognized, but I can see a reason why NAD27 cannot be transformed into a system based on another datum. Transforming coordinates from/to NAD27 needs a grid based transformation. For USA, this grid based transformation uses a grid format which is not supported in CTS (ctable produced by proj4 nad2bin program). See https://svn.osgeo.org/metacrs/proj/tags/proj_4_4_9/proj/html/gen_parms.html for more info.
Seems that the grid are now free as open licence for distribution : https://www.rncan.gc.ca/cartes-outils-publications/outils/systemes-reference-geodesiques/18768 But as you point ,CTS must be able to read the grid format. Another good point, some data will be available here https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ntv2.php?locale=fr#GRIDSELECTOR to build CTS tests. Note : You must create an account to use the resources.
The canadian case is not so difficult as they use the NTv2 grid format (indeed this format has been developped in Canada), but NAD27 covers also USA (original ticket is about Louisiana South), which is more problematic as USA use another grid format (ctable or los/las ?) : https://github.com/OSGeo/proj-datumgrid/tree/master/.github
los/las files are included in the dowloadable nadcon java software and transformations from/to NAD27 are available in gdal so I think that resources are free and open, but I did not find much documentation about this format.