[PROJ] Vertical Transformations?

[Even Rouault]

Paul,

yet another interesting use case that exhibited a few defects that should be 
adressed by https://github.com/OSGeo/proj.4/pull/1279

To get the best results for the NGVD29 height (ftUS) to NAVD88 height 
transformation, you need to install in $install_prefix/share/proj the VERTCON 
grids that are now part of the 
https://download.osgeo.org/proj/proj-datumgrid-north-america-1.2RC1.tar.gz 
package 
(see the README.md of https://github.com/OSGeo/proj-datumgrid/tree/master/
north-america)

With the vertconc.gtx grid available, you'll get

$ echo "45 -100 100" | cs2cs -f "%.8f" EPSG:7406 EPSG:5500 
44.99998071	-100.00039555 30.85853056

Without it, you'll get just us-ft to metre for the vertical part:
44.99998071	-100.00039555 30.48006096

Even

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