Unspecified +vunits no longer scaled the same amount as +units

[dmahr1]

Problem description + examples

I am upgrading some old code that unprojects + transforms coordinates from an arbitrary input to WGS84. With PROJ 6.2.1 it correctly converts the input 650.000 foot elevation to 198.120 meters:

$ docker run --rm -it osgeo/proj:6.2.1 bash
root@aa66cbfb0aed:/# echo 3200000 10200000 650 | cs2cs "epsg:6578" "epsg:4326" -f %.3f
30.618  -97.461 198.120

But starting with PROJ 6.3.0 the z-coordinate is unchanged at 650.000:

$ docker run --rm -it osgeo/proj:6.3.0 bash
root@24492e9f6418:/# echo 3200000 10200000 650 | cs2cs "epsg:6578" "epsg:4326" -f %.3f
30.618  -97.461 650.000

It appears that PROJ used to assume that the +vunits for horizontal CRSs was the same as the +units. But this is no longer the case. Is this expected behavior, or a regression?

The only related PR or issue I found was #1731.

Environment Information

• Docker versions osgeo/proj:6.2.1 and osgeo/proj:6.3.0

Installation method

• Docker

[rouault]

The issue here is that you convert between two 2D CRS. EPSG:6578 has US-survey-foot for horizontal components, but we know nothing about the vertical direction: is it ellipsoidal height or orthometric height, in meters, us-survey-foot, whatever... So the behaviour of the transform of the Z component is completely unspecified. There was indeed a change of behaviour in the 6.x series regarding that, but neither, nor the new behaviour are correct. To get reliable transformation of Z, you need to use 3D CRS, that is either Compound CRS (Projected/Geographic + Vertical), Projected 3D CRS (with ellipsoidal height) or Geographic 3D CRS (with ellipsoidal height).

[dmahr1]

@rouault Thank you for confirming my suspicion! It seems like the old code was relying on undefined behavior. I will change the code to either specify the +vunits explicitly or use a 3D CRS.