[PROJ] ESRI:102655 - z-axis in meter insted of us-ft

[Even Rouault]

On vendredi 5 juin 2020 17:09:15 CEST Kilian Demeulemeester wrote:
> Hi there!
> 
> I am using proj c++ api and I have the follwoing question:
> 
> When using ESRI:102655, it look like it do not convert the vertical axis
> to us-ft. My program output the |z| in meters.

Well, this is even worse than that. You should consider that whatever value you get as the z is 
undefined. ESRI:102655 is a projected 2D CRS, so there is no Z axis. PROJ transformation 
code might do things on the z value (or not), but you shouldn't rely on that

If you want a reliable behaviour regarding the Z axis, you need to build a CompoundCRS if the 
Z value is an elevation related to a VerticalCRS, or a Projected 3D CRS if it relates to an 
ellipsoidal height.

> If I generate the proj4 string via |proj_as_proj_string|, I retrieve the
> following string :
> 
> +proj=lcc +lat_0=36.6666666666667 +lon_0=-105.5 +lat_1=37.2333333333333
> +lat_2=38.4333333333333 +x_0=914401.828899998 +y_0=304800.6096
> +datum=NAD83 +units=us-ft +no_defs +type=crs
> 
> Then, if I add |vunits=us-ft |to the projString, I get the expected
> result (z in us-ft!)

Caution: ingesting such a string with +vunits=us-ft will be understood as a CompoundCRS 
with a VerticalCRS with a unknown datum (hint: look at the output of projinfo on such a 
string). Any transformations done with that will never apply any geoid model or other 
transformation of the Z value, except basic unit conversions. So you might be off several tens 
of meters.

Even

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