Hello all,
I've been working with planar orthomosaic also and I think that wanting Z axis of such projection to represent height MSL is not possible. In the planar ortho the local xy axii correspond to a plane parallel to façade which is almost vertical so local Z can never represent H amsl as it is orthogonal to plane represented by façade. Of course you can always give origin marker a Z corresponding to its height amsl but tthen all points on façade will have about the same Z which does not correspond to reality. I hope that this makes some sense.
For example in attached example, I have a planar projection ortho of a side of a roof. The planar projection was defined using 3 markers where point 1 is the origin of local coordinates (0,0,0), point 1 to point 2 > local x.
I clicked on Point 1 and Point 3 in ortho to get their chunk CRS coordinates and of course their Z amsl is quite different though in the local sustem they are on same plane (side roof) and thus have local z =0...
PS. in case of vertical façade, then the planar ortho can be shifted so that local y corresponds to Z amsl. In 2nd example, I have a a façade done by planar projection with 3 markers (1, 2, 3). In origina ortho (right half of screen), the local coordinates of marker 1 are (0,0). But in shifted ortho (left side), the marker 1 local coordinates are (0,13.4) where the y corresponds to Z amsl...