[Proj] How grids work in PROJ4

[Michal Seidl]

Hello,
I moved forward on implementing quite new Czech projection called "Krovak
Modified" which is used in new ProjectedCRS EPSG:5224. I also want to
implement vertical datum "Bpv" by use of +geoidgrids but I found myself
lacking some basic information which I can not easily find  on the web.

1)  What kind of coordinate transformation is supported by PROJ 4.8.0?
    - towgs84?
    - nadgrids?
    - geoidgrids?
    - others?


2) Can I combined these transformation? And how they are applied? Look at
these situations:

cs2cs ... +proj=latlong  +nadgrids=... +geoidgrids=...  +towgs84=...  +to 
+towgs84 ... +geoidgrids=... +nadgrids=...  +proj=latlong

What happens?


LLH->(dL,dL)->LLH->(dH)->LLH->XYZ->(3D)->XYZ(wgs84)->(3D)->XYZ->LLH->(dH)->LLH->(dL,dL)->LLH 

I mean by this:
a) if there is +nadgrids  and +towgs84 on the LEFT SIDE PROJ firstly changes
LL by dL,dL from grid before 3D transformation
b) if there is +geoidgrids on the LEFT SIDE PROJ thinks there is orthometric
height on input so firstly it changes H by dh from grid to get ellipsoid
height before 3D transformation
c) on the RIGHT SITE it should work in a opposite order. Firstly do 3D
transformation and than apply +nadgrids and +geoidgrids as there is
ellipsoid height on the imput from 3D transformation

I think PROJ should do this but I am not sure about it. I tried to do some
test but I got confused. Is there some document how it exactly works?

Thanks Michal

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