[PROJ] Wrong transformation in NL?

[Even Rouault]

> [GT]: So was the bug confusing orthometric and ellipsoidal?
>
> No, PROJ used an inappropriate geoid. The 2D Amersfoort datum of the 
> 2D target CRS RD has an ellipsoidal height within 1 m of MSL in the 
> Netherlands. PROJ nevertheless thought it was a good idea to take the 
> geoid for the vertical target CRS NAP that is to be used on 3D ETRS89 
> and apply it on the Amersfoort ellipsoidal height.
>
A bit more subtle than that. It did:

- WGS 84 + EGM 2008 -> WGS 84 3D geographic using EGM2008 height: OK

- use of "|Amersfoort to WGS 84 (4)" Helmert transformation in reverse 
direction, promoting the Amersfoort CRS to a 3D geographic one and do 
the Helmert transformation in 3D. That's the part that was questionnable 
(yeah, wrong) in this context since that Helmert transformatioon  is 
likely only intended for 2D. Unfortunately the EPSG database doesn't 
distinguish cases where this is valid and those where it is not. The 
only hint is that if there's no official record for a 3D geographic CRS, 
doing the Helmert transformation in 3D is likely to not be correct|

|- apply the "Amersfoort to ETRS89-NLD [AGRS2010]" transformation using 
the 2D grid|

|- apply the "||ETRS89-NLD [AGRS2010] to NAP height (2)" transformation 
using the geoid grid|

|- apply the ||"Amersfoort to ETRS89-NLD [AGRS2010]" in the reverse 
direction, to finally get Amersfoort + NAP height coordinates|


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