[PROJ] Wrong transformation in NL?

[Lesparre, Jochem]

Hi,



[ER]: PROJ synthetized a new operation (operation 3) that has the same accuracy than the "correct" (operation 1), and a smaller area of use, thus it was selected.



Thanks for this fix.



[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.



[GT]: Am I reading this right that 0 m in EGM2008 is 20 cm in NAP?  (I get it that this is super close and we expect close since they are both trying to match mean sea level, with the caveat that mean sea level is a difficult concept, with EGM2008 tied to some global MSL and NAP to some measured sea level over some period at one(?) tide gauge.)



Yes, NAP and EGM2008 are within 0,4 m. NAP is mean summer hightide in Amsterdam in 1683-1684, way before its open connection to the sea was dammed. Nowadays, MSL in the North Sea is close to NAP.



GT: It seems it was about preference for transforms that were all sort of ok.



Yes, the transformations are more or less correct. I did however start a discussion with EPSG on the Amersfoort to WGS 84 transformation https://epsg.org/transformation_4833/Amersfoort-to-WGS-84-4.html this month. At least, the accuracy should be corrected from 1 m to 1.25 m and the extent enlarged to Netherlands onshore + offshore. Other options are to remove or to supersede this transformation by a copy of https://epsg.org/transformation_9282/Amersfoort-to-ETRS89-NLD-AGRS2010-9.html or to create a concatenated operation or pipeline to enforce the transformation route Amersfoort - ETRF2000 - WGS 84 (where the second step is a null transformation).



Regards, Jochem






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