[mapserver-users] Coordinates shifted for EPSG:28992

[Even Rouault]

Hi Just,

> On Ubuntu 20.4 with Proj Rel. 6.3.1, February 10th, 2020 and MapServer
> 7.4.3 with Proj, the transformation from EPSG:4326 to EPSG:28992 is
> shifted.
> 
> Findings:
> - projinfo for EPSG:28992 shows correct proj-string
> - cs2cs gives correct results
> - when adding the proj-def explicitly to each layer: correct results
> - did not occur on older MS+proj.4-versions on Ubuntu 16.
> 
> Looks like a Datum shift problem, shift is around 80m NE, but towgs84 is
> in the proj-string from projinfo.
> 
> The proj string is:
> <28992> +proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889
> +k=0.9999079 +x_0=155000 +y_0=463000 +ellps=bessel
> +towgs84=565.417,50.3319,465.552,-0.398957,0.343988,-1.8774,4.0725
> +units=m +no_defs <>
> 

Complicated subject, but basically you should try to stick to one of the following 
combinations:
- With MapServer 7.4 or 7.6: GDAL 2.x + PROJ 5.x
- With MapServer >= 7.6: GDAL 3.x + PROJ >= 6.x

Any other combination will lead to surprising results as you got, although I'm not completely 
sure why you get yours if projinfo EPSG:28992 includes the +towgs84 string. Well actually, 
checking the output of projinfo EPSG:28992 with PROJ 6 or 7, it uses
+towgs84=565.2369,50.0087,465.658,-0.406857330322398,0.350732676542563,-1.87034738
36068,4.0812
which comes from "Amersfoort to WGS 84 (3)", whereas your above towgs84 comes from 
"Amersfoort to WGS 84 (4)". But from a quick test the difference between both is on the 
order of 1 cm, not 80m. So it looks more like you don't get any datum shift at all.

I'd note that with a change I've just queue in
https://github.com/OSGeo/PROJ/pull/2357 for PROJ 7.2, EPSG:28992 as a PROJ.4 string will 
no longer include any +towgs84 term, as there are actually 2 candidate transformations 
EPSG:15934 "Amersfoort to WGS 84 (3)" and EPSG:4833 "Amersfoort to WGS 84 (4)", and as 
they have the same extent and accuracy, a "random" one could be used in theory...
"Amersfoort to WGS 84 (4)" mentions "Replaces Amersfoort to WGS 84 (3)" in comments, but 
it is not marked explictly as superseding it, hence PROJ presents both...
Because I love Dutch people, I've added a change in my pull request so that "Amersfoort to 
WGS 84 (4)" is now prefered over "Amersfoort to WGS 84 (3)", when presenting results by 
order of relevance...

Gosh, I'm confident we'll still have such datum related issues until I'm retired :-)

Even


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