[PROJ] NTv2 grid file from Bessel ellipsoid to Bessel ellipsoid

[Jochem]

Hi all,

Thanks for the quick reply, Even.


> Yes this is somewhat logical given what the code does 

Ah, then the behaviour is different from what I expectated. 


> For what you describe in your needs, you'd better use PROJ 5.2 or the
> nearly released PROJ 6.0 and use PROJ pipelines 

I know, but I would like to be able to do the same with Proj.4, to share it
with the many users that still use this version. 


> given what the code does (which of course is true of any bug :-)).

I thought it was a bug, since I also noticed that de code ignores the
setting for GS_TYPE in the header of de NTv2 (.gsb) file: if this is not set
to seconds, it still assumes seconds.


Anyway, thanks for the explanation. Now I am able to get what I need like
this with Proj.4:

cs2cs +proj=sterea +lat_0=52.156160556 +lon_0=5.387638889 +k=0.9999079
+x_0=155000 +y_0=463000 +ellps=bessel +towgs84=0,0,0,0,0,0,0 +units=m
+no_defs +to +proj=longlat +ellps=bessel +towgs84=0,0,0,0,0,0,0 +units=m
+no_defs -f %.9f < inputRD.txt > step1.txt

cs2cs +proj=longlat +ellps=WGS84 +wktext +nadgrids=rdcorr2018beta.gsb
+units=m +no_defs +to +proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0
+units=m +no_defs -f %.9f < step1.txt > step2.txt

(replace all heights by 0.0000) 

cs2cs +proj=longlat +ellps=bessel
+towgs84=565.7381,50.4018,465.2904,-0.395026,0.330772,-1.876074,4.07244
+units=m +no_defs +to +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0
+units=m +no_defs -f %.9f < step2H0.txt > outputERS89.txt

which works for the entire world and within the bounding box of the grid it
gives the same (diff. < 1 mm) as:

cs2cs +proj=sterea +lat_0=52.156160556 +lon_0=5.387638889 +k=0.9999079
+x_0=155000 +y_0=463000 +ellps=bessel +wktext +nadgrids=rdtrans2018beta.gsb
+units=m +no_defs +to +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0
+units=m +no_defs -f %.9f < inputRD.txt > outputETRS89.txt

Kind regards, Jochem





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