pyproj crs definition lacking the towgs84 parameter

[hesmebye]

THIS POST IS ALSO POSTED AS AN ISSUE

I am trying to transform from ED50 UTM32 to Euref89 UTM32 using pyproj. With just using the crs 23032 for ED50 and 25832 for Euref89 I get an error at about 76 meters in East-West and 52 in South-North. When applying the same CRS definition as postgis uses I get about 2 and 0.4 meters.

The difference is that postgis has defined the towgs84=-87,-98,-121,0,0,0,0 parameters which takes the difference of origo for the ED50 and Euref89 elipse into account.

Why do the pyproj projection definition for ED50 not include this - or am I missing something here?

I have been to the norwegian mapping agency and epsg.io to check the correct answer which is included in the code. Here is my testcode I used to verify that it is this towgs84 parameter that is the problem.

from pyproj import Transformer, CRS
#Starting coordinates
fromEast = 597111.0449
fromNorth = 6648897.1099

#correct answer
ed50SKEast = 597192.480
ed50SKNorth = 6649105.064

ed50crsWithToWGS = CRS.from_epsg(23032)
x = ed50crsWithToWGS.to_proj4()
ed50crsWithToWGS = CRS.from_proj4(x + " +towgs84=-87,-98,-121,0,0,0,0")

transformerWithToWGS = Transformer.from_crs("epsg:25832", ed50crsWithToWGS)

transformer = Transformer.from_crs("epsg:25832", "epsg:23032")

eastPyProj, northPyProj = transformer.transform(fromEast, fromNorth)
print(f"pyproj: eastPyProj:{eastPyProj}, northPyProj:{northPyProj}")

diffKartvX = ed50SKEast - eastPyProj
diffKartvY = ed50SKNorth - northPyProj
print(f"pyproj: diffKartvX:{diffKartvX}, diffKartvY:{diffKartvY}")


eastPyProj, northPyProj = transformerWithToWGS.transform(fromEast, fromNorth)
print(f"pyproj towgs84: eastPyProj:{eastPyProj}, northPyProj:{northPyProj}")

diffKartvX = ed50SKEast - eastPyProj
diffKartvY = ed50SKNorth - northPyProj
print(f"pyproj towgs84: diffKartvX:{diffKartvX}, diffKartvY:{diffKartvY}")

This print the following text:

pyproj: eastPyProj:597115.9013242763, northPyProj:6649052.5260300655
pyproj: diffKartvX:76.57867572363466, diffKartvY:52.537969934754074
pyproj towgs84: eastPyProj:597194.6075596083, northPyProj:6649104.665756716
pyproj towgs84: diffKartvX:-2.1275596083141863, diffKartvY:0.39824328385293484