[PROJ] Possible error with +datum=potsdam and PROJ >= 5.0.0

Wed May 8 06:46:43 PDT 2019

On Tue, 2019-05-07 at 21:52 +0200, Even Rouault wrote:
> Actually, I realize there must be something fishy with your PROJ
> installation. 
> The BETA2007.gsb grid is part of the
> https://download.osgeo.org/proj/proj-datumgrid-1.8.zip package which
> is 
> normally installed together with PROJ with usual binary
> distributions, so the 
> changed to expanding to +nadgrids=@BETA2007.gsb was reasonable.
> If you compile PROJ from source, you must unzip its content in
> ${proj_install_prefix}/share/proj

Ok, thank you. That makes a lot of sense. It is a little unfortunate
that there is no warning or anything when BETA2007.gsb is not
available. 

I was using a PostGIS Docker image based on Alpine Linux and indeed,
the grid file is not there. I will try to contact the maintainer of the
PROJ package for Alpine Linux, and ask him if he could update the
package. 

Thanks again.

Best,

Alexander

> 
> > Alexander,
> > 
> > In PROJ 4.9.3 and earlier, +datum=potsdam expanded to
> > +ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7
> > 
> > In PROJ 5.0.0 and later, +datum=postdam was changed to expand to
> > +ellps=bessel +nadgrids=@BETA2007.gsb
> > 
> > That is it will use the more precise BETA2007.gsb grid rather than
> > the
> > towgs84 clause, but this require the grid to be installed alongside
> > other
> > PROJ grids. If not present, then no datum shift is done, hence the
> > result
> > you see.
> > 
> > So with PROJ 5.0.0 and later, you could get the same result as PROJ
> > 4.9.3 if
> > you don't have the grid installed, by replacing "+datum=potsdam"
> > with
> > "+ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7"
> > (I don't understand why you'd get a correct result with just
> > +ellps=bessel
> > without a +towgs84 or +nadgrids. That doesn't make sense to me)
> > 
> > With proj 5.2.0
> > 
> > $ echo "4395886.918912 5819485.694352" | cs2cs -f "%.4f"
> > +proj=tmerc
> > +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=bessel
> > +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +to
> > +init=epsg:25832
> > 599474.7609	5817502.6096 42.5689
> > 
> > With proj 6, things will work transparently:
> > 
> > - with BETA2007.gdb installed, you get exactly the result from the
> > .csv
> > file:
> > 
> > $ echo "4395886.918912 5819485.694352" | cs2cs -f "%.4f" EPSG:31464
> > EPSG:25832 599474.9342	5817502.6270 0.0000
> > 
> > - without it installed, it fallbacks to the Helmert transformation
> > 
> > $ echo "4395886.918912 5819485.694352" | cs2cs -f "%.4f" EPSG:31464
> > EPSG:25832 599474.7609	5817502.6096 0.0000
> > 
> > Even
> > 
> > > Hi,
> > > 
> > > we noticed differences in transforming geographic data from
> > > EPSG:31468
> > > to WGS 84 using ST_Transform on different PostGIS installations
> > > and it
> > > seems to come from different PROJ versions.
> > > 
> > > For example, we have a point from CAD drawings of the Nuremberg
> > > subway
> > > network with the coordinates 4435877.4477166822
> > > 5475816.0547993957, so
> > > we know it must be on the train tracks.
> > > 
> > > Converting this point using PROJ 4.9.3 gives us this point [1]
> > > with the
> > > coordinates 11.1148165905971 49.4159011654427 and using PROJ
> > > 5.0.1
> > > gives us this point [2] with the coordinates 11.1161576118633
> > > 49.4169712194789. However, updating the proj4text in the
> > > spatial_ref_sys table for the srid 31468 from
> > > 
> > > +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0
> > > +datum=potsdam
> > > +units=m +no_defs
> > > 
> > > to
> > > 
> > > +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0
> > > +ellps=bessel
> > > on the PostGIS installation with version 5.0.1 of PROJ again
> > > gives
> > > correct results.
> > > 
> > > PROJ version >= 5.0.0 seems to incorrectly handle conversions
> > > using
> > > +datum=potsdam. From [3] one can download test data to verify
> > > correct
> > > conversion from DHDN / GK to ETRS89 / UTM. Testing conversion of
> > > the
> > > value in line 7 of the downloadable CSV file [4] the results for
> > > PROJ
> > > version 4.9.3 and versions >= 5.0.0 differ. I have executed the
> > > following command using PROJ versions 4.9.3, 5.0.0, 5.1.0, and
> > > 6.0.0
> > > 
> > > echo 4395886.918912 5819485.694352 | cs2cs +proj=tmerc +lat_0=0
> > > +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +datum=potsdam +units=m
> > > +no_defs +to
> > > +init=epsg:25832
> > > 
> > > The output for 4.9.3 is:
> > > 
> > > 599474.76	5817502.61 42.57
> > > 
> > > The output for the other versions is:
> > > 
> > > 599559.03	5817663.04 0.00
> > > 
> > > Is this a bug in PROJ? And if so, what would be the recommended
> > > workaround?
> > > 
> > > Kind Regards,
> > > 
> > > 
> > > Alexander Njemz
> > > 
> > > 
> > > 
> > > [1]	https://www.google.de/maps/place/49%C2%B024'57.2%22N+1
> > > 1%C2%B
> > > 006'53.3%22E/@49.4158889,11.1126169,751m/data=!3m2!1e3!4b1!4m5!3m
> > > 4!1s0x
> > > 0:0x0!8m2!3d49.4159012!4d11.1148166
> > > 
> > > [2]	https://www.google.de/maps/place/49%C2%B025'01.1%22N+1
> > > 1%C2%B
> > > 006'58.2%22E/@49.4169722,11.113978,750m/data=!3m2!1e3!4b1!4m5!3m4
> > > !1s0x0
> > > 
> > > :0x0!8m2!3d49.4169712!4d11.1161576
> > > 
> > > [3]	http://crs.bkg.bund.de/crseu/crs/descrtrans/BeTA/de_dh
> > > dn2etr
> > > s_beta.php
> > > 
> > > [4]	http://crs.bkg.bund.de/crseu/crs/descrtrans/BeTA/BETA2
> > > 007tes
> > > tdaten.csv
> 
> 
-- 
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Alexander Njemz
Mondula GmbH

Geschäftsführer Michael Looft, Dr. Jan F. Ortmann
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mailto:office at mondula.com
http://www.mondula.com
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