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

[Alexander Njemz]

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+11%C2%B
006'53.3%22E/@49.4158889,11.1126169,751m/data=!3m2!1e3!4b1!4m5!3m4!1s0x
0:0x0!8m2!3d49.4159012!4d11.1148166

[2]	https://www.google.de/maps/place/49%C2%B025'01.1%22N+11%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_dhdn2etr
s_beta.php

[4]	http://crs.bkg.bund.de/crseu/crs/descrtrans/BeTA/BETA2007tes
tdaten.csv


-- 
------------------------------------
Alexander Njemz
Mondula GmbH

Geschäftsführer Michael Looft, Dr. Jan F. Ortmann
Amtsgericht Hamburg HRB 68092
Gerichtsstand Hamburg

Behringstrasse 28 a, H1, OG2,
22765 Hamburg
fon: ++49 (0)40 - 35 70 33 - 00
fon: ++49 (0)40 - 35 70 33 - 12

mailto:alexander.njemz at mondula.com
mailto:office at mondula.com
http://www.mondula.com
------------------------------------
Diese Email enthält vertrauliche und/oder rechtlich geschützte
Informationen. Wenn Sie nicht der richtige Adressat sind oder diese
Email irrtümlich erhalten haben, informieren Sie bitte sofort den
Absender und vernichten Sie diese Mail. Das unerlaubte Kopieren sowie
die unbefugte Weitergabe der Mail ist  nicht gestattet.
------------------------------------
This e-mail may contain confidential and/or privileged information. If
you are not the intended recipient (or have received this e-mail in
error) please notify the sender immediatley and destroy this e-mail.
Any unauthorised copying, disclosure or distribution of the material in
this e-mail is strictly forbidden.
------------------------------------