Etienne,
I see this stuff from a user's point of view. I am currently not working
with different projections but my projects are EPSG:31468 and my data,
too. So this is my perspective. I am neither a geodesist nor did I dig
into how different software handles srs or prj files. All that I know is
that QGIS (1.7.4) imports EPSG:31468 shape files as EPSG:2167 because it
does neither evaluate the DATUM["D_Deutsches_Hauptdreiecksnetz" nor the
SPHEROID["Bessel_1841" infos given.
Trunk creates a user-defined SRS and does not apply EPSG:31468 either.
I would prefer QGIS to recognize the shape file as being EPSG:31468.
Am 30.03.2012 03:23, schrieb Etienne Tourigny:
[snip]
You can also play with the command line and see what comes up.
GDAL_FIX_ESRI_WKT=GEOGCS gdalsrsinfo file.prj
PROJ.4 : '+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 +units=m
+no_defs '
GDAL_FIX_ESRI_WKT=TOWGS84 gdalsrsinfo file.prj
PROJ.4 : '+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 +units=m
+no_defs '
GDAL_FIX_ESRI_WKT=NO gdalsrsinfo file.prj
PROJ.4 : '+proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0
+datum=potsdam +units=m +no_defs '
This would be my preferred parameters as datum=potsdam can be different
towgs84 parameters in different locations
How would this be decided? By the user or some magic way?
The user can change it with the plugin "Coordinate Systems Updater". You
see: if the datum gets translated into a towgs84 parameter and the
projection is matched based on this then if a different user-defined
towgs84 paramter exists the right projection is not found.
according to http://mapref.org/GeodeticReferenceSystemsDE.html#Zweig733
the towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7
is applicable for the whole country of Germany
but local towgs84 parameters may differ
This sort of contradicts your statement that it should be recognized
as EPSG:31468 because that code is NOT potsdam datum (in GDAL/OGR
anyway)
AFAIK the DHDN is the realisation of the Potsdam Datum see e.g. here:
http://georepository.com/datum_6314/Deutsches-Hauptdreiecksnetz.html
so in the prj file it is called DATUM["Deutsches_Hauptdreiecksnetz"
whereas Proj4 says +datum=potsdam ; both mean the same
However the problem persists that QGIS does not recognize the projection as
EPSG:31468 no matter if I set it to GEOGCS, TOWGS84 or NO!
Neiter does it detect the correct projction from the prj file QGIS itself
created.
The GEOGCS option to GDAL_FIX_ESRI_WKT fixes the GEOGCS authority
node, but not the root node which is a PROJCS (see example below)!
Trying to match all possible projections can be more challenging than
matching the possible geogcs definitions. It is possible, but I didn't
get there yet.
Just to make sure I understand - is the expected behaviour in QGis
(from you POV) to see the file as EPSG:31468 with the TOWGS84
parameters seen above? Or the more generic 'potsdam' datum?
Maybe it would be better to see the towgs84 parameters (as defined by
QGIS default or the user via the plugin) but
1) it must be imported as EPSG:31468 (and not as user-defined)
2) if saving to shapefile only the DATUM["Deutsches_Hauptdreiecksnetz"
must be written (as QGIS does) and IMHO not the
TOWGS84[598.1,73.7,418.2,0.202,0.045,-2.455,6.7]
===>
Perhaps QGis could be modified to scan existing proj4 definitions for
a match with the layer's proj4 string as Berhard suggests.
That should probably work in this case (because all that is missing is
the authority nodes - which is probably the comparison inside Qgis) -
am I right Jurgen?
Example of working GEOGCS detection:
$ GDAL_FIX_ESRI_WKT=GEOGCS gdalsrsinfo tmp1.prj
PROJ.4 : '+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
+units=m +no_defs '
OGC WKT :
PROJCS["Transverse_Mercator",
GEOGCS["DHDN",
DATUM["Deutsches_Hauptdreiecksnetz",
SPHEROID["Bessel 1841",6377397.155,299.1528128,
AUTHORITY["EPSG","7004"]],
TOWGS84[598.1,73.7,418.2,0.202,0.045,-2.455,6.7],
AUTHORITY["EPSG","6314"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4314"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",12],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",4500000],
PARAMETER["false_northing",0],
UNIT["Meter",1]]
this is intersting as the definition has a datum and a towgs84 parameter
AFAIK the towgs84 values are translating the datum for the use in a
7-parameter datum transformation. This becomes relevant when my project
has a srs using a WGS84 ellipsoid and I need otf transformation of data
defined in EPSG:31468
I doubt it is useful to store a DATUM _and_ a TOWGS84 node, only the
datum is part of the srs and thus part of the data whereas the question
how to transform to wgs84 is more the responsibility of the software
that performs the transformation.
and compare with EPSG:31468, the GEOGCS node has correct authority (4314)
$ gdalsrsinfo EPSG:31468
PROJ.4 : '+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
+units=m +no_defs '
OGC WKT :
PROJCS["DHDN / 3-degree Gauss-Kruger zone 4",
GEOGCS["DHDN",
DATUM["Deutsches_Hauptdreiecksnetz",
SPHEROID["Bessel 1841",6377397.155,299.1528128,
AUTHORITY["EPSG","7004"]],
TOWGS84[598.1,73.7,418.2,0.202,0.045,-2.455,6.7],
AUTHORITY["EPSG","6314"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4314"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",12],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",4500000],
PARAMETER["false_northing",0],
UNIT["metre",1,
AUTHORITY["EPSG","9001"]],
AXIS["X",NORTH],
AXIS["Y",EAST],
AUTHORITY["EPSG","31468"]]
Etienne
[more snip]
How does QGIS work? What happens when it is confronted with a prj file?
How does the matching work if no authority is given for the PROJCS?
With EPSG:31468 we have the problem that the srs string uses a different
name for the datum than Proj4 and that this datum can obtain different
towgs84 paramters for different parts of the world.
Bernhard
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