Is it possible to redefine a projection in QGIS?

Question

Is it possible to redefine a projection in QGis?

It seems like the shape files offered by Agiv (a Flemish agency) differ slightly from the definition that QGis has of EPSG:31370.

I'm NOT looking to create a custom projection, only to replace the existing one.

Update: the definition Agiv (official Flemish agency) uses, becomes this in QGis if you add a shapefile. It is considered a user defined coordinate system, generated CRS, USER:100000.

• Generated CRS (+proj=lcc +lat_1=49.8333339 +lat_2=51.16666723333333 +lat_0=90 +lon_0=4.367486666666666 +x_0=150000.01256 +y_0=5400088.4378 +ellps=intl +towgs84=-106.869,52.2978,-103.724,0.3366,-0.457,1.8422,-1.2747 +units=m +no_defs)

The definition of the EPSG:31370 in QGis for EPSG:31370 is: +proj=lcc +lat_1=51.16666723333333 +lat_2=49.8333339 +lat_0=90 +lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl +towgs84=-106.869,52.2978,-103.724,0.3366,-0.457,1.8422,-1.2747 +units=m +no_defs

So there's only a very very minor difference, but it seems QGis won't recognize it by default. That's what I'd like to change. (I know I could define it manually). That's how the files are provided, and how they are recognized in ArcGIS, which shows the same False_Easting and False_Northing.

Answer 1

There is only one official definition of the EPSG:31370 projection but several "dialects" to represent it (look at Spatial Reference: EPSG:31370 or epsg io: 31370)

The official OGC WKT string is:

PROJCS["Belge 1972 / Belgian Lambert 72",GEOGCS["Belge 1972",DATUM["Reseau_National_Belge_1972",SPHEROID["International 1924",6378388,297,AUTHORITY["EPSG","7022"]],TOWGS84[-106.869,52.2978,-103.724,0.3366,-0.457,1.8422,-1.2747],AUTHORITY["EPSG","6313"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4313"]],PROJECTION["Lambert_Conformal_Conic_2SP"],PARAMETER["standard_parallel_1",51.16666723333333],PARAMETER["standard_parallel_2",49.8333339],PARAMETER["latitude_of_origin",90],PARAMETER["central_meridian",4.367486666666666],PARAMETER["false_easting",150000.013],PARAMETER["false_northing",5400088.438],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["X",EAST],AXIS["Y",NORTH],AUTHORITY["EPSG","31370"]]

It is used in the .qpj files of QGIS.

ESRI use its own WKT string:

PROJCS["Belge_1972_Belgian_Lambert_72",GEOGCS["GCS_Belge 1972",DATUM["D_Belge_1972",SPHEROID["International_1924",6378388,297]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["standard_parallel_1",51.16666723333333],PARAMETER["standard_parallel_2",49.8333339],PARAMETER["latitude_of_origin",90],PARAMETER["central_meridian",4.367486666666666],PARAMETER["false_easting",150000.013],PARAMETER["false_northing",5400088.438],UNIT["Meter",1]]

It is used in the .prj files

If you look at the boreholes of AGIV: G3Dv2_boringen, for example, they use the ESRI dialect in their .prj files and they are recognised by QGIS:

Other libraries and applications use different "dialects" as the Proj4 format, as QGIS in the definition of a projection:

Answer 2

If the shapefile comes with a .prj file, you can edit that with any text editor.

Once you have saved the file with QGIS, make sure to delete the .qpj file, which will be loaded instead of the .prj file when present.

Note that the .prj file normally does not offer datum shifts. If you don not like the one that QGIS bundles with an EPSG code, you have to build a custom CRS.

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Update

Alternatively, you can Rightclick -> Set CRS for Layer to ovveride the custom CRS that QGIS has created.

Or install the SQLite Database Browser GUI, open .../apps/qgis/resources/srs.db, look up 31370 in field srid of table tbl_srs and change the values in the parameters field. After restarting QGIS this will be permanently on your side, until you install the next QGIS version.