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I'm new to GIS. I've been given a shapefile that I've added as a new vector layer in QGIS and it displays OK, but it says it has an Unknown CRS. The CRS from the prj file is:

PROJCS["Transverse_Mercator",GEOGCS["GCS_Airy 1830",DATUM["D_unknown",SPHEROID["airy",6377563.396,299.3249753150316]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER["central_meridian",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",400000],PARAMETER["false_northing",-100000],UNIT["Meter",1]]

and the one displayed in the details window of the CRS selector is:

Unknown CRS
WKT
PROJCRS["Transverse_Mercator",
    BASEGEOGCRS["GCS_Airy 1830",
        DATUM["D_unknown",
            ELLIPSOID["airy",6377563.396,299.324975315032,
                LENGTHUNIT["metre",1,
                    ID["EPSG",9001]]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["Degree",0.0174532925199433]]],
    CONVERSION["unnamed",
        METHOD["Transverse Mercator",
            ID["EPSG",9807]],
        PARAMETER["Latitude of natural origin",49,
            ANGLEUNIT["Degree",0.0174532925199433],
            ID["EPSG",8801]],
        PARAMETER["Longitude of natural origin",-2,
            ANGLEUNIT["Degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["Scale factor at natural origin",0.9996012717,
            SCALEUNIT["unity",1],
            ID["EPSG",8805]],
        PARAMETER["False easting",400000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8806]],
        PARAMETER["False northing",-100000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8807]]],
    CS[Cartesian,2],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]]]
Proj4
+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +a=6377563.396 +rf=299.324975315032 +units=m +no_defs +type=crs
Extent
Extent not known

I'd like to convert it to a well known CRS within QGIS but when I do select a different CRS from the options, all the features disappear, with the exception of EPSG:3994 which displays the right map but inverted N/S. Does anyone have any advice on what I'm doing wrong or what CRS I should be changing it to?

5
  • 1
    What happens when you save the data to a new file and select a different CRS when doing so?
    – Erik
    Commented Apr 14, 2020 at 14:01
  • Referring to epsg.io/9001, it is a geocentric Cartesian coordinate system. AFAIK QGIS supports lat/lon and projected coordinate systems and 9001 is neither in proj epsg nor in QGIS srs.db. I think you should convert geocentric coordinates to lat/log to use it in QGIS.
    – Zoltan
    Commented Apr 14, 2020 at 15:32
  • Hi @Zoltan, EPSG:9001 is the code of the metre unit of measure. This is a custom Transverse Mercator projection system. Commented Apr 14, 2020 at 19:04
  • 1
    What is the real EPSG code for your layer? In QGIS there are 10 Transverse Mercator projections (6312, 3109, ...). If the coordinates are projected then you have to assign the right projection to your layer. EPSG:27700 (OSGB 1936/British National Grid) has the same definition in QGIS/proj database. Please assign this to your layer in the layer properties on the source tab or create a custom CRS.
    – Zoltan
    Commented Apr 15, 2020 at 6:58
  • There is a minor difference with the flattening provided in the original CRS definition and the value defined in the EPSG database for the Airy 1830 ellipsoid, and because of that PROJ assigns 25%, instead of 70%, of identification with EPSG:27700. Commented Apr 16, 2020 at 6:20

1 Answer 1

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What you need to do is save your custom CRS.

Unknown CRS is an unsaved one. A custom CRS is a saved CRS that does not matches any other of the known CRSes.

Go to Settings ~> Custom Projections and click on the add sign button. Give a name to it, and use any of the Proj4 string or the WKT definition. Validate it to see that will be recognized and click OK.

From there, the shapefile layer will have a Custom CRS with the code USER:100xxx.

Do not change its CRS. If you want, you can export the layer and reproject it to another CRS.

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