Rotating labels based on custom CRS QGIS 3

Question

I'm trying to rotate labels based on CRS of a Europe map EPSG:3035 ETRS89-extended / LAEA Europe but with Longitude of origin 18° and Latitude origin 49°.

According to the question Parameters of the coordinate system for calculating the rotation of labels along latitude lines in the EPSG:3035 is working perfectly:

After i change the project CRS EPSG:3035 to USER:100000 (custom CRS based on LAEA +proj=laea +lat_0=49 +lon_0=18 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +units=m +no_defs +type=crs) in the expression window appears an error:

Any suggestion how to fix this ?

Answer 1

Curved label along a parallel

See below for variant of rotated label of a point.

To create a curved label along parallels for a point layer, create a parallel through each point you want to label. You can achieve that using geometry generator in the label placement tab (details below) to create an invisible auxiliary line used for curved label placement.

Screenshot: World map in a custom CRS with point layer in EPSG:3857 that is labeled along parallels; the dotted parallel lines are just added for visualization purpose and will not appear on the map:

1. In the label placement tab, activate Geometry generator > LineString and paste this expression - in line 3, change 3857 to the EPSG-code of your point layer.

   This expression creates correct parallels through the point, regardless what CRS you use for map canvas (project CRS) and whatever layer CRS you have for the input points layer (if you adapt this once in line 3):

    with_variable (
        'mycrs',
        'EPSG:3857',  -- change here: EPSG-code of the CRS of your layer
    with_variable(
        'mypoint',
        transform ($geometry, @mycrs, 'EPSG:4326'),
        transform (
            densify_by_count( 
                make_line (
                    @mypoint,
                    project (
                        @mypoint,
                        360,
                        radians(90)
                    )
                ), 
                100
            ),
            'EPSG:4326',
            @mycrs
        )
    ))
   

2. Set Mode to Curved.

3. Expand label Anchoring > Settings and set the position to Start of Line and Clipping to Use Entire Line

---

Variant: rotated label of a point

If you want to have not a curved label along the parallel, but simply rotate a label the angle of the parallel at the current point, you can use a variant of the solution above:

1. Go to Label Settings > Placement tab. Set Mode = Parallel

2. Activate Geometry Generator > LineString and insert this expression. In line 3, replace 3857 with the EPSG-code of the layer's CRS:

    with_variable (
         'mycrs',
         'EPSG:3857',  -- change here: EPSG-code of the CRS of your layer
     with_variable(
         'mypoint',
         transform ($geometry, @mycrs, 'EPSG:4326'),
         transform (
             densify_by_count( 
                 make_line (
                     @mypoint,
                     project (
                         @mypoint,
                         1,
                         radians(90)
                     )
                 ), 
                 1
             ),
             'EPSG:4326',
             @mycrs
         )
    ))
   

3. Increase Label Overrun > Overrun distance to 9999 [Millimeters].

You can now show the map canvas in any projection (change project CRS) and the label will be rotated to follow the direction of the parallel going through the point.

Screenshot: parallel lines only for visualization purpose:

Answer 2

First of all, clarify that the grid convergence is not the angle of rotation of the parallel, but that of the meridian. In most cases both lines are perpendicular and the function can be used to find the rotation of the parallel.

---

The custom grid_convergence() function uses the coordinate reference system definitions from the PROJ library database.

When calling that function, the third argument you are using is the @map_crs variable, which contains the value 'USER:100000', which is the identifier within QGIS internal database. But PROJ doesn't know about that identifier and returns the error.

However, if instead of the @map_crs variable, you were to pass the map projection's custom proj string ('+proj=laea +lat_0=49 +lon_0=18 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +units=m +no_defs +type=crs') to the function, it should be interpreted by PROJ without any problem.