Calculate X & Y projected values from a table into another projection with QGIS

Question

I have a entities layer in projection A with positionned labels coordinates based on X,Y and Rotation angle in the attribute table :

ETIKT_X, ETIKT_Y, ETIKT_ROTA

Those labels are defined into the geometry displayed, I have positionned manually those labels with QGIS label tools and data defined attributes.

Now, I wish to reproject my layer in projection B but, if I do that, my labels will still be in projection A and therefore will not appear correctly.

My actual manual solution is :

• Use "Point from table" tool to create points in my label position in projection A
• Project my label point layer from projection A to B
• Calculate $x and $y of my projected points into my attribute table
• Project my layer from projection A to B
• Join my label point layer to my layer and update the label X and Y field with my proj B calculated values.

Is there a already-made tool in QGIS that can do that without doing all this manipulation ?

If not, I think I'll try to make a processing script that will take as argument : "X field", "Y field", "proj A code" and "proj B code", optionnaly X and Y new Proj B field and do the automation of my process. I'll put it into a solution if I manage to do it.

Answer 1

The calculation was quite simple in fact, thanks to the wonderful field calculator. I did some research and below the solution I found :

• In the Field Calculator, add a new field X_ProjB or Y_ProjB for example and enter as a parameter the following codes for each attribute column :

  x(transform( make_point( "ETIKT_X" , "ETIKT_Y" ) ,'EPSG:27572','EPSG:2154'))

  y(transform( make_point( "ETIKT_X" , "ETIKT_Y" ) ,'EPSG:27572','EPSG:2154'))

Explanation :

1. make_point() : creates a point geometry from my 2 fields X and Y in proj A.
2. transform() projects the geometry generated in (1) from EPSG:proj A to EPSG:proj B
3. x() and y() gets the coordinates of the projected point generated in (2)

The result is the projection B coordinates values. So simple !