I have a shapefile with this information embedded:

Projected Coordinate System:

[+proj=somerc +a=6377397.155000 +b=6356078.962818 +x_0=2600000.0 +y_0=1200000.0 +k_0=1.0 +alpha=90.0 +lon_0=7.439583333333333 +lat_0=46.95240555555556 +no_defs]

How would I convert a point from this shape (as example: (2883434.7562500015, 1208358.5424999967, -1.4551915228366852E-11) to a normal longitude/latitude) ?

Anyone know the correct formula?

  • Do you need transformed values in a new field of the attributes table for the same layer or do you need a new layer with the transformed geometries features? Dec 5, 2019 at 11:04
  • It's far easier to choose a GIS tool or code library than to code your own.
    – Vince
    Dec 5, 2019 at 12:39

1 Answer 1


What you see there is a PROJ string.

somerc is the Swiss Oblique Mercator projection.

You can find all the mathematical details about it in SwissTopo's "Formulas and constants for the calculation of the Swiss conformal cylindrical projection and for the transformation between coordinate systems" available at https://www.swisstopo.admin.ch/en/knowledge-facts/surveying-geodesy/reference-systems/map-projections.html#documents_publicatio.

Converting the coordinates by yourself using these formulas is a significant undertaking and I would recommend that you use readily available software solutions instead. Good luck!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.