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In QGIS 2.8 I was able to successfully do a transformation using the following proj4 syntax :

+proj=utm +zone=18 +ellps=clrk66 +datum=WGS84 +towgs84=-11.0584,163.8703,190.541,0,0,0,0 +units=m +no_defs +wktext

With QGIS 3.10 this no longer seem to work in the customised projection window. Reading some of the documentation it seems QGIS 3.10 is now using proj6. Is there a change that needs to be done in the syntax?

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    In what way does it not work? Are points in the wrong place? Slightly? Completely? I tried putting the proj4 string you provided into the QGIS 3.10.2 custom CRS definition window, with format="Proj string", and it seems to validate OK in the window.
    – Houska
    Feb 8, 2020 at 2:08
  • probably the problem was related to this: github.com/qgis/QGIS/pull/34258
    – pigreco
    Feb 8, 2020 at 14:39
  • The points are shifted but not to the same coordinates as when I use QGIS 2.8. As I understand it, the points were collected in the 70's in a modified local grid (sub-arctic in Quebec) and then registered as nad27. It's supposed to be a pretty standard Molodensky transformation used to convert these modified nad27 points to nad83.
    – Frederic
    Feb 9, 2020 at 15:31

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