[Qgis-psc] How to apply a correction to inaccurate contour elevations

[David Brownrigg]

I have a dsm made by WebODM from a drone image set with inaccurate gps.
Contours made from the dsm have a +45cm error compared to known vales.

In Raster->extract->contour I tried setting 'Set offset from zero' to -0.45
BUT resulting contours have the same numerical values as before.

What should I be doing (e,g for a contour labelled 10.50 to be labelled 10.05 in my case)?
Thanks in advance for any help.
David
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