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QGIS 3.4: My project uses Pseudo Mercator as the CRS (datum: WGS 1984). I now added a shapefile which uses a local coordinate system from Austria (datum: MGI). I did not define a datum transformation in the general QGIS settings, neither did i define a datum transformation in the project settings->CRS (it is empty).

I would expect that no transformation is used and that there is an offset between my map and the shapefile (like in ArcGIS), but the shapefile aligns correctly with my background map (which is Pseudo Mercator, like the project CRS).

I assume that there is a default transformation, even if I did not define anything, but where can I see the default datum transformation or how does QGIS deal with that?

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    IMO, one of the advantages of QGIS over ArcGIS is that the user doesn't have to worry about datums, only CRS (everyone who ever took a beginner Arc course came away completely confused about projections vs datums). If you want to torture yourself, this github thread has a summary of how QGIS formerly handled datum transformations, back in August 2017. Read through all the comments to follow the changes that have been made since then.
    – csk
    Nov 8, 2018 at 19:08
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    @csk you're still dealing with datums in QGIS--they're part of the geographic CRS (and thus projected CRS too). Anyway, GIS7 does the particular MGI definition you're using include a +wgs84 keyword or +nadgrds? That could also set a transformation.
    – mkennedy
    Nov 8, 2018 at 23:55
  • thank you for your feedback: @mkennedy: I guess that this is the "standard" transformation (epsg.io/1618). I know I could change it, but how can I be sure which transformation has been used by default? It is not visible.
    – GIS7
    Nov 9, 2018 at 7:21
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    If you right-click on the layer, then Set CRS, Set Layer CRS, you see what projection parameters are used. The +towgs84 parameters indicate the datum transformation.
    – AndreJ
    Nov 14, 2018 at 6:57
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    @GIS7 this will be implemented as soon as PROJ v6 can be used by GDAL and QGIS: gdalbarn.com
    – AndreJ
    Nov 14, 2018 at 10:51

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Comment from AndreJ: "If you right-click on the layer, then Set CRS, Set Layer CRS, you see what projection parameters are used. The +towgs84 parameters indicate the datum transformation"

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