Use "meters at scale" with custom CRS

Question

[QGIS V.3.10.3 A Coruña]

For streets in my fantasy world map I want to use a line ESRI shapefile.

My custom CRS is a lonlat one and looks like the following:

GEOGCRS["unknown",
  DATUM["unknown",
    ELLIPSOID["Yartol",6370997,326.249334289226,
      LENGTHUNIT["metre",1,ID["EPSG",9001]]]],
PRIMEM["Kangorr",0,
  ANGLEUNIT["degree",0.0174532925199433,
    ID["EPSG",9122]]],
CS[ellipsoidal,2],
AXIS["longitude",east,
  ORDER[1],
  ANGLEUNIT["degree",0.0174532925199433,
    ID["EPSG",9122]]],
AXIS["latitude",north,
  ORDER[2],
  ANGLEUNIT["degree",0.0174532925199433,
    ID["EPSG",9122]]]]

For my understanding LENGTHUNIT["metre"... should do the stuff. The measuring bar and tools show good results. In my project properties everything is set to give me correct measurements when scrolling around and measuring stuff, but the lines of my shapefile fill up the whole screen when I set the line thickness to 1 meter at scale, even when I should see my whole world map! The CRS is set both for the project and the shapefile layer. The (apparently unchangeable) properties of the layer indicate, that the unis is degrees though.

What am I missing here?

DonMeles · Answer

The solution lies in the difference between geographic coordinate systems (Unit: degrees) such as a longlat crs and projections such as Mercator, Winkel-Tripel, etc. (Unit: meters, if defined so).
So if you want to visualise lines with a thickness defined in meters at scale, you need to draw them in a layer with some projection as layer-crs. I did it in a custom Winkel-Tripel-crs. Note though, that the width of the lines only will be shown with the correct width, when also the project crs is set to a real projrection. As soon as you switch to a geographical coordinate system as project crs, a street with a width of 4m will cover more than the whole world map!

Use "meters at scale" with custom CRS

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