Tip: Planetary Projections in QGIS

When using QGIS with planetary datasets in current versions (3.x), we get this or similar error messages:

CRITICAL No transform available between SINUSOIDAL MARS and EPSG:4326
CRITICAL No transform available between SINUSOIDAL MARS and Custom CRS

(here we used this image as an example data set)
As a result, coordinate display in latitude and longitude is not possible, the scale bar and the measurement tools do not correctly work.
This can be solved by providing a geographic reference system matching to our data for the QGIS project. In our example (watch out for the “truncated” Mars radius of 3396 km), we create a custom CRS in QGIS (settings->custom projections) with the following WKT (adapt the ellipsoid radius for your needs):

GEOGCRS["MARS Geographic",
DATUM["D_MARS",
    ELLIPSOID["MARS",3396000,0,
        LENGTHUNIT["metre",1,
            ID["EPSG",9001]]]],
PRIMEM["Reference meridian",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]]]]

Then choose it as your project’s CRS (Project->Properties->CRS) and the problems should be gone. Some similar error messages are still shown, but these can be safely ignored, they are related to some widgets in the status bar. Lat/lon values should be correct, also the scale bar and measurement should work (double check with a known-sized feature).
More information here and here.
Tested with QGIS 3.16.1 and QGIS 3.19-nightly 2021/03/23

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