In the length() function QGIS uses the CRS unit of the layer to calculate the
result. When I use for example a layer in a local CRS like EPSG:31370 or
EPSG:5439 and a layer in EPSG:3857 (that all use meters as unit) the results
are very different (+40% difference). Changing the project CRS makes no
difference. When I check the length using the Measure Tool the results are very
similar to the local CRS.
How can I measure the length of a line and use it as a label independent from
the layer CRS and get results similar to the results of the Measure Tool?
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