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I have a layer containing multiple non-overlapping lines (MultiLineString). I want to calculate the length of each line. However, I get three different answers depending on the method used to calculate in Qgis3.28 (Mac). I wouldn't mind if they were a few meters different, but they are a km different in some cases. All my lines are about 10km or so. The layer is native projected in EPSG:4326 - WGS 84. The project is reprojected on the fly in UTM53S. An example of a line measurement;

  1. using the Measure Line tool on the toolbar - 7991m. I expect this to be least accurate.
  2. using Vector->Geometry Tools->Add Geometry - 0.0788 which seems to be orders of magnitude different?
  3. Using the field calculator Geometry->$Length - 8774m

I've tried reprojecting the layer into UTM35S which makes a very small difference in some instances.

I expect that the three different methods have various mathematical trade-offs.

Can anyone advise which would be the best method?

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    Take a look at gis.stackexchange.com/a/348856/107424. It's about area, but it's the same for length. About your second one: Thats the length in degrees.
    – MrXsquared
    Commented Jan 24, 2023 at 12:56
  • Thanks for the link. I figured it would be something like that. So in this instance I'll stick with $Length.
    – user2175481
    Commented Jan 24, 2023 at 13:25

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