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I have a shapefile with a MultiLineStringZ geometry type that I created using the grass v.drape function to attach z-values from a DEM. In order to calculate the 3d length of a line segment, can I use the $length function in the QGIS field calculator, or will this function only report the 2D length?

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    A simple test with two identical features but with wildly different Z values at the points gives the same $length value. Maybe submit a documentation issue to make this explicit in the help if it is the case? Also, I can't find a function or plugin that says it can measure 3d line lengths for things with Z geometry.
    – Spacedman
    Commented Feb 23, 2021 at 18:30
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    @Spacedman you are right, I did this simple test over a volcano (one 2d one 3d) and they yielded the same length when length was calculated via $length function in the field calculator. I'll submit a documentation issue to try and clarify this.
    – jamierob
    Commented Feb 23, 2021 at 22:41

2 Answers 2

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For QGIS versions QGIS 3.20 and greater

Now one can calculate the 3D length of a MultiLineStringZ with the length3D($geometry) function, see the Changelog for QGIS 3.20 for more details.

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For QGIS versions lower than QGIS 3.20

No, the $length function only reports lengths from 2d geometries.

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