I would like to calculate line sinuosity for line segments.

The sinuosity is defined as a ratio of the distance between start and end point along a line (line length) and the euclidean distance between both points. A large sinuosity signifies a large detour between start and end point and is shown as the red line in the next image:

enter image description here

Caculating this measure is straightforward in GRASS GIS with:

v.to.db map=lines option=sinuous columns=sinuosity

However, v.to.db is not part of QGIS processing and cannot be called from within the QGIS Python shell.

Is there another option to calculate line sinuosity in PyQGIS?

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1 Answer 1


Since the calculation of the sinuosity is quiet simple, one option might be to calculate it with the QGIS fieldcalculator which is callable with PyQGIS:

                      {"INPUT_LAYER": lines,
                       "FIELD_NAME": "sinuosity",
                       "FIELD_TYPE": 0,
                       "FORMULA": 'length($geometry)/distance(start_point($geometry), end_point( $geometry))',
                       "NEW_FIELD": True,
                       "OUTPUT_LAYER": line_sinuosity})
  • 1
    The formula may give an incorrect result since $length measures using ellisoid and units from the QGIS settings. To get the line length in the CRS units use "length($geometry)". also, you can use the field calculator distance function like so: length($geometry)/distance(start_point($geometry), end_point( $geometry))
    – amcaninch
    Dec 30, 2017 at 14:27
  • Thanks @amac I changed the former formula ("length" / sqrt((xat(-1) - xat(0))^2 + (yat(-1) - yat(0))^2)) to (length($geometry)/distance(start_point($geometry), end_point( $geometry))) Jan 5, 2018 at 12:09

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