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I have

  • a polygon vector layer containing four polygons, corresponding to four Protected Areas in a state in India.
  • a line vector corresponding to all the rivers in that state.

Desired output: I want to find out the proportion of the total river length flowing through Protected Areas.


Results so far:

  1. I tried using 'Sum Line Lengths' under Vector Analysis to get at this, and then used the 'Basic Statistics for Numeric Tools' to get the summed length of all the rivers falling within the Protected Areas, by putting the river layer in the 'Lines' layer and the Protected Area layer in the 'Polygons' layer, in the 'Sum Line Lengths' dialog box. The value I got was 368.62 km.

However, the new layer looks the same as the polygon layer of Protected Areas in the workspace, so I decided to use 'Clip' as well, where the river layer was the Input layer and the Protected Area layer was the Clip layer.

  1. I then ran a '$length' operation on the field calculator to get the length of all the river segmens, and then used the 'Basic Statistics for Numeric Tools' to get the summed lengths. Now I'm getting a value of 552.33 km.

I'm a little confused with this, as I was assuming I should get the same value for both. Does the 'Sum Line Lengths' work differently than a straight up 'Clip'?

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