What I want to do: create a new line (named linestring in Postgis?) from existing multipolygon layer in Postgis.

Purpose: once these lines are created, I'll run some distance calcs from specific points.

What I'm using: Postgres 9.3 with Postgis enabled. Quantim GIS 2.2 Valmiera.

The existing multipolygon layer are neighborhoods in Rio de Janeiro city. The lines I want to create are clean beaches that some neighborhoods have.

What I'm doing: Like in the image below, I select the layer in QGIS, click on the button "Toggle Editing", click on "Node Tool", select the neighborhood to show its points with the Node Tool, select the points based unpon a rectangle I draw, the poinst become highlighted in blue, and then I don't know how to save them as a new line.


UPDATE: I've found this tutorial to manually create a new feature. It will be my backup technique if there is no other.

2 Answers 2


Use the polygon to lines tool and then remove those parts of the line layer which you don't want to keep.

  • Will this method alter the current polygon selected? If so, I think the safest way would be copy the features table and work on the copy.
    – srodriguex
    Jul 21, 2014 at 3:15
  • The polygon to line tool will not alter the polygons. It creates a new line layer anyway.
    – underdark
    Jul 21, 2014 at 17:26

Given the nature of the lines you're trying to create versus the geometry you have, I think it will probably be fastest/simpler to just create new features as in the tutorial you have linked to. Any method that actually converts the polygons to lines will involve splitting the lines at certain points and/or deleting a lot of extra nodes/lines/etc.

There is a plugin that might help speed things up a bit more called AutoTrace, which helps trace existing features rather than snapping to and clicking on each vertex you want to follow individually.

  • @cris-w, I've found somewherelse the tip on AutoTrace. I was hoping there would be a "handless" way of doing this.
    – srodriguex
    Jul 21, 2014 at 3:16

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