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.