Identifying overlapping lines within layer

Question

I am trying to identify the overlapping lines within a layer that consists of polygons of land parcels.

I have already converted my data to lines and exploded those lines. Indeed, I have run a "no duplicates" rule with the Topology checker and it identifies the overlapping lines (shown as red in the image below). But I do not know how to select the data identified by the Topology Checker.

When I run "intersect" on the layer, it seems to return all my data, including the non-overlapping portions. Is there any built-in tool that can help me here? If not, I assume this would be a straightforward SQL query?

Answer 1

I honestly don't know how to extract overlapping lines through the Topology checker. Let me suggest another tool - SAGA: Shared polygon edges, which is available in the Processing Toolbox.

1. Activate Shared polygon edges tool from Processing Toolbox > SAGA > Vector polygon tools.
2. Select a unique id field in your polygon data (e.g. "fid" in this example).
3. Give a small Tolerance to ensure the tool recognizes all overlapping edges... you may have to give a trial and error... 1.0 meter worked for me in this case.
4. Tick on Check vertices
5. Run the tool.

You will then obtain a line layer Edges, which would look like below:

• I have changed the Edge layer style to green dashed line.
• Open the attribute table to examine which edge has come from which polygon(s).
• If you select any of the edge, corresponding attribute record will show id (ID_A and ID_A) from the original polygons to which the edge belonged.