Creating an intersecting linestring layer from two polygon layers using some tolerance

Question

I have two postgis layers:

• The first layer contains the sidewalks of my town (polygons in blue).
• The second one contains the pavements (roads) of my town (polygons in brown).

The map looks like this in QGIS with both layers activated:

Ok, now I have to obtain a layer with the curbs (lines) in the town starting from these two layers. The new layer must be composed of linestrings. Each element in this new layer should be a linestring where every of the greometries of the two layers intersect.

In the following snapshot I manually painted the geometries that should be created in this particual area:

Sorry about my bad pulse. I hope it helps to understand me better.

Ok, if I just add the layers to QGIS, for example, and intersect one layer with another I get the following layer:

The new green layer is the obtained curbs layer. I have two snags with this layer:

• It is composed of polygons
• Not all the curvs are drawn.

With this last evidence I discover that the sidewalks and roads layers were not precisely delineated one next to the other, so I need some kind of tolerance in the operation to get the layer I am looking for. As I told before it should be a linestring layer.

I don't mind if the query is directly done in postgis or using QGIS or other software's tools, I just can't figure out right now how to solve this.

Answer 1

In PostGIS you can use ST_Buffer to add a little margin to one of the polygon layers, then use ST_Intersection to get the curbs polygon. Now the ST_StraightSkeleton may give you what you want (haven't actually tested but it should work).

As noted below skeletonize produces wrong result. The better way should be to do ST_DumpRings on the roads polygon and get ST_Intersection of rings and the curb polygon.