select polyline intersections from one shapefile using QGIS

Question

I'd like to find out if there is a way to select all of the polyline intersections within one shapefile. The lines are a road network. I've looked through the posts, and the only tip I can find is the intersections tool. That requires two layers, though.

Thanks, Ward

Answer 1

In a spatial database such as Spatialite you can do this easily. Polyline intersections are points, so import your roads network into spatialite, and create a new points spatial table:

.loadshp roads_network roads LATIN1 <SRID>
CREATE TABLE intersections (pk_uid INTEGER PRIMARY KEY AUTOINCREMENT)
SELECT AddGeometryColumn('intersections','Geometry',<SRID>,'POINT','XY');

The <SRID> parameter is the EPSG code of your coordinate reference system. Now run a query such as (assuming the roads table has a PRIMARY KEY column pk_uid):

INSERT INTO intersections (Geometry) 
SELECT ST_Intersection(r1.Geometry, r2.Geometry) 
FROM roads AS r1, roads AS r2
WHERE r1.pk_uid <> r2.pk_uid;

The WHERE clause insures that you are ignoring the cases of a road intersecting itself.