PostGIS – How to Split OSM Roads into Individual Segments at Intersections

postgispostgis-2.0postgresqlsplitting

I want to create a road-network for use with pgRouting using OpenStreetMap data. I loaded a shapefile from GeoFabrik into a Postgres table (with PostGIS enabled). However, one problem I had was that the roads do not always end at intersections, so I decided to split them all at every intersection or crossing.

To identify all the interesections where roads crossed or intersected I used the following SQL (similar to a previous question):

CREATE TABLE split_points as
SELECT DISTINCT    
   ST_GeometryN(ST_Intersection(a.geom, b.geom),1) as geom      
FROM
   roads as a,
   roads as b
WHERE
    ST_Touches(a.geom, b.geom)
OR
    ST_Crosses(a.geom, b.geom)    
    AND a.gid != b.gid
GROUP BY
   ST_Intersection(a.geom, b.geom);

I now want to split the roads using these points. I used the following approach:

CREATE TABLE split_roads as
SELECT     
    ST_GeomFromEWKB((ST_Dump(ST_Split(g.geom, blade.geom))).geom) As geom,
    generate_series(1,ST_NumGeometries((ST_Split(g.geom, blade.geom)))) as gid
FROM    
    split_points as blade,
    roads as g
WHERE
    ST_Intersects(g.geom, blade.geom);

The problem with this split approach is that the full road length remains in addition to all of the split pieces. To remove these un-split road geometries that were included I used the ST_Equals() function to identify them, and to delete them:

DELETE FROM split_roads USING roads
WHERE ST_Equals(split_roads.geom, roads.geom)

However, this approach does not remove all of the original unsplit geometries (although it does remove some of them). Is there a better approach for deletion (or overall) so that I have only the split geometries in a table?

Best Answer

Simple answer: Don't. You shouldn't do it that way.

From the OSM road Shapefiles, it is impossible to distinguish between intersections and over/underpasses. You'll create intersections that don't exist in reality if you split all seemingly crossing roads.

You'll need to get your hands dirty with the original OSM file, if you don't want to use existing tools such as osm2pgrouting (where the network is small enough) or osm2po.

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