pgRouting – Creating Network Topology with Grade Separation

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I have built a network using Ordnance Survey's ITN data and I can solve routes across the network. What I need is to build in grade separation topology for overpasses and underpasses on the network so that my routing solutions do not drop off bridges onto roads underneath in an attempt to make the shortest path possible. Does anyone have any pointers as to where to start?

I am using PostGIS 2.0.3, pgRouting 1.0.7dev and Postgresql 9.2 on Windows.

Best Answer

When loading data to PostGIS, there's no topology information associated with them. To create a useful topology the data needs to be “noded”. This means that where two or more roads form an intersection there it needs to be a node at the intersection and all the road segments need to be broken at the intersection, assuming that you can navigate from any of these segments to any other segment via that intersection. (Extract from pgrouting 2.0 documentation).

In your case, it seems that some nodes are common between bridges over roads and underneath roads. I don't know if those nodes were given in the ITN data or if you created them.

So you have to pass a clean network to pgrouting :

Let's say you have a bridge (A-B-C) and a road underneath (D-B-E) :

Network example

The table you pass to pgRouting is like :

source | target | geometry           | cost
   A   |   B    | LINESTRING (A,B)   | cost_ab
   B   |   C    | LINESTRING (B,C)   | cost_bc
   D   |   B    | LINESTRING (D,B)   | cost_db
   B   |   E    | LINESTRING (B,E)   | cost_be

You have to modify it to 

source |target | geometry            | cost
   A   |   C   | LINESTRING (A,B,C)  | cost_ab+cost_bc
   D   |   E   | LINESTRING (D,B,E)  | cost_db+cost_be

You can create a temporary table and use some case statements to fill columns/filter your grade and generate the modified network.

Note that pgRouting 2.0 documentation gives an example (but with new methods) : pgRouting 2.0 Routing topology

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