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I would like to calculate a graph similiar to an isochrone using pgsql. Therefore, I already used the algorithm pgr_drivingDistance. You provide a starting point and a distance value and receives an isochrone.

The output using the algorithm is received with code which looks something like: 

SELECT * FROM pgr_drivingDistance(
        'SELECT id, source, target, cost FROM edge_table',
        2, 2, false -- starting point, distance, directed
      );

The red star represents the starting point.

Now, I want a graph which works the same way, like starting at one point and get routes in all directions. The difference is, that I don't want to provide a travel distance, but a list with point coordinates, which are lying on the road network. The route in every direction has to stop at the first reached point lying on each route. The distance on every route is different and I don't know which points are the closest ones.

The desired output using the "stopping" points, which are visualized in green, is supposed to look like this.

I tried already:

  • Using the given algorithm pgr_drivingDistance and raising the distance value every time no point is reached -> problem here: the distance is equal for all directions and not individual for each route.
  • Using the algorithm pgr_dijkstra for each route -> problem here: because you don't know which point is affected you don't know which end point to choose for the calculation. You also cannot take the closest one in the immediate vicinity because you need the closest one on the specific route.

I know that I have to build an almost complete new algorithm, but maybe someone has an idea how to start or even experience with this kind of problem.

Thank you in advance!

isikl
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1 Answers1

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This is a one to many routing problem. You have to compute the route to each end point to find the shortest one. I have not looked at the pgRouting function recently, but I believe there is a one to many, many to one and many to many Dijkstra function(s). You should be able to use the one to many to compute all the routs in one go and then you can sort the routs based on length to find the shortest one.

Stephen Woodbridge
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