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I use OSM data imported into PgRouting database. I developped a web application with a map to use OSM data.

I trace routes using shooting_star_sp() function. For example, to have time cost between 2969 and 70890 nodes, I use :

SELECT sum(time_cost_car) FROM ways, (SELECT * FROM shootingstar_sp('ways', 2969, 70890, 0.1, 'time_cost_car', true, true))as rt WHERE ways.gid=rt.gid;

And the result is 861 (seconds). I think it's correct.

Now, I want to create polygon on my map to create isochron area. I followed this tutorial : http://underdark.wordpress.com/2011/09/25/a-closer-look-at-alpha-shapes-in-pgrouting/ It uses driving_distance() function. But the produced polygon seems to be wrong. I have performed the previous time cost calculation using the tutorial approach :

SELECT cost FROM driving_distance('SELECT gid AS id, source::int4 AS source, target::int4 AS target, time_cost_car::float8 AS cost FROM ways', 2969, 1000, false, false) WHERE vertex_id=70890;

And the result is 600 (seconds). That's wrong.

So what is the difference between shootingstar_sp() and driving_distance() ? How can I create a correct isochron polygon ?

Best regards,

RudyWI


I think a part of the problem is resolved. The following request returns 1337 (seconds), it seems to be correct :

SELECT cost FROM driving_distance('SELECT gid AS id, source::int4 AS source, target::int4 AS target, time_cost_car::float8 AS cost FROM ways', 2969, 100000, true, false) WHERE vertex_id=70890;

However, the polygon produced by the following request seems to be wrong :

SELECT * FROM alphashape('SELECT id, ST_X(the_geom) AS x, ST_Y(the_geom) AS y FROM nodes JOIN (SELECT * FROM driving_distance(''SELECT gid AS id, source::int4 AS source, target::int4 AS target, time_cost_car::float8 AS cost FROM ways'', 2969, 30, true, false)) AS route ON nodes.id = route.vertex_id');

Do you kwow why ?

I'm not sure about JOIN with "nodes.vertex_id". What is "vertex_id" ? Why not "edge_id" ? how to be sure "nodes.id" semantic corresponds to "vertex_id" ?

The source parameter used into "driving_distance()" ("2969" in my example) is vertex_id ? edge_id ? or another id ?


I have performed other tests to illustrate the problem :

`SELECT sum(cost) FROM (SELECT * FROM shortest_path('SELECT gid AS id, source::int4 AS source, target::int4 AS target, length::float8 AS cost FROM ways', 25069, 13530, true, false)) AS foo;

Result => 16.7790377846424

SELECT sum(length) FROM ways, (SELECT gid, the_geom AS the_geom FROM shootingstar_sp('ways', 25069, 13530, 0.1, 'length', true, false)) as rt WHERE ways.gid=rt.gid;

Result => 63.5863605213523`

The result retuned by shootingstar_sp() seems to be OK. But to generate the polygon, I use driving_distance() which returns the same result as shortest_path(). This result seems to be wrong.

So, what is the difference between shortest_path() (or driving_distance()) and shootingstar_sp() ?

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1 Answer

Checking the parameters in your function calls, it seems like the first call instructs pgRouting to assume a directed graph (the "true,true"), while the second one tells it that the graph is undirected (the "false,false").

This could explain why the second call finds a faster route.

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It was a good idea, but it does not seem to be the problem. I have performed the following request : SELECT cost FROM driving_distance('SELECT gid AS id, source::int4 AS source, target::int4 AS target, time_cost_car::float8 AS cost, time_cost_car::float8 AS reverse_cost FROM ways', 2969, 1000, true, true) WHERE vertex_id=70890; The result is still 600 (seconds). –  RudyWI Oct 3 '12 at 15:45
    
shouldn't you be summing up the costs? –  underdark Oct 3 '12 at 16:23
    
The same request without "reverse_cost" : SELECT cost FROM driving_distance('SELECT gid AS id, source::int4 AS source, target::int4 AS target, time_cost_car::float8 AS cost FROM ways', 2969, 1000, true, false) WHERE vertex_id=70890; Does not return any result (0 line). I don't understand. Do you have any idea ? –  RudyWI Oct 4 '12 at 8:45
    
In a directed graph, edges without reverse cost will only be traversable in one direction. It's well possible that no it's impossible to find a route in such a setting. –  underdark Oct 4 '12 at 16:26
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