[Cartoweb-users] Help with pgdijkstra

[Darren Houston]

Hello everyone,

I received good help from Marc on the PostGIS Users' mailing list. Basically 
he told me that pgdijkstra is working as it should. From his explanation, a 
shared vertex doesn't necessarily mean two lines have a common node (think a 
bridge crossing a street). Only start and end nodes are taken into 
consideration.

Long story short; my current dataset will not work with pgdijkstra. I have 
written some routines that automatically clean up my dataset and apply the 
pgdijkstra functions. It's hit and miss, but I am getting some expected 
results.

Darren H.

On Tuesday 06 February 2007 14:55, Darren Houston wrote:
> Hello everyone,
>
> I am trying out the pgdijkstra library and am having difficulties getting
> it to work with my test dataset. Included is the sql to load the test
> dataset, and a SVG and PNG output of what the dataset looks like.
>
> Running this sql works (finding the path from the start of the green line
> to the end of the yellow line)
>
> SELECT gid,astext(the_geom) FROM shortest_path_as_geometry('line_test',
> (select v.id from line_test_vertices v join line_test g on v.geom_id =
> g.source_id where g.ident=1),
> (select v.id from line_test_vertices v join line_test g on v.geom_id =
> g.target_id where g.ident=3));
>
>
> outputs
>
>
> 5|"MULTILINESTRING((665249.377865961 5662895.69047619,670545.577160494
> 5659332.79276896,668812.275573192 5654999.53880071,670449.282627866
> 5649607.04497354,674589.947530864 5645658.9691358))"
>
> 6|"MULTILINESTRING((674589.947530864 5645658.9691358,671701.111552028
> 5635259.15961199,663419.781746032 5641325.71516755,652634.794091711
> 5639688.70811287))"
>
> 7|"MULTILINESTRING((652634.794091711 5639688.70811287,650708.903439153
> 5631022.20017637,656486.575396825 5624859.35008818,663227.192680776
> 5629096.30952381,667560.44664903 5623222.34303351,671315.933421517
> 5623126.04850088,677767.667107584 5629481.48765432))"
>
>
> Running this sql does not work (finding the path from the start of the
> green line to the end of the red line)
>
> SELECT gid,astext(the_geom) FROM shortest_path_as_geometry('line_test',
> (select v.id from line_test_vertices v join line_test g on v.geom_id =
> g.source_id where g.ident=1),
> (select v.id from line_test_vertices v join line_test g on v.geom_id =
> g.target_id where g.ident=4));
>
>
> outputs
>
>
> ERROR:  Error computing path: No path found
> CONTEXT:  PL/pgSQL function "shortest_path_as_geometry_internal_id" line 9
> at for over execute statement
> PL/pgSQL function "shortest_path_as_geometry" line 24 at for over select
> rows
>
>
> Looking at the tables and columns  pgdijkstra creates / fills, it seems
> that only the start and end vertices of a line are taken into
> consideration, hence pgdijkstra thinks that the red line is not connected
> to the network. Is this true? If so, any way to get pgdijkstra to work in
> this situation? Any help / ideas form seasoned pgdijkstra users is greatly
> appreciated.
>
> Thank you,
>
> Darren H.