[postgis-users] Nearest Aid station to accident

[William Temperley]

Hi Matthew,


>What I am finding hard to implement is the part where to find the shortest
>distance on the road network.  It is taking more or a less a minute there.
>Any idea how I can improve it please?

Any way in which you can cut down the size of the graph PGRouting
works on will give you a faster calculation - just pass a query
representing a subset of your street data to the shortest path
function, e.g.

	SELECT edge_id FROM shortest_path_astar(''
				SELECT gid AS id,
				source::int4,
				target::int4,
				to_cost as cost,
				reverse_cost,
				x1, y1, x2, y2
				FROM '||table_name||' where
				(
				(the_geom
				&& <something to cut down the result set>)
				)
					
				''
				,'||start_node.node_id||','||end_node.node_id||', true, true
)

A while ago I was attempting to precalculate routes between strongly
connected nodes and keeping these in the routing table, then selecting
only streets near the start and end points, plus the relevant
pre-calculated routes and passing these to the A* function, but this
never really got past proof of concept, there being a lot of
complications with the precalculation itself. This way I think you
could get sub-second queries on any sized network.

Also- I think Shooting* is the slowest method and the only extra you
get over A* is right turn restrictions.

Hope this helps

Will


Hope this helps