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I would like to know if it is possible to make a graph with pgrouting 1.05 on polygons. What I mean is that I have to make a graph on a database without roads but with polygon geometry. I would like to make a graph on this geometry.

Thanks for your help and I'm sorry for my english

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3 Answers 3

up vote 5 down vote accepted

Okay, I looked further into the idea of Steve above. I'll try to demontrate his idea in a QGIS/PostGIS/pgrouting environment. You will get results such as this:

enter image description here

First, let's assume you have a geodata table with your shelves/obstacles looking like this (I made them up for this purpose):

enter image description here

Make sure your shelf data is in a projected coordinate system with meters as units (I used a Mercator CRS(SRID=3785). If it's in lat/lng you have to reproject your data.

Then, open up your database connection and import your shelves geodata (either use a QGIS plugin like SPIT or the command line tool shp2pgsql). Now, create a point grid. Use the function from here: Create regular point grid in Postgis :

CREATE OR REPLACE FUNCTION makegrid(geometry, integer)
RETURNS geometry AS
generate_series(floor(st_xmin($1))::int, ceiling(st_xmax($1)-st_xmin($1))::int, $2) as x
,generate_series(floor(st_ymin($1))::int, ceiling(st_ymax($1)-st_ymin($1))::int,$2) as y 
where st_intersects($1,ST_SetSRID(ST_POINT(x,y),ST_SRID($1)))'

Following SQL gives you a table containing a point grid with spacing of one meter between each point:

SELECT nextval('seq') AS gid, (ST_Dump(makegrid(ST_Transform(ST_SetSRID(ST_Extent(the_geom),3785),3785),1))).geom AS the_geom FROM shelves;

This grid only covers the extent of your shelves file:

enter image description here

Now remove every grid point which is intersected by/contained by a shelf (you could also do this directly in the grid query above) and create an updated table:

CREATE TABLE open_space_grid AS 
grid left join
  shelves ON
WHERE shelves.gid is null;

this should give you this:

enter image description here

Now you have to connect the points to form a network. This query connects each point pair with a distance of one meter, forming a grid network.

SELECT nextval('seq') AS gid, the_geom 
    select ST_SetSRID(ST_MakeLine(a.the_geom, b.the_geom),3785) AS the_geom 
    FROM open_space_grid AS a,open_space_grid AS b) AS tmp WHERE ST_Length(ST_Transform(ST_SetSRID(tmp.the_geom,3785),3785)) = 1;

Then throw out all duplicates:

WHERE gid IN (
    SELECT e1.gid 
    FROM edges e1, edges e2
    WHERE st_equals(e1.the_geom, e2.the_geom)
    AND e1.gid < e2.gid

Now we are here:

enter image description here

Now create the routable network (Everything here taken from http://underdark.wordpress.com/2011/02/07/a-beginners-guide-to-pgrouting/)

   SELECT *, ST_StartPoint(ST_SetSRID(the_geom,3785)) as startpoint, ST_EndPoint(ST_SetSRID(the_geom,3785)) as endpoint
   FROM edges;

   SELECT row_number() OVER (ORDER BY foo.p)::integer AS id, 
          foo.p AS the_geom
   FROM (         
      SELECT DISTINCT net_ext.startpoint AS p FROM net_ext
      SELECT DISTINCT net_ext.endpoint AS p FROM net_ext
   ) foo
   GROUP BY foo.p;

   SELECT a.*, b.id as start_id, c.id as end_id, ST_Length(ST_MakeLine(a.startpoint, a.endpoint)) as length
   FROM net_ext AS a
      JOIN node AS b ON a.startpoint = b.the_geom
      JOIN node AS c ON a.endpoint = c.the_geom;

Now try it out (e.g. execute the query below in QGIS via pgQuery). Input your start and end node id's (in my case, those are 1 and 590):

   FROM network
   (SELECT * FROM shortest_path('
      SELECT gid AS id, 
          start_id::int4 AS source, 
          end_id::int4 AS target, 
          length AS cost
      FROM network',
      false)) AS route
   network.gid = route.edge_id;
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+1 thanks for this great step-by-step guide –  underdark May 9 '12 at 19:07
Thanks a lot for your help. It's very very kind of you :) –  raphael May 10 '12 at 12:20
Sure, no problem. It was a rather interesting task and I was curious how it could be solved ;) –  chriserik May 10 '12 at 12:41

Just to clarify - your roads are polygons? I'm afraid you have to convert them to lines. It's the nature of a routable network to only have edges (lines) and nodes (points). As for how to convert polygons to lines, look here: Simplifying polygons to linestring

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My roads are not polygons. Actually, I'm working on a project and I have to create a route in a building between some shelves. In my database I only have the shelves with a polygon geometry and I don't really know how to do. Thanks a lot for your answer –  raphael May 9 '12 at 12:24
Maybe you could calculate the centroids of your shelves and connect them based on a nearest-neighbour solution. Then you would have a routable network. Of course, this would not take obstacles into account. –  chriserik May 9 '12 at 12:37

From your comment in an earlier answer it sounds like the polygons you have are not the available routes, but the boundaries of areas where travel is not possible. In the space between the shelves you could create a grid (or "mesh") with lines and use that as your routing network.

Here is an example of how you might create that grid: Create regular point grid in Postgis

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Thanks a lot, I'll try that –  raphael May 9 '12 at 13:54
I don't see how to do that? :s –  raphael May 9 '12 at 14:36
Can you be more specific? What don't you see how to do? Creating a grid between your polygons? –  Steve Horn May 9 '12 at 17:41

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