[GIS] How to set the tolerance value in pgr_createTopology function of pgrouting

pgroutingpostgresqltopology

I am making a map based on the floor plan of a building.
I made the shapefile and added it to the PostgreSQL database.

I created the topology for the map using the following code:

SELECT pgr_createTopology('ways', 0.00001, 'the_geom', 'gid');

The tolerance provided here is 0.00001. I didn't get many vertices when I checked it up (just 14). How can I set a proper tolerance value? What does it depend on?

Best Answer

The tolerance value is in the same unit as your geometry. When the distance between start/end point of two linestring geometries is smaller than the tolerance, then the topology function assumes, that these two points are connected. This means, both points get snapped together.

The unit of the tolerance value is in most cases either meter or degree, which usually makes a big difference.

In your case, if the projection unit would be meter, then 2 points must be very very close to make a connection between the road segments.

Related Solutions

[GIS] How to use the pgRouting TSP function

According to the examples and the documentation, you should only be loading the five nodes you are interested in in the select query, so I added a where clause to your select query. You also had a trailing comma, following the list of ids in parameter two that I removed.

So give this a try and see if it works better:

SELECT vertex_id
FROM tsp('SELECT id as source_id, lon AS x, lat AS y 
          FROM point_layer where id in (23,221,63,432,335)','23,221,63,432,335',63);

Here is a link to the doc: http://pgrouting.org/docs/1.x/tsp.html

[GIS] Doubts about how pgRouting topology is built and how flexible it is

I'm trying to give a brief reply to your questions:

The topology function assigns a source and a vertex ID for each road segment. To do this a vertex table is created, which contains all source and target IDs. You can use this vertex table, but it's not required.
If there is no shared node at an intersection, then the topology script will not connect these 2 roads.
You can use any network data you want.
You need to handle intermediate points yourself. PostGIS provides various functions to find nearest roads and split them, see ST_LineLocatePoint or ST_LineSubstring. pgRouting is a set of basic functions, which add routing or network analysis functionality.
Turn restrictions are complex. You should first start with something simple.
PostGIS and pgRouting topology functions are different. pgRouting cares about a valid "network".

You probably best start with the pgRouting workshop.

Best Answer

Related Solutions

[GIS] How to use the pgRouting TSP function

[GIS] Doubts about how pgRouting topology is built and how flexible it is

Related Question