osm2pgrouting is a convenient tool, and its focus to work
on OpenStreetMap data. There are several cases where osm2pgrouting can’t be used. Some network data already comes with a network
topology that can be used with pgRouting out-of-the-box. Often network data is
stored in Shape file format (.shp
) and we can use PostGIS』
shp2pgsql
converter to import the data into a PostgreSQL database.
But what to do then?
In this chapter you will learn how to create a basic Routing Network Topology from a network data that does not have a routing Topology create the minimum attributes needed the Routing Network Topology.
Chapter contents
At first we will load OpenStreetMap sample data with osm2pgsql.
CITY="DS_TS"
cd ~/Desktop/workshop
cp ~/data/osm/$CITY.osm.bz2 .
createdb -U user osm_data
psql -U user -d osm_data -c "CREATE EXTENSION postgis;"
psql -U user -d osm_data -c "CREATE EXTENSION pgrouting;"
osm2pgsql -U user -c -d osm_data --latlong --cache 5 --cache-strategy sparse $CITY.osm.bz2
Let’s see which tables have been created:
Run: psql -U user -d osm_data -c "\d"
The table containing the road network data has the name planet_osm_roads
.
It consists of large amount of attributes.
Run: psql -U user -d osm_data -c "\d planet_osm_roads"
It is common that road network data provides at least the following information:
This allows to display the road network as a PostGIS layer in GIS software, for example in QGIS. Though it is not sufficient for routing, because it doesn’t contain network topology information.
The next steps will use the PostgreSQL command line tool.
psql -U user osm_data
Having your data imported into a PostgreSQL database might require one more step for pgRouting.
Make sure that your data provides a correct Routing Network Topology, which consists of information about source and target identifiers for each road link. The results above, show that the network topology does not have any source and target information.
Creation of the Routing Network Topology is necessary.
警告
PostGIS topology is not suitable for Routing.
pgRouting provides a general way for creating the Routing Network Topology
with the pgr_createTopology
function.
This function:
source
and a target
identifiers to each road linkpgr_createTopology('<table>', <tolerance>, '<geometry column>', '<gid>')
For additional information see pgr_createTopology.
First add source and target column, then run the pgr_createTopology
function
… and wait.
The dimension of the tolerance parameter depends on your data projection. Usually it’s either 「degrees」 or 「meters」. In our example the geometry data projection to determine the tolerance:
SELECT find_srid('public','planet_osm_roads','way');
find_srid
-----------
4326
(1 row)
Based on this result the tolerance will be 0.00001
-- Add "source" and "target" column
ALTER TABLE planet_osm_roads ADD COLUMN "source" integer;
ALTER TABLE planet_osm_roads ADD COLUMN "target" integer;
-- Run topology function
SELECT pgr_createTopology('planet_osm_roads', 0.00001, 'way', 'osm_id');
To verify that there is a basic Routing Network Topology:
\d planet_osm_roads
Also a new table containing the vertices information was created:
\d planet_osm_roads_vertices_pgr
id
is the vertex identifierthe_geom
is the geometry considered for that particular vertex identifier.source
and target
from the planet_osm_roads
correspond to an
id
in planet_osm_roads_vertices_pgr
tableNow we are ready for our first routing query with Dijkstra algorithm!
Analyzing the topology with pgr_analyzeGraph:
SELECT pgr_analyzeGraph('planet_osm_roads', 0.000001, the_geom := 'way', id := 'osm_id');
Adjusting the topology is not an easy task:
Depending on the application some adjustments need to be made.
Some topology manipulation functions help to detect and fix some of the topological errors in the data.