source: sydneycoastwalks.com.au

Disclaimer: This tutorial was developed on Mac OSX 10.15.6 and tested on Ubuntu 18.04 as well as Ubuntu 20.04. Windows compatibility cannot be guaranteed.

"Dear routing service, please get me from A to B!". If you are sitting in a car or truck you would usually expect the fastest route and not care too much about the environment around you.

However, if you move away from motorized vehicles and think more about leisure and outdoor activities you may prefer scenic routes instead of walking alongside polluted and noisy main roads in metropolitans.

In this small technical tutorial we want to demonstrate you how easy it is to adapt the behaviour of a least cost path algorithm consuming the topology with the support of pgRouting using the full power of PostgreSQL.

In a nutshell and algorithmically speaking: if you want to prefer footways over primary roads, you can make this subset of roads "cheaper" to use - and by doing so guide the routing algorithm (we will be using Dijkstra's in this tutorial) to prefer those ways. If you need a primer or refresher on the terminology you can look at the glossary of our comprehensive FOSS routing engine article.

• PostgreSQL instance with PostGIS and pgRouting enabled, we recommend Kartoza's docker image (see this tutorial)
• osmconvert (installation instructions): easily clip OSM files
• osm2po: create pgRouting topology from raw OSM data (later on you'll be provided with the SQL topology file, so you could skip this step)
• osm2pgsql (installation instructions): import OSM to PostgreSQL to update the edge costs of the topology

First of all, please navigate to your working directory on your host machine and download the OSM data with:

wget https://download.geofabrik.de/australia-oceania/australia-latest.osm.pbf

Using osmconvert we will use this data to clip a part of Sydney between Coogee and Bondi from it.

osmconvert australia-latest.osm.pbf -b=151.2463,-33.9274,151.2956,-33.8302 -o=sydney-coast.pbf

Alternatively, feel free to download the output of this step here.

As mentioned above, we will use osm2po to generate the topology from the OSM data we generated in the previous step. It is important to understand that OSM data is not routable in its raw form. The geniality of this software is that it isn't only a light-weight routing engine, it also processes the OSM data and outputs a SQL file which can directly be imported to your PostgreSQL database and be used with pgRouting. For this tutorial our task is to make sure we output a topology including all highways in the area of interest as we will want to post-process these in the database.

source: osm2po

Change your directory where the osm2po jar file is located. By default osm2po will not include OpenStreetMap highways with pedestrian or cycleway tags which is why we have to make some small changes to the osm2po.config file:

1. We want to process OSM data for these profiles, so change line 190 to wtr.finalMask = car,foot,bike
2. Uncomment lines 221 .default.wtr.tag.highway.service to 230 .default.wtr.tag.railway.rail

Additionally, we want to make sure the topology is produced which can be directly consumed by pgRouting - you will have to uncomment line 341 (postp.0.class = de.cm.osm2po.plugins.postp.PgRoutingWriter) if you are running version 5.1 or greater.

Afterwards run the following command:

java -Xmx512m -jar osm2po-core-[5.5.2]-signed.jar cmd=c prefix=syd your/path/to/sydney-coast.pbf

The prefix syd will produce a new folder in the directory you are currently in which will hold the generated files osm2po will output.

root@5d93dd759514:~/osm2po/syd# ls -la
total 3540
drwxr-xr-x 2 root root    4096 Jan 18 12:05 .
drwxr-xr-x 6 root root    4096 Jan 18 12:05 ..
-rw-r--r-- 1 root root    6461 Jan 18 12:05 jw_orphans.2po
-rw-r--r-- 1 root root  400334 Jan 18 12:05 jw_S040E150.2po
-rw-r--r-- 1 root root       1 Jan 18 12:05 jw_shared.2po
-rw-r--r-- 1 root root  111367 Jan 18 12:05 sv_all.2po
-rw-r--r-- 1 root root  533689 Jan 18 12:05 sw_all.2po
-rw-r--r-- 1 root root 2050861 Jan 18 12:05 syd_2po_4pgr.sql
-rw-r--r-- 1 root root    6848 Jan 18 12:05 syd_2po.log
-rw-r--r-- 1 root root     113 Jan 18 12:05 tm_info.2po
-rw-r--r-- 1 root root  214418 Jan 18 12:05 tn_S040E150.2po
-rw-r--r-- 1 root root    4681 Jan 18 12:05 tr_raw.2po
-rw-r--r-- 1 root root  259173 Jan 18 12:05 tw_raw.2po

The file we are interested in is syd_2po_4pgr.sql which consists of a set of columns including the source osm_id, source and target. Download the osm2po output here if you want to skip this step. Import this data with psql.

psql -h [IP] -U [USER_NAME] -d [DB_NAME] -q -f syd_2po_4pgr.sql

Last but not least we want to import the raw OSM-data to be able to join the topology with it's origin feature using the osm_id to determine what kind of highway type (e.g. primary road, secondary road, footpath, cycleway..) we are dealing with.

The following command will generate a set of tables holding the OSM data (lines planet_osm_line, polygons planet_osm_polygon & points planet_osm_point) and all of their corresponding tags.

osm2pgsql --create --database [DB_NAME] --username [USER_NAME] --host [IP] --port [PORT] --password sydney-coast.pbf

Objective of this tutorial is to guide Dijkstra's algorithm along the coast of Sydney. The way we will be approaching this will bring in quite an imbalance into the network consisting of broken up roads (edges). However, for the purpose of this tutorial this will give you a basic understanding. Later, if and when you are tailoring your own topology, you can think about more sophisticated approaches to adapt the costs in a more delicate fashion, for instance using gravity-based ideas, where points of interest (points or even polygons) may attract the routing algorithm "their way".

The choice being Sydney's coastal walk is quite convenient. There exist many roads you could walk on which are more or less parallel to the coastal footpaths, obviously getting you faster to the destination. The reason is straightforward: the distance is shorter. If you take a quick glimpse at what common routing services usually compute, such as Google Maps, you will understand what we mean and why you may want to do some customizing.

source: Google Maps

If you wanted to have full control over how the algorithm determines the least cost connection, you will have to start thinking about how to update the individual costs of the edges in your topology. The precious thing about OpenStreetMap is that the data is fairly structured and streets will feature a specific class. While a highway may be tagged as a "primary road", a path forbidden for motorized vehicles may be tagged as "footway". With this distinction we can start doing some fun things to the data.

In the most simple use case the cost of an edge could simply be the length of it. If we executed the following query using our topology generated by osm2po in our database and feeding it to pgRouting...

-- find he nearest vertex to the start longitude/latitude
WITH start AS (
  SELECT topo.source --could also be topo.target
  FROM syd_2po_4pgr as topo
  ORDER BY topo.geom_way <-> ST_SetSRID(
    ST_GeomFromText('POINT (151.261360 -33.915983)'),
  4326)
  LIMIT 1
),
-- find the nearest vertex to the destination longitude/latitude
destination AS (
  SELECT topo.source --could also be topo.target
  FROM syd_2po_4pgr as topo
  ORDER BY topo.geom_way <-> ST_SetSRID(
    ST_GeomFromText('POINT (151.28241 -33.890510)'),
  4326)
  LIMIT 1
)
-- use Dijsktra and join with the geometries
SELECT ST_AsText(ST_Union(geom_way))
FROM pgr_dijkstra('
    SELECT id,
         source,
         target,
         ST_Length(ST_Transform(geom_way, 3857)) AS cost
        FROM syd_2po_4pgr',
    array(SELECT source FROM start),
    array(SELECT source FROM destination),
    directed := false) AS di
JOIN   syd_2po_4pgr AS pt
  ON   di.edge = pt.id;

...your result should look like the following (which is very similar to what we got using Google Maps up top):

Well, it's the shortest path and we aren't satisfied with this result as we want to see the coast non-stop. This is where we will exploit the nature of the algorithm and guide it onto footpaths as much as we can (and guess what: the coastal tracks are foopaths). If you remember we imported the OpenStreetMap data at the beginning which we will now join to the topology allowing us to decrease the cost of the edges in question. To this end, we will add an auxiliary column cost_updated to our topology table (which will be consumed by pgRouting later on). Subsequently we will populate this column using a very straightforward logic. If the source osm_id of the record (edge) is tagged footway, pedestrian, living_street, cycleway, track or steps (in planet_osm_line or planet_osm_polygon), we will divide the length of the edge by 1000, otherwise the cost remains the metric length. The division ultimately makes the cost very cheap, i.e. the algorithm will be guided onto this subset of edges.

-- Add aux cost column
ALTER TABLE syd_2po_4pgr ADD COLUMN "cost_updated" float;

-- The edge may either be in planet_osm_line
UPDATE syd_2po_4pgr as t
SET cost_updated =
CASE WHEN
	(l.highway IN ('footway', 'pedestrian', 'living_street', 'cycleway', 'track', 'steps'))
THEN
	ST_Length(ST_Transform(geom_way, 3857))/1000
ELSE
	ST_Length(ST_Transform(geom_way, 3857))
END
FROM planet_osm_line AS l
WHERE t.osm_id = l.osm_id;

-- Or in planet_osm_polygon
UPDATE syd_2po_4pgr as t
SET cost_updated =
CASE WHEN
	(l.highway IN ('footway', 'pedestrian', 'living_street', 'cycleway', 'track', 'steps'))
THEN
	ST_Length(ST_Transform(geom_way, 3857))/1000
ELSE
	ST_Length(ST_Transform(geom_way, 3857))
END
FROM planet_osm_polygon AS l
WHERE t.osm_id = l.osm_id;

Executing the query once again from above and instead of using ST_Length(ST_Transform(geom_way, 3857)) AS cost but cost_updated AS cost, your route will look something like this - enjoy the walk!

Congratulations for completing this tutorial. By now you will have learnt how you can influence Dijkstra's routing algorithm by tinkering with edge costs. With this being a bold and simple tutorial we hope that you get the idea and understand the vast possibilities of different open source tools and frameworks in the global ecosystem of OpenStreetMap and routing.

Please feel free to get in touch with us at enquiry[at]gis-ops.com if you have any further questions, need support or have ideas for other tutorials on pgRouting!