Building on the work of previous posts looking at building routable networks and finding the shortest path in a network, this post will look at creating buffers around nodes based on a cost attribute specifed by the user.
The driving distance function, which comes with pg routing 2.0, creates an alpha shape around a node, normally based on the distance someone could travel from that node, or how far someone could go in a time interval.
For this example, we are looking at where someone could travel in 1km along the network from a central node. The following query selects all the nodes that are within 1 km of node 2000.
Once a routable network has been set up we can start to find the shortest path (using Dijkstra’s algorithm) between two points. From here on, I will be using the OSM data for the London road network.
The query use’s a ‘cost’ column – which could be anything thing you like – to calculate the shortest cost from one node to another. In this example I decided to use travel time (rather than distance).
First, I changed the distance of each vertex (the way table) from km to miles, and then calculated the time it would take to traverse each vertex whether walking, cycling or driving. To make this more accurate, I updated the max speed (forward and backward) along each vertex dependent on its class (as listed in the class table).
This post highlights some very useful commands for entering spatial data into a PostgreSQL database ready for analysis within GIS or using Postgis.
For entering spatial data into PostgreSQL you can either use the ogr tools or pgsql2shp commands. After download, both need to be added to your windows environment variable:
ogr2ogr: input csv
ogr2ogr -overwrite -f "postgresql" pg:"dbname=postgres host=localhost port = 5632 user=postgres password = XXXXX" C:\Link\To\file.csv -lco OVERWRITE=YES
ogr2ogr: input shapefile
ogr2ogr –overwrite –nlt MULTIPOLYGON –f PostgreSQL PG:"dbname=Nathan_spatial host=localhost port=5632 user=postgres password=XXXXX" example.shp
ogr2ogr: input ESRI FileGDB
ogr2ogr –overwrite – skipfailures –f PostgreSQL PG:"dbname=Nathan_spatial host=localhost port=5632 user=postgres password=XXXXX" "C:\somefolder\somegeodatabase.gdb" "example feature class"