Generating the shortest paths for all students

So far, we have only looked at the shortest path between all the given segments of road in the city. Now, given the student location, let's look at where student traffic will accumulate.

Finding the associated segment for a student location

By following these steps, we will join attributes from the closest road segment—including the associated topological and travel attributes—to each student location. Perform the following steps:

Install the NNJoin plugin.
Navigate to Plugins | NNJoin | NNJoin.
Enter the following parameters:
- Input vector layer: students
- Join vector layer: newark_osm
- Output layer: students_topology
Click on OK.
Import students_topology into the packt_c4 database using Database Manager.

The following image shows the parameters as entered into the NNJoin plugin:

Finding the associated segment for a student location

Calculating the accumulated shortest paths by segment

We want to find which routes are the most popular given the student locations, network characteristics, and school location. The following steps will produce an accumulated count of the student traffic along each network segment:

Go to Database | DB Manager | DB Manager.
Select the database to be updated.
Navigate to Database | SQL window.

We want to run a SQL command that will do a shortest path calculation for each student and find the total number of students traveling on each road segment. This query may be very slow. Enter the following code:

SELECT id, geom, count(id1)
FROM
(SELECT *
  FROM pgr_kdijkstraPath(
      'SELECT id, source, target, traveltime_min as cost FROM newark_osm',
      1, (SELECT array_agg(join_target) FROM students_topology), false, false  
  ) a,
  newark_osm b
WHERE a.id3=b.id) x
GROUP BY id, geom

Select the Load as new layer option.
Select id as your Column with unique integer values and geom as your Geometry column, as shown in the following screenshot:

Flow symbology

We want to visualize the number of students on each segment in a way that really accentuates the segments that have a high number of students traveling on them. A great way to do this is with a symbology expression. This produces a graduated symbol as would be found in other GIS packages. Perform the following steps:

Navigate to Layer | Properties | Style.
Click on Simple line to access the symbology expressions, as shown in the following screenshot:
In the Pen width section, click on the advanced menu to edit the symbology expression.
Natural log is a good function to use to get a more linear growth rate when a value grows exponentially. This helps us to produce a symbology that varies in a more visually appealing way. Enter the following expression into the Expression string builder dialog:
```
ln("count")
```

Now that we have mapped the variable sized symbol to the natural log of the count of students traveling on each segment, we will see a pleasing visualization of the "flow" of students traveling on each road segment. The student layer, showing the student locations, is displayed alongside the flow to better illustrate what the flow visualization shows.

Previous Chapter

Creating the travel time isochron polygons

Next Chapter

Web applications – creating safe corridors

Table of Contents for QGIS: Becoming a GIS Power User

Generating the shortest paths for all students

Finding the associated segment for a student location

Calculating the accumulated shortest paths by segment

Flow symbology

Table of Contents for
QGIS: Becoming a GIS Power User