Wichtig

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# 6.3. Lesson: Network Analysis

Calculating the shortest distance between two points is a common GIS task. Tools for this can be found in the Processing Toolbox.

**The goal for this lesson:** learn to use
Network analysis algorithms.

## 6.3.1. ★☆☆ Follow Along: The Tools and the Data

You can find all the network analysis algorithms in the

menu. You can see that there are many tools available:Open the project `exercise_data/network_analysis/network.qgz`

.
It contains two layers:

`network_points`

`network_lines`

The network_lines layer has already a style that helps to understand the road network.

The shortest path tools provide ways to calculate either the shortest or the fastest path between two points of a network, given:

start and end points selected on the map

start point selected on the map and end points taken from a point layer

start points taken from a point layer and end point selected on the map

Let’s start.

## 6.3.2. ★☆☆ Calculate the shortest path (point to point)

The

allows you to calculate the shortest distance between two manually selected points on the map.In this example we will calculate the **shortest** (not fastest) path
between two points.

Open the Shortest path (point to point) algorithm

Select network_lines for Vector layer representing network

Use

`Shortest`

for Path type to calculateUse these two points as starting and ending points for the analysis:

Click on the … button next to Start point (x, y) and choose the location tagged with

`Starting Point`

in the picture. Enable the snapping options for an accurate selection. The coordinates of the clicked point are added.Do the same thing, but choosing the location tagged with

`Ending point`

for End point (x, y)Click on the Run button:

A new line layer is created representing the shortest path between the chosen points. Uncheck the

`network_lines`

layer to see the result better:Open the attribute table of the output layer. It contains three fields, representing the coordinates of the start and end points and the

**cost**.We chose

`Shortest`

as Path type to calculate, so the**cost**represent the**distance**, in layer units, between the two locations.In our case, the

*shortest*distance between the chosen points is around`1000`

meters:

Now that you know how to use the tool, feel free to test other locations.

## 6.3.3. ★★☆ Try Yourself: Fastest path

With the same data of the previous exercise, try to calculate the fastest path between the two points.

How much time do you need to go from the start to the end point?

Answer

Open

and fill the dialog as:Make sure that the Path type to calculate is

`Fastest`

.Click on Run and close the dialog.

Open now the attribute table of the output layer. The cost field contains the travel time between the two points (as fraction of hours):

## 6.3.4. ★★☆ Follow Along: Advanced options

Let us explore some more options of the Network Analysis tools.
In the previous exercise we
calculated the **fastest** route between two points.
As you can imagine, the time depends on the travel **speed**.

We will use the same layers and starting and ending points of the previous exercises.

Open the Shortest path (point to point) algorithm

Fill the Input layer, Start point (x, y) and End point (x, y) as we did before

Choose

`Fastest`

as the Path type to calculateOpen the Advanced parameter menu

Change the Default speed (km/h) from the default

`50`

value to`4`

Click on Run

Once the algorithm is finished, close the dialog and open the attribute table of the output layer.

The

*cost*field contains the value according to the speed parameter you have chosen. We can convert the*cost*field from hours with fractions to the more readable*minutes*values.Add a new field to store the path cost in minutes.

That’s it!
Now you know how many minutes it will take to get from one point to
the other one if the whole network speed is at `4 km/h`

.

## 6.3.5. ★★★ Shortest path with speed limit

The Network analysis toolbox has other interesting options. Looking at the following map:

we would like to know the **fastest** route considering the **speed
limits** of each road (the labels represent the speed limits in km/h).
The shortest path without considering speed limits would of course be
the purple path.
But in that road the speed limit is 20 km/h, while in the green road
you can go at 100 km/h!

As we did in the first exercise, we will use the

and we will manually choose the start and end points.Open the

algorithmSelect network_lines for the Vector layer representing network parameter

Choose

`Fastest`

as the Path type to calculateSelect the Start point (x, y) and End point (x, y) as we did before

Open the Advanced parameters menu

Choose the

*speed*field as the Speed Field parameter. With this option the algorithm will take into account the speed limits for each road.Click on the Run button

Turn off the

`network_lines`

layer to better see the result

As you can see the fastest route does not correspond to the shortest one.

## 6.3.6. ★★☆ Service area (from layer)

The

algorithm can answer the question: given a point layer, what are all the reachable areas given a distance or a time value?Bemerkung

The

is the same algorithm, but it allows you to manually choose the point on the map.Given a distance of `250`

meters we want to know how far we can go
on the network from each point of the network_points layer.

Uncheck all the layers except

`network_points`

Open the

algorithmChoose

`network_lines`

for Vector layer representing networkChoose

`network_points`

for Vector layer with start pointsChoose

`Shortest`

in Path type to calculateEnter

`250`

for the Travel cost parameterClick on Run and close the dialog

The output layer represents the maximum path you can reach from the point features given a distance of 250 meters:

Cool isn’t it?

## 6.3.7. Zusammengefasst

Now you know how to use Network analysis algorithm to solve shortest and fastest path problems.

We are now ready to perform some spatial statistic on vector layer data. Let’s go!

## 6.3.8. Was als nächstes?

Next you’ll see how to run spatial statistics algorithms on vector datasets.