6.3. Lesson: Analiza Rețelelor

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. basic Follow Along: The Tools and the Data

You can find all the network analysis algorithms in the Processing ► Network Analysis menu. You can see that there are many tools available:

../../../_images/select_network_algorithms.png

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.

../../../_images/network_map.png

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. basic Calculate the shortest path (point to point)

The Network analysis ► Shortest path (point to point) 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.

  1. Open the Shortest path (point to point) algorithm

  2. Select network_lines for Vector layer representing network

  3. Use Shortest for Path type to calculate

    Use these two points as starting and ending points for the analysis:

    ../../../_images/start_end_point.png

  4. 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.

  5. Do the same thing, but choosing the location tagged with Ending point for End point (x, y)

  6. Click on the Run button:

    ../../../_images/shortest_point.png

  7. A new line layer is created representing the shortest path between the chosen points. Uncheck the network_lines layer to see the result better:

    ../../../_images/shortest_point_result.png

  8. 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:

    ../../../_images/shortest_point_attributes.png

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

6.3.3. moderate 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?

Răspuns

  1. Open Network Analysis ► Shortest Path (Point to Point) and fill the dialog as:

    ../../../_images/fastest_path_result.png

  2. Make sure that the Path type to calculate is Fastest.

  3. Click on Run and close the dialog.

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

    ../../../_images/fastest_path_attribute.png

6.3.4. moderate 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.

  1. Open the Shortest path (point to point) algorithm

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

  3. Choose Fastest as the Path type to calculate

  4. Open the Advanced parameter menu

  5. Change the Default speed (km/h) from the default 50 value to 4

    ../../../_images/shortest_path_advanced.png

  6. Click on Run

  7. 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.

  8. Open the field calculator by clicking on the calculateField icon

  9. Add a new field to store the path cost in minutes.

    ../../../_images/shortest_path_conversion.png

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. hard Shortest path with speed limit

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

../../../_images/speed_limit.png

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 Network analysis ► Shortest path (point to point) and we will manually choose the start and end points.

  1. Open the Network analysis ► Shortest path (point to point) algorithm

  2. Select network_lines for the Vector layer representing network parameter

  3. Choose Fastest as the Path type to calculate

  4. Select the Start point (x, y) and End point (x, y) as we did before

  5. Open the Advanced parameters menu

  6. Choose the speed field as the Speed Field parameter. With this option the algorithm will take into account the speed limits for each road.

    ../../../_images/speed_limit_parameters.png

  7. Click on the Run button

  8. Turn off the network_lines layer to better see the result

    ../../../_images/speed_limit_result.png

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

6.3.6. moderate Service area (from layer)

The Network Analysis ► Service area (from layer) algorithm can answer the question: given a point layer, what are all the reachable areas given a distance or a time value?

Notă

The Network Analysis ► Service area (from point) 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.

  1. Uncheck all the layers except network_points

  2. Open the Network Analysis ► Service area (from layer) algorithm

  3. Choose network_lines for Vector layer representing network

  4. Choose network_points for Vector layer with start points

  5. Choose Shortest in Path type to calculate

  6. Enter 250 for the Travel cost parameter

  7. Click on Run and close the dialog

    ../../../_images/service_area.png

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

    ../../../_images/service_area_result.png

Cool isn’t it?

6.3.7. In Conclusion

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. What’s Next?

Mai departe, veți vedea cum să rulați algoritmii statisticilor spațiale asupra seturilor de date vectoriale.