7.2. Lesson: Vector Analysis

Vector data can also be analyzed to reveal how different features interact with each other in space. There are many different analysis-related functions in GIS, so we won't go through them all. Rather, we'll pose a question and try to solve it using the tools that QGIS provides.

The goal for this lesson: To ask a question and solve it using analysis tools.

7.2.1. The GIS Process

Before we start, it would be useful to give a brief overview of a process that can be used to solve any GIS problem. The way to go about it is:

1. State the Problem

2. Get the Data

3. Analyze the Problem

4. Present the Results

7.2.2. The Problem

Let's start off the process by deciding on a problem to solve. For example, you are an estate agent and you are looking for a residential property in Swellendam for clients who have the following criteria:

1. It needs to be in Swellendam

2. It must be within reasonable driving distance of a school (say 1km)

3. It must be more than 100m squared in size

4. Closer than 50m to a main road

5. Closer than 500m to a restaurant

7.2.3. The Data

To answer these questions, we're going to need the following data:

1. The residential properties (buildings) in the area

2. The roads in and around the town

3. The location of schools and restaurants

4. The size of buildings

All of this data is available through OSM and you should find that the dataset you have been using throughout this manual can also be used for this lesson.

If you want to download data from another area jump to Introduction Chapter section to read how to do it.

注解

Although OSM downloads have consistent data fields, the coverage and detail does vary. If you find that your chosen region does not contain information on restaurants, for example, you may need to chose a different region.

7.2.4. Follow Along: Start a Project and get the Data

We first need to load the data to work with.

1. Start a new QGIS project

2. If you want you can add a background map. Open the Browser and load the OSM background map from the XYZ Tiles menu.

   

3. In the training_data.gpkg Geopackage database load all the files we will use in this chapter:

   1. landuse

   2. buildings

   3. roads

   4. restaurants

   5. schools

4. Zoom to the layer extent to see Swellendam, South Africa

Before proceeding we should filter the roads layer in order to have only some specific road types to work with.

Some of the roads in OSM dataset are listed as unclassified, tracks, path and footway. We want to exclude these from our dataset and focus on the other road types, more suitable for this exercise.

Moreover, OSM data might not be updated everywhere and we will also exclude NULL values.

1. Right click on the roads layer and choose Filter....

2. In the dialog that pops up we can filter these features with the following expression:

   "highway" NOT IN ('footway','path','unclassified','track') OR "highway" != NULL
   

   The concatenation of the two operators NOT and IN means to exclude all the unwanted features that have these attributes in the highway field.

   != NULL combined with the OR operator is excluding roads with no values in the highway field.

   You will note the icon next to the roads layer that helps you remember that this layer has a filter activated and not all the features are available in the project.

The map with all the data should look like the following one:

7.2.5. Try Yourself Convert Layers' CRS

Because we are going to be measuring distances within our layers, we need to change the layers' CRS. To do this, we need to select each layer in turn, save the layer to a new one with our new projection, then import that new layer into our map.

You have many different options, e.g. you can export each layer as a new Shapefile, you can append the layers to an existing GeoPackage file or you can create another GeoPackage file and fill it with the new reprojected layers. We will show the last option so the training_data.gpkg will remain clean. But feel free to choose the best workflow for yourself.

注解

In this example, we are using the WGS 84 / UTM zone 34S CRS, but you may use a UTM CRS which is more appropriate for your region.

1. Right click the roads layer in the Layers panel;

2. Click Export --> Save Features As...;

3. In the Save Vector Layer As dialog choose GeoPackage as Format;

4. Click on ... of File name parameter and name the new GeoPackage as vector_analysis;

5. Change the Layer name as roads_34S;

6. Change the CRS parameter to WGS 84 / UTM zone 34S;

7. Finally click on OK:

   

   This will create the new GeoPackage database and fill it with the roads_34S layer.

8. Repeat this process for each layer, creating a new layer in the vector_analysis.gpkg GeoPackage file with _34S appended to the original name and removing each of the old layers from the project.

   注解

   When you choose to save a layer to an existing GeoPackage, QGIS will append that layer in the GeoPackage.

9. Once you have completed the process for each layer, right click on any layer and click Zoom to layer extent to focus the map to the area of interest.

Now that we have converted OSM's data to a UTM projection, we can begin our calculations.

7.2.6. Follow Along: Analyzing the Problem: Distances From Schools and Roads

QGIS allows you to calculate distances from any vector object.

1. Make sure that only the roads_34S and buildings_34S layers are visible, to simplify the map while you're working

2. Click on the Processing ‣ Toolbox to open the analytical core of QGIS. Basically: all algorithms (for vector and raster) analysis are available within this toolbox.

3. We start by calculating the area around the roads_34S by using the Buffer algorithm. You can find it expanding the Vector Geometry group.

   

   Or you can type buffer in the search menu in the upper part of the toolbox:

   

4. Double click on it to open the algorithm dialog

5. Set it up like this

   

6. The default Distance is in meters because our input dataset is in a Projected Coordinate System that uses meter as its basic measurement unit. You can use the combo box to choose other projected units like kilometers, yards, etc.

   注解

   If you are trying to make a buffer on a layer with a Geographical Coordinate System, Processing will warn you and suggest to reproject the layer to a metric Coordinate System.

7. By default Processing creates temporary layers and adds them to the Layers panel. You can also append the result to the GeoPackage database by:

   1. clicking on the ... button and choose Save to GeoPackage...

   2. naming the new layer roads_buffer_50m

   3. and saving it in the vector_analysis.gpkg file

   

8. Click on Run and then close the Buffer dialog.

Now your map will look something like this:

If your new layer is at the top of the Layers list, it will probably obscure much of your map, but this gives you all the areas in your region which are within 50m of a road.

However, you'll notice that there are distinct areas within your buffer, which correspond to all the individual roads. To get rid of this problem:

1. Uncheck the roads_buffer_50m layer and re-create the buffer using the settings shown here:

   

   Note that we're now checking the Dissolve result box

2. Save the output as roads_buffer_50m_dissolved

3. Click Run and close the Buffer dialog again

Once you've added the layer to the Layers panel, it will look like this:

Now there are no unnecessary subdivisions.

注解

The Short Help on the right side of the dialog explains how the algorithm works. If you need more information, just click on the Help button in the bottom part to open a more detailed guide of the algorithm.

7.2.7. Try Yourself Distance from schools

Use the same approach as above and create a buffer for your schools.

It needs to be 1 km in radius. Save the new layer in the vector_analysis.gpkg file as schools_buffer_1km_dissolved.

Check your results

7.2.8. Follow Along: Overlapping Areas

Now we have areas where the road is 50 meters away and there's a school within 1 km (direct line, not by road). But obviously, we only want the areas where both of these criteria are satisfied. To do that, we'll need to use the Intersect tool. You can find it in Vector Overlay group within Processing ‣ Toolbox.

1. Set it up like this:

   

   • The input layers are the two buffers

   • The saving location is, once again, the vector_analysis.gpkg GeoPackage

   • And the output layer name is road_school_buffers_intersect

2. Click Run.

   In the image below, the blue areas show us where both distance criteria are satisfied at once!

   

3. You may remove the two buffer layers and only keep the one that shows where they overlap, since that's what we really wanted to know in the first place:

   

7.2.9. Follow Along: Extract the Buildings

Now you've got the area that the buildings must overlap. Next, you want to extract the buildings in that area.

1. Look for the menu entry Vector Selection ‣ Extract by location within Processing ‣ Toolbox

2. Set up the algorithm dialog like in the following picture

   

3. Click Run and then close the dialog

4. You'll probably find that not much seems to have changed. If so, move the well_located_houses layer to the top of the layers list, then zoom in.

   

   The red buildings are those which match our criteria, while the buildings in green are those which do not.

5. Now you have two separated layers and can remove buildings_34S from layer list.

7.2.10. Try Yourself Further Filter our Buildings

We now have a layer which shows us all the buildings within 1km of a school and within 50m of a road. We now need to reduce that selection to only show buildings which are within 500m of a restaurant.

Using the processes described above, create a new layer called houses_restaurants_500m which further filters your well_located_houses layer to show only those which are within 500m of a restaurant.

Check your results

7.2.11. Follow Along: Select Buildings of the Right Size

To see which buildings are of the correct size (more than 100 square meters), we first need to calculate their size.

1. Select the houses_restaurants_500m layer and open the Field Calculator by clicking on the button in the main toolbar or within the attribute table

2. Set it up like this

   

   We are creating the new field AREA that will contain the area of each building square meters.

3. Click OK. The AREA field has been added at the end of the attribute table.

4. Click the edit mode button again to finish editing, and save your edits when prompted.

5. Build a query as earlier in this lesson

   

6. Click OK.

Your map should now only show you those buildings which match our starting criteria and which are more than 100m squared in size.

7.2.12. Try Yourself

Save your solution as a new layer, using the approach you learned above for doing so. The file should be saved within the same GeoPackage database, with the name solution.

7.2.13. In Conclusion

Using the GIS problem-solving approach together with QGIS vector analysis tools, you were able to solve a problem with multiple criteria quickly and easily.

7.2.14. What's Next?

In the next lesson, we'll look at how to calculate the shortest distance along the road from one point to another.