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6.2. 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, so we won’t go through them all. Rather, we will pose a question and try to solve it using the tools that QGIS provides.

O objetivo desta lição: Fazer uma pergunta e respondê-la usando ferramentas de análise.

6.2.1. ★☆☆ The GIS Process

Antes de começarmos, seria útil dar uma breve visão geral de um processo que pode ser usado para resolver um problema. A maneira de fazer isso é:

1. Definir o Problema

2. Coletar os Dados

3. Analisar o Problema

4. Demonstrar os Resultados

6.2.2. ★☆☆ The Problem

Comecemos o processo definindo o problema a ser solucionado. Por exemplo, você é um funcionário público e está procurando por propriedades residenciais em Swellendam por clientes que se encaixam nos seguintes critérios:

1. It needs to be in Swellendam

2. Deve estar a uma distância razoável de uma escola (digamos 1 km)

3. Deve ter mais de 100m quadrados de tamanho

4. A menos de 50m de uma estrada principal

5. A menos de 500m de um restaurante

6.2.3. ★☆☆ The Data

Para responder a essas perguntas, vamos precisar dos seguintes dados:

1. As propriedades residenciais (edifícios) na área

2. As estradas dentro e ao redor da cidade

3. A localização de escolas e restaurantes

4. O tamanho dos prédios

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

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

Nota

Aunque hay coherencia en los campos de datos que encontramos en las descargas de OSM, pueden variar en su cobertura y detalle. Si ves, por ejemplo, que la región que has elegido no contiene información sobre restaurantes, quizás necesitas elegir otra región.

6.2.4. ★☆☆ Follow Along: Start a Project and get the Data

Primeiro precisamos carregar os dados para trabalhar.

1. Inicie um novo projeto QGIS

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, you will find most the datasets we will use in this chapter:

   1. prédios

   2. estradas

   3. restaurantes

   4. escolas

   Load them, and also landuse.sqlite.

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

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

   Some roads in OSM datasets 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.

5. Right click on the roads layer and choose Filter….

6. Na caixa de diálogo exibida, filtramos essas feições com a seguinte expressão:

   "highway" NOT IN ('footway', 'path', 'unclassified', 'track') AND "highway" IS NOT NULL
   

   The concatenation of the two operators NOT and IN excludes all the features that have these attribute values in the highway field.

   IS NOT NULL combined with the AND operator excludes roads with no value in the highway field.

   Note the icon next to the roads layer. It helps you remember that this layer has a filter activated, so some features may not be available in the project.

O mapa com todos os dados deve se parecer com o seguinte:

6.2.5. ★☆☆ Try Yourself: Convert Layers’ CRS

Como vamos medir distâncias dentro de nossas camadas, precisamos alterar o SRC das camadas. Para fazer isso, precisamos selecionar cada camada por vez, salvar a camada em uma nova com nossa nova projeção e importar essa nova camada para o nosso mapa.

You have many different options, e.g. you can export each layer as an ESRI Shapefile format dataset, 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. Feel free to choose the best workflow for yourself.

Nota

In this example, we are using the WGS 84 / UTM zone 34S CRS, but you should 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 … for the File name, and name the new GeoPackage vector_analysis

5. Change the Layer name to roads_34S

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

7. Click on OK:

   

   This will create the new GeoPackage database and add 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.

   On macOS, press the Replace button in the dialog that pops up to allow QGIS to overwrite the existing GeoPackage.

   Nota

   When you choose to save a layer to an existing GeoPackage, QGIS will add that layer next to the existing layers in the GeoPackage, if no layer of the same name already exists.

9. Remove each of the old layers from the project

10. Once you have completed the process for all the layers, 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 data to a UTM projection, we can begin our calculations.

6.2.6. ★☆☆ Follow Along: Analyzing the Problem: Distances From Schools and Roads

O QGIS permite calcular distâncias entre qualquer objeto vetorial.

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 in this toolbox.

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

   

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

   

4. Clique duas vezes nele para abrir a caixa de diálogo do algoritmo

5. Select roads_34S as Input layer, set Distance to 50 and use the default values for the rest of the parameters.

   

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.

   Nota

   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. Saving it in the vector_analysis.gpkg file

   

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

   Ahora tu mapa se parece un poco a esto:

   

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.

Notice that there are distinct areas within your buffer, which correspond to each individual road. To get rid of this problem:

1. Uncheck the roads_buffer_50m layer and re-create the buffer with Dissolve results enabled.

   

2. Save the output as roads_buffer_50m_dissolved

3. Click Run and close the Buffer dialog

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

Ahora no hay subdivisiones innecesarias.

Nota

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.

6.2.7. ★☆☆ Try Yourself: Distance from schools

Usa el mismo enfoque que anteriormente y crea un buffer para tus colegios.

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

Responda

• Your buffer dialog should look like this:

  

The Buffer distance is 1 kilometer.

• The Segments to approximate value is set to 20. This is optional, but it’s recommended, because it makes the output buffers look smoother. Compare this:

  

  Para isso:

  

The first image shows the buffer with the Segments to approximate value set to 5 and the second shows the value set to 20. In our example, the difference is subtle, but you can see that the buffer’s edges are smoother with the higher value.

6.2.8. ★☆☆ Follow Along: Overlapping Areas

Now we have identified areas where the road is less than 50 meters away and areas where there is 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 will need to use the Intersect tool. You can find it in Vector Overlay group in the Processing Toolbox.

1. Use the two buffer layers as Input layer and Overlay layer, choose vector_analysis.gpkg GeoPackage in Intersection with Layer name road_school_buffers_intersect. Leave the rest as suggested (default).

   

2. Haz clic en Run.

   Na imagem abaixo, as áreas azuis são onde ambos os critérios de distância são satisfeitos.

   

3. Usted puede borrar las dos capas buffer y solo mantener la que muestra la superposición, dado que eso era lo que queriamos conocer en primer lugar:

   

6.2.9. ★☆☆ Follow Along: Extract the Buildings

Agora você tem a área em que as construções devem se sobrepor. Em seguida, você deseja extrair as construções nessa área.

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

2. Select buildings_34S in Extract features from. Check intersect in Where the features (geometric predicate), select the buffer intersection layer in By comparing to the features from. Save to the vector_analysis.gpkg, and name the layer well_located_houses.

   

3. Click Run and close the dialog

4. You will 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.

   

   Os prédios vermelhos são aqueles que atendem aos nossos critérios, enquanto os prédios em verde são aqueles que não atendem.

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

6.2.10. ★★☆ Try Yourself: Further Filter our Buildings

Ahora tenemos una capa que nos muestra los edificios en un radio de 1km de una escuela y a menos de 50m de una carretera. Ahora tenemos que reducir la selección para que sólo nos muestre los edificios que están a menos de 500 metros de un restaurante.

Usando os processos descritos acima, crie uma nova camada chamada casas_restaurantes_500m, que filtra ainda mais a sua camada casas_bem_localizadas para mostrar apenas aqueles que estão a 500m de um restaurante.

Responda

To create the new houses_restaurants_500m layer, we go through a two step process:

1. Primeiro, criar um buffer de 500m ao redor dos restaurantes e adicionar a camada ao mapa:

   

   

2. Em seguida, extrair edifícios dentro dessa área de buffer:

   

Seu mapa agora deve mostrar apenas os prédios que estão a 50m de uma estrada, 1km de uma escola e 500m de um restaurante:

6.2.11. ★☆☆ Follow Along: Select Buildings of the Right Size

Para ver quais edifícios são do tamanho correto (mais de 100 metros quadrados), precisamos calcular seu tamanho.

1. Select the houses_restaurants_500m layer and open the Field Calculator by clicking on the ^{Open Field Calculator} button in the main toolbar or in the attribute table window

2. Select Create a new field, set the Output field name to AREA, choose Decimal number (real) as Output field type, and choose $area from the Geometry group.

   

   O novo campo AREA conterá a área de cada edifício em metros quadrados.

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

4. Click the ^{Toggle Editing} button to finish editing, and save your edits when prompted.

5. In the Source tab of the layer properties, set the Provider Feature Filter to "AREA >= 100.

   

6. Clique em OK.

Seu mapa agora deve mostrar apenas os edifícios que correspondem aos nossos critérios iniciais e que têm mais de 100 metros quadrados de tamanho.

6.2.12. ★☆☆ Try Yourself:

Salve sua solução como uma nova camada, usando a abordagem que você aprendeu acima para fazer isso. O arquivo deve ser salvo dentro do mesmo banco de dados GeoPackage, com o nome solução.

6.2.13. In Conclusion

Usando a abordagem de solução de problemas GIS junto com as ferramentas de análise vetorial do QGIS, você conseguiu resolver um problema com vários critérios de maneira rápida e fácil.

6.2.14. What’s Next?

Na próxima lição, veremos como calcular a distância mais curta ao longo das estradas de um ponto a outro.