ANINTRODUCTIONTOGIS


Combine vector layers


After having seen the basics of vector manipulation with QGIS, we will now see how the different layers can be combined.

As a case study, we will identify the buildings most likely to be flooded. The procedure is similar to the one summarized in the animation below:


But, for this case study, we will focus only on the municipality of Joze (63), along the Allier river.

Data come from the BD TOPO 63, which contains a vectorial description, structured in objects, of the elements of the elements of the Puy de Dôme department.

I. Crop vector layers

First, open QGIS and set the system of coordinates of your project as Lambert 93 (EPSG:2154), which is the reference coordinate system for France:

  • Project > Properties > CRS

To simplify, the main vector layers for this tutorial are available here.

You may download and add two layers (Layer > Add layer > Add vector layer) to your project:

  • COMMUNE: layer of the municipalities of the Puy-de-Dôme;

  • COURS_D_EAU: layer of the waterways of the Puy-de-Dôme.


Figure 1 Add vector layer


Practise: From the “COMMUNE” layer, select the municipality of Joze (using the ‘NOM’ column) and create a new layer with only this municipality.


We will now keep only the waterways included in the municipality of Joze. To do this, we are going to perform an Intersection between the layer with the borders of Joze and the layer of the rivers of the Puy-de-Dôme.

To open the Intersection window:

  • Vector > Geoprocessing tools > Intersection

You may then fill the window as follows:

Figure 2 Perform an intersection


II. Create buffer layer

Not all rivers have the same importance: “l’Allier” is the main river of the municipality.

Practise: From the layer of the waterways of Joze, select the river “l’Allier” (using the ‘TOPONYME’ column) and create a new layer with only this river.


We will now create a 500 meters Buffer around this river which will represent the risk of flooding zone.

To open the Buffer window:

  • Vector > Geoprocessing tools > Buffer

You may then fill the window as follows:

Figure 3 Create a buffer layer


Practise: Perform an intersection to crop the newly created buffer layer with the border of Joze.


III. Identify buildings likely to be flooded

Add a new layer with the buildings of Joze to your project:

  • Bati_Joze: layer with the building of Joze.
Figure 4 Add buildings to the map


We can now estimate the number of buildings likely to be flooded.

Practise: Perform an intersection to select the building inside the flooding zone. How many buildings are inside this zone (you may use the attribute table to answer this question)?