If you need to analyse map data in order to find areas which intersect or points of interest within a boundary for example, then you have a need for spatial analysis.
QGIS uses the geographical elements within your layers to enable you to make more sense of your data. This principle is based on the fact that two vector layers can have some sort of relationship between their respective geometries: some points are within a polygon, a polygon has some lines crossing or two polygons overlap partially or totally.
Several functions such as buffers and SQL selections allow you to use the geographic elements of your layers in your analysis.
The SQL selection function within QGIS can be used to extract data from a layer which corresponds to certain criteria.
QGIS uses the geographical elements within your layers to enable you to make more sense of your data. This principle is based on the fact that two vector layers can have some sort of relationship between their respective geometries: some points are within a polygon, a polygon has some lines crossing or two polygons overlap partially or totally.
Several functions such as buffers and SQL selections allow you to use the geographic elements of your layers in your analysis.
The SQL selection function within QGIS can be used to extract data from a layer which corresponds to certain criteria.
Geographic joins
Intersects: this geographic function compares data within two tables.
A point, line or polygon intersects another point, line or polygon when they have at least a node in common: a network crosses another network for example a road or a railway line.
The intersection means a geographical geometry touches or goes through another.
Contains and its reciprocal Within means geographical geometries are within another geometry.
A geometry A contains a geometry B when it contains the centroid of B.
When B is within A: for example a catchment area contains clients represented by point geometries.
Entirely within and its reciprocal Contains entire.
A geometry A is entirely within a geometry B when the group of nodes of A are within B.
According to the type of the compared geometry, according to needs, depends on which of these operators are chosen.
Generally, use the Intersects function to compare polygons or from polygons to lines.
Intersects: this geographic function compares data within two tables.
A point, line or polygon intersects another point, line or polygon when they have at least a node in common: a network crosses another network for example a road or a railway line.
The intersection means a geographical geometry touches or goes through another.
Contains and its reciprocal Within means geographical geometries are within another geometry.
A geometry A contains a geometry B when it contains the centroid of B.
When B is within A: for example a catchment area contains clients represented by point geometries.
Entirely within and its reciprocal Contains entire.
A geometry A is entirely within a geometry B when the group of nodes of A are within B.
According to the type of the compared geometry, according to needs, depends on which of these operators are chosen.
Generally, use the Intersects function to compare polygons or from polygons to lines.
Use Contains/Within to compare points within polygons.
Spatial analysis example
After having created some areas of interest, let’s analyse the impact of these areas. As a starting point look at those areas which intersect with the buffers PA1 to PA7.
Spatial analysis example
After having created some areas of interest, let’s analyse the impact of these areas. As a starting point look at those areas which intersect with the buffers PA1 to PA7.
Here we have opened a project containing three layers. The population information is stored in a layer called Ilots_Saumur within a field called PSDC99. We can use the virtual layers function to find those areas which intersect with the buffer zones.. We can also find the sum of the populations for the areas which intersect the buffers. Additionally, we can create a geographic join by using the Intersects option. Prior to the introduction of the virtual layers function the individual layers in this project would have had to have been converted into a relational database format such as PostGIS or Oracle Spatial in order to access the SQL functionality needed.
To access the virtual layers function open the DB Manager from the top menu.
To access the virtual layers function open the DB Manager from the top menu.
Click on the > icon next to the Virtual Layers option.
Next click on the icon > next to the Project layers option
The dialogue box in the Info section states that Project layers: Not connected.
In the Providers section double click on the Project layers option.
The various layers are now displayed
Click on the Ilots_Saumur option and view the Info, Table and Preview tabs
From the top menu click on Database and choose the SQL window option
When the dialogue box appears click on the SQL query builder icon which is just below the Info tab
In the Tables drop down list choose the Ilots_Saumur and Buffer_Stop_Points tables
Click in the Columns section and from the Columns drop down list choose
Ilots_Saumur.PSDC99
Buffer_Stop_Points.ID_PA
Ilots_Saumur.geometry
Buffer_Stop_Points.geometry
Ilots_Saumur.PSDC99
Buffer_Stop_Points.ID_PA
Ilots_Saumur.geometry
Buffer_Stop_Points.geometry
Click within the Where section and select ST_Intersects from the Functions drop down list
Choose Ilots_Saumur.geometry from the Columns section
Put a comma after this entry
Then choose Buffer_Stop_Points.geometry and add a closing bracket
Click in the Group by section and choose Buffer_Stop_Points.ID_PA from the Columns drop down list as in the following screen shot
Then choose Buffer_Stop_Points.geometry and add a closing bracket
Click in the Group by section and choose Buffer_Stop_Points.ID_PA from the Columns drop down list as in the following screen shot
Click on the OK button
Click on the Execute button or Ctrl + R
Click on the Execute button or Ctrl + R
Select Load as new layer by ticking in the box
The Co-ordinate Reference System dialogue box appears. Click OK to accept the default co-ordinate system
The Co-ordinate Reference System dialogue box appears. Click OK to accept the default co-ordinate system
Name the layer as “Population data where buffer intersects” in the Layer Name section
The layer is loaded into QGIS
The layer is loaded into QGIS
If you wanted the find the population data for the areas which intersect the seven buffers we could modify the SQL as follows
This will create a new layer similar to the previous one but with a different attribute table as shown below.
With virtual layers you can create more complex SQL statements than is possible with the QGIS field calculator. Now it is possible to use ESRI shape and MapInfo mif files to create SQL statements previously only feasible with PostGIS, MySQL and Oracle Spatial formats.
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