Performing Spatial Queries
Spatial queries are core to many types of GIS analysis. In QGIS, this
functionality is available via the Spatial Query plugin.
Overview of the task
We will be working with 2 datasets - a lines layer representing rivers and a
point layer representing cities. The task is to run a spatial query to find all
cities that are within 10 kms of a river.
Other skills you will learn
- Opening .zip files directly in QGIS.
- Choosing an appropriate projection and re-projecting vector data.
- Creating buffers.
- Selecting features using SQL-like expressions.
- Coverting a shpefile to a KML file.
- Validating your results using Google Earth.
- Once you have downloaded the data, open QGIS. Go to
- Click Browse and navigate to the folder where you downloaded the
- Hold the
Shift key and click on both the zip files to select them.
- You will be asked to choose a layer from the zip archive.
ne_10m_rivers_lake_centerlines.shp and click OK.
- Since you have selected multiple files, repeat the process for the next file.
Select 10m_populated_places_simple.shp and click OK.
- You will see both the shapefiles now loaded in QGIS.
- We will be created buffers around the point and line layers. The Buffer
geoprocessing tool in QGIS uses layer units to calculate buffer distances.
The layers we have are in Geographic Coordinate Reference System (CRS) with
the unit of degrees. This is not appropriate as we want our analysis to use
metres or kilometres. To achive this, we must re-project our layers to a
Projected Coordinate Reference System (CRS). Right-click on the
10m_populated_places_simple layer and choose Save As.
- In the Save vector layer as... dialog, click Browse
next to Save as and select the output file location. Name the
output file as
populated_places_reprojected.shp. Next, click the
Browse button next to CRS.
- Now we must choose an appropriate CRS for our purpose. For creating buffers,
a Azimuthal Equidistant projection would be best suited as radial distances
around the center of the projection are accurate. In our case, since the
dataset is global, we will choose a world projection. In the
Coordinate Reference System Selector dialog, start searching for
world az.. and you will see the results show up. Select the
World_Azimuthal_Equidistant and click OK.
The World_Azimuthal_Equidistant projection spans 90 degrees from the
origin. Here the origin being 0 degrees longitude, the only data contained
within +/- 90 degrees longitude will be converted.
- Back in Save vector layer as ... dialog, check the box next to
Add saved file to map and click OK.
- Repeat the re-projection process for the
layer and save the new layer as
- Now you will have 4 layers in your Layers Panel. Un-check the
boxes next to the original layers to display only the re-projected layers.
The re-projected layers are still being shown in the Geographic CRS
because of a setting. Let’s turn that off. Click on the
Project Properties button. This setting can also be accessed
- In the CRS tab of the Project Properties dialog,
un-check the box next to Enable on-the-fly CRS transformation.
- Back in the main QGIS window, right-click on any one of the re-projected
layers and select Zoom to Layer Extent.
- Now you will see the data in the layer’s CRS. We will now create buffers for
both the datasets. Click
- In the Buffer tool, select
layer as Input. Enter the buffer distance as 10000. Note that we want a
buffer of 10kms and since the CRS units are metres, we need to enter 10,000.
Enter the output file name as
- Once the buffer processing is over, click the Yes to add the
newly created layer to the TOC.
- Repeat the same buffer process for the
rivers_lake_reprojected layer and
create an output file named
rivers_lake_buffer contains features that are both rivers as well as
lakes. Our analysis calls for using only river features, so we will run a
query to select only river features. Right-click on the
rivers_lake_buffer layer and select Open Attribute Table.
- You will see that the featurecla attribute contains the information we can
use to select the river features. Click on Select features using an
- Enter the expression “featurecla” = “River” and click Select
and then click Close to back to the main QGIS window.
- Now we are ready to perform the spatial query. You need to enable the
Spatial Query plugin to use this functionality. See Using Plugins
for more details. Once enabled, go to .
- For our query, we want to select features from the buffered places that
intersect with the buffered river lines. Make sure the checkbox next to
selected geometries is checked. This is to ensure the query uses only
river features that we selected previously. Click Apply.
- Once the query is complete, you will see a new section named
Selected features. Click on the Create layer with
selected button. A new layer will be added to the Layers Panel. Click
- Zoom-in to any area and compare the results. You will notice that the new
layer contains only the features that intersect with river buffers.
- We should always verify my results to ensure the analysis is not flawed. One
way to verify the results is to export this layer as a KML file and load it
up in Google Earth. You can check if the areas you found really are within
10kms of a river. Right-click the layer and Save As....
- In the Save vector layer as..., choose WGS84 as the CRS. This
because KML format needs the coordinates to be in this CRS. Name your KML as
- Open Google Earth and verify that the cities represented by these buffers
are indeed close to rivers.