GIS is very useful in analyzing spatial relationship between features. One such
analysis is finding out which features are closest to a given feature. QGIS has
a tool called Distance Matrix which helps with such analysis. In this
tutorial, we will use 2 datasets and find out which points from one layer are
closest to which point from the second layer.
Open Layer ‣ Add Layer ‣ Add Delimited Text Layer and
browse to the downloaded signif.txt file.
Since this is a tab-delimited file, choose Tab as the
File format. The X field and Y field
would be auto-populated. Click OK.
Note
You may see some error messages as QGIS tries to import the file. These are
valid errors and some rows from the file will not be imported. You can ignore
the errors for the purpose of this tutorial.
As the earthquake dataset has Latitude/Longitude coordinates, it will be
imported with the default CRS of EPSG:4326. Verify that is the case in
the bottom-right corner. Let’s also open the Populated Places layer. Go to
Layer ‣ Add Layer ‣ Add Vector Layer.
Browse to the downloaded ne_10m_populated_places_simple.zip file and
click Open.
Zoom around and explore both the datasets. Each purple point represents the
location of a significant earthquake and each blue point represents the
location of a populated place. We need a way to find out the nearest point
from the populated places layer for each of the points in the earthquake
layer.
Go to Vector ‣ Analysis Tools ‣ Distance Matrix.
Here select the earthquake layer signif as the Input point layer and the
populated places ne_10m_populated_places_simple as the target layer. You
also need to select a unique field from each of these layers which is how
your results will be displayed. In this analysis, we are looking to get only
1 nearest point, so check the Use only the nearest(k) target
points, and enter 1. Name your output file matrix.csv, and
click OK. Once the processing finishes, click Close.
Note
A useful thing to note is that you can even perform the analysis with only 1
layer. Select the same layer as both Input and Target. The result would be a
nearest neighbor from the same layer instead of a different layer as we have
used here.
Once the processing finishes, click the Close button in the
Distance Matrix dialog. You can now view the matrix.csv file
in Notepad or any text editor. QGIS can import CSV files as well, so we
will add it to QGIS and view it there. Go to Layer ‣ Add
Layer ‣ Add Delimited Text Layer….
Browse to the newly created matrix.csv file. Since this file is just
text columns, select No geometry (attribute only table) as the
Geometry definition. Click OK.
You will see the CSV file loaded as a table. Right-click on the table layer
and select Open Attribute Table.
Now you will be able to see the content of our results. The
InputID field contains the field name from the Earthquake
layer. The TargetID field contains the name of the feature from
the Populated Places layer that was the closest to the earthquake point.
The Distance field is the distance between the 2 points.
Note
Remember that the distance calculation will be done using the layers’
Coordinate Reference System. Here the distance will be in decimal degrees
units because our source layer coordinates are in degrees. If you want
distance in meters, reproject the layers before running the tool.
This is very close to the result we were looking for. For some users, this
table would be sufficient. However, we can also integrate this results in
our original Earthquake layer using a Table Join. Right-click on the
Earthquake layer, and select Properties.
Go to the Joins tab and click on the + button.
We want to join the data from our analysis result to this layer. We need to
select a field from each of the layers that has the same values. Select
matrix as the Join layer` and InputID as the
Join field. The Target field would be I_D.
Leave other options to their default values and click OK.
You will see the join appear in the Joins tab. Click
OK.
Now open the attribute table of the signif layer by right-clicking and
selecting Open Attribute Table.
You will see that for every Earthquake feature, we now have an attribute
which is the nearest neighbor (closest populated place) and the distance to
the nearest neighbor.
We will now explore a way to visualize these results. First, we need to
make the table join permanent by saving it to a new layer. Right-click the
signif layer and select Save As….
Click the Browse button next to Save as label and
name the output layer as earthquake_with_places.shp. Make sure the
Add saved file to map box is checked and click OK.
Once the new layer is loaded, you can turn off the visibility of the
signif layer. As our dataset is quite large, we can run our
visualization analysis on a subset of the data. QGIS has a neat feature
where you can load a subset of features from a layer without having to
export it to a new layer. Right-click the earthquake_with_places layer
and select Properties.
In the General tab, scroll down to the Feature
subset section. Click Query Builder.
For this tutorial, we will visualize the earthquakes and their nearest
populated places for Mexico. Enter the following expression in the
Query Builder dialog.
"COUNTRY" = 'MEXICO'
You will see that only the points falling within Mexico will be visible in
the canvas. Let’s do the same for the populated places layer. Right-click
on the ne_10m_populated_places_simple layer and select
Properties.
Open the Query Builder dialog from the General tab.
Enter the following expression.
"adm0name" = 'Mexico'
Now we are ready to create our visualization. We will use a plugin named
MMQGIS. Find and install the plugin. See Using Plugins for more
details on how to work with plugins. Once you have the plugin installed, go
to MMQGIS ‣ Create ‣ Hub Lines.
Select ne_10m_populated_places_simple as the Hub Point
Layer and name as the Hub ID Attribute. Similarly, select
earthquake_with_places as the Spoke Point Layer and
matrix_Tar as the Spoke Hub ID Attribute. The hub lines
algorithm will go through each of earthquake points and create a line that
will join it to the populated place which matches the attribute we
specified. Click Browse and name the Output
Shapefile as earthquake_hub_lines.shp. Click OK to start
the processing.
The processing may take a few minutes. You can see the progress on the
bottom-left corner of the QGIS window.
Once the processing is done, you will see the earthquake_hub_lines
layer loaded in QGIS. You can see that each earthquake point now has a line
that connects it to the nearest populated place.
