Spatial Analysis

QGIS offers a multitude of geoprocessing tools that allow you to manipulate vector and raster data. Elements such as Buffer, Clip, Dissolve, or Intersection allow you to create zones surrounding a border, cut the layer by a particular boundary, dissolve a specific inner geographical area, or obtain a common area between two or more layers, respectively. These tools give us a way to extract more insightful analyses from geospatial data.

Objectives

Learn how to use the buffering tool in QGIS.
Use the zonal statistics tool to calculate the total population within a particular region.

Buffering

The Buffering tool is one of the most commonly used geoprocessing tools. Practically speaking, it allows you to define two areas: a region that falls within the specified distance, and a region that falls outside of the specified distance. The buffer zone is the region that falls within the defined distance. When you establish a buffer, it is important that you check what type of units your data uses. The type of units is defined by the data’s projection. You can check the projection by right-clicking on the layer and going to Properties > Information and scroll down to the Coordinate Reference System (CRS) section.

The buffer zone does not have to be defined by a single number – it can vary based on the values defined in a specific Attribute Table column. Furthermore, you can define multiple buffers for a single object, as well as buffers with overlapping or dissolving boundaries.

[IMAGE buffer w variation, multiple buffers, dissolved vs overlapping buffers]

Exercise 3.1. Delineate historical flood risk extent.

We will visualize flood risk zones by establishing a buffer around the major waterways in Puerto Rico based on historical flood extents.

Open the pr-elev-analysis project.
Ensure that the following layers are loaded into the project:
1. For analysis: pr-rivers-corrected-reproj
2. For visualization: PRI_adm0, pr-elev-clipped, pr-hillshade.
Uncheck any other layers so they are not visible to keep things organized.
Double-check that all the layers and the map project are set to the CRS EPSG:32630 setting (this is important so that we have a projection that uses meters as its distance metric). If they are not, you will need to change the CRS:
1. Navigate to Processing > Toolbox and search for Reproject Layer. Choose an input vector layer that needs to be changed, and set EPSG:32630 - WGS 84 / UTM zone 20N as the Target CRS. For raster layers, you will need to use Rasters > Projections > Warp (reproject)....
2. Choose to save the file as LAYER-NAME-HERE-reproj in the intro-gis-data/analysis folder.
3. Press Run and then close the window. Save the project.
4. Repeat steps 1-3 for any other vector layers that need to be reprojected.
5. To change the map canvas projection, click on the lower right corner where it says the current projection (most likely EPSG:4326). Choose WGS 84 / UTM zone 20N and choose OK.
6. Remove any layers in the wrong projection from the map and and adjust the symbology as needed.
Navigate to Processing > Toolbox and use the search bar to locate the Tapered buffers tool listed under the Vector Geometry category.
Rivers do not flood uniformly. Thus, instead of defining a fixed value to establish the buffer zone, we will use a buffer that gradually increases from the start of the line segment to the end of the line segment such that a greater flood extent is defined at lower elevations. The rivers in the pr-rivers-reproj.gpkg file have already been configured so that their start point is at a higher elevation and their end point at a lower elevation.
Define the “Tapered buffers” parameters.
1. Input layer: pr-rivers-reproj
2. Start width: 0.00
3. End width: 3200.00
4. Segments: Select the dropdown menu button. Hover over the Field type: int, double, string option and select maxelev. This choice will create a more gradual looking buffer.
Leave the default option to save the layer as a temporary layer as press Run, close the window, and save the project.

You should see a new layer called Buffered appear on the canvas. You may notice that it looks a little messy – a number of the rivers have overlapping buffer zones. Let’s dissolve the overlapping borders to create a cleaner flood extent boundary.

Exercise 3.2. Dissolve buffer boundaries.

Navigate to Vector > Geoprocessing Tools > Dissolve...
Select Buffered as the input layer.
Next to the Dissolved field, select the ... button and save the file as pr-flood-extent in the intro-gis-data/analysis folder.
Press Run and then close the window. Save the project.

Now you have a layer that indicates the possible flooding extent from rivers in Puerto Rico. Obviously this layer is a very rough flooding estimate, but it will serve our purposes for the remainder of the lesson.

Zonal Statistics

More than just visualizing data, geospatial analysis requires the ability to gain numerical insights from the data to better understand patterns and trends occurring within the study region and time period. A helpful tool for conducting this type of analysis is the Zonal Statistics tool. This process allows you to calculate the mean, median, sum, minimum, maximum, or range of a particular feature within a specified boundary. The tool is applied to raster data, but the boundary can be specified with vector or raster data.

[IMAGE zonal stats visualization]

Exercise 3.3. Calculate total population at risk of flooding.

For our next exercise, we will use the population layer to determine the number of people within the country that fall within the potential flood zone, putting them at risk of being affected by flooding events.

Open up the pr-elev-analysis project in QGIS if it is not already open.
You should have the same layers loaded into the project as in Exercise 3.1 and 3.2.
Go to Layer > Add Layer > Add Raster Layer... and select the pr-population-2018.tif file in the intro-gis-data folder. Press Add and close the window.
Check the projection and reproject the layer to EPSG:32620 if necessary.
Now click on Processing > Toolbox to open the Toolbox panel. Search for Zonal Statistics and double-click on the name to open it. The tool should be listed under the Raster analysis category.
Set the following parameters:
1. Input layer: pr-flood-extent
2. Raster layer: pr-population-2018-reproj
3. Raster band: This field should auto populate. Leave it as is.
4. Statistics to calculate: Click the ... next to the statistics field and check only the Sum option. Press OK to return to the parameters window.
Next to the zonal statistics field, press ... and save the file in intro-gis-data/outputs as pr-total-pop-flood-risk.
Click Run and close the window. Save the project.
A new layer called pr-total-pop-flood-risk should appear. I will look the same as the pr-flood-extent layer, but it contains more information in the attribute table. Right-click on the layer name and select Open Attribute Table.
Scroll to the right to view the last column in the table _sum. This number is the total number of people who live in flood risk zones, or about 651,000 people.

That’s quite a few people! If you wanted to know the number of people for each river feature, you could run the same operation on the non-dissolved buffer layer.

Challenge: Calculate percentage of national population at risk for flooding.

See if you can take the analysis one step further – you know the number of people who live within flood risk zones, but what percentage of the population is that? Knowing that value could aid in resource allocation or population relocation. Calculate the percentage of the national population that falls within our defined flood risk zones.

Hint 1: Calculate the total country population and join it to the layer from Exercise 3.3
Hint 2: The tools Join attributes by field value and the Field calculator will be useful in this exercise.

Congratulations! You completed the Introduction to GIS workshop. You can refer back to this material anytime you want throughout the learning series, and don’t forget to reach out with feedback or questions!