Vector Tools

This page will introduce some vector tools common in map making and spatial analysis. Vector tools can be searched for in the Processing Toolbox which can be opened from the Processing menu at the top of your screen. They can also be accessed from the Vector menu, where they are grouped by task: Geoprocessing, Geometry, Analysis, Research, and Data Management.

Geoprocessing

Geoprocessing tools are useful for modifying the spatial extent of features, particularly in relationship to other layers. Geoprocessing is often done in the beginning of a QGIS project to prepare the data layers for further analysis. The Geoprocessing tools we will introduce today are: Clip, Buffer, Difference, and Dissolve.

Geometry

Geometry tools are useful for operations to do with the geometric shape of the feature layer. The Geometry tool we will introduce today is: Centroids.

Analysis

Analysis tools are useful for performing basic statistical analysis on vector layers. The Analysis tool we will introduce today is: Count points in polygon.

Research

Research tools support further exploration of your data by running selections and generating random points for test scenarios. The Research tools we will introduce today are: Select by Location and Select within Distance.

Data Management

Data Management tools is useful for modifying the data layer itself. The Data Management tools we will introduce today are: Merge Vector Layers, Join Attributes by Location, and Reproject Layer.

Page Contents

• Vector Tools
  • Clip
  • Buffer
  • Difference
  • Dissolve
  • Merge
  • Count points in polygon
  • Centroids
  • Select by location
  • Select within Distance
• Designing Workflows

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Open the Processing Toolbox

For today, we’ll search for tools from the Processing Toolbox. Go ahead and open the Processing toolbox now.

• If you don’t see the Processing menu at the top of your screen, you may have to enable the processing plugin. Click on the Plugins menu at the top of your screen, and then on Manage and Install Plugins…. In the search bar, type in “Processing”. Make sure to select the Processing box, and then click Close. You should now see the Toolbox icon and be able to proceed with the next steps. Once enabled, you will be able to access the Processing menu anytime you open this or any other QGIS project.

Make sure your map canvas is zoomed to Vancouver.

Clip

The first tool we’ll use is Clip, one of the most frequently used tools. Like a cookie cutter, Clip takes an Input layer (the cookie dough) and an Overlay layer (the cookie cutter), clipping the Input layer to the extent of the Overlay layer. Clip helps identify a set of points from a larger dataset within a particular area. It is a useful tool for highlighting a particular area of your map, honing your extent to certain area of interest.

To Do

As it stands, local-area-boundaries, the layer visualizing Vancouver’s neighborhoods juts out past the shoreline.

The census-tracts layer on the other hand nicely hugs the shore. Let’s practice the clip tool by clipping local-area-boundaries to census-tracts. That way we can have a layer visualizing Vancouver neighborhoods where the shoreline is still visible.

In the Processing Panel, search for “Clip”. Make sure you open the tool under Vector Overlay.

Clicking a tool will open a dialogue window specific to that tool. On the right hand side will be a description of what the tool does, and on the left, prompts for selecting input layers as well as saving the output layer to a file.

• Set local-area-boundaries as your Input layer
• Set census-tracts as your Overlay layer

The choice to save the output layer as a permanent or temporary file depends on whether you are running an intermediary step in your workflow. Temporary files will be deleted when you quit your QGIS project, whereas permanent files are new datasets you saved and stored on your computer. Temporary layers load with the layer name of the tool — in this case, “Clip”.

Since we’re just practicing, we could leave the output as a temporary layer. However, since this will be a useful layer to have, let’s save it as a permanent file before running by clicking “Save to file”. Remember to give it a location (by clicking the tree dots ...) as well as a name, such as local-areas-clip or neighborhoods. Save the output as a shapefile.

• The tool window might disappear after setting the output filepath. Find the window again and click Run. Ignore any warning saying “No spatial index exists for the input layer”; this is how the data came from the City of Vancouver.

• Close the tool (it might have jumped behind your main QGIS interface), and return to your map view. Toggle off public-art and cultural-centers for a moment so you can see the new Clip layer alone.

The attribute table will be the same. The only thing different is the outline of the layer.

Buffer

Buffer is probably the second most used/useful tool. Like the name implies, buffer creates a new layer that buffers a distance around points, lines, or polygons, and includes the area of the feature(s) buffered. Buffer is therefore useful for determining spatial proximity but defining a distance zone around features. For example, you could use Buffer areas prone to flooding around a water feature, or to determine a radio signal’s geographic influence or the area of a neighborhood disturbed by construction sounds.

To Do

Find the Buffer tool under Vector Geometry.

The Input Layer is the layer you want to buffer. If we were to try and buffer cultural-spaces — or public-art or van-parks for that matter — we would get an error. This is because the input layer must be in a projected coordinate system (PCS) in order to buffer a distance around it. Currently, these layers are in WGS 84, a geographic coordinate system (GCS) only. If the layer is in a geographic coordinate system, you will see an error telling you QGIS cannot buffer distance in degrees.

We need to Reproject our cultural-spaces layer before we can buffer it.

• Search for the Reproject Layer tool.

• Reproject cultural-spaces to the following CRS: NAD83 / UTM zone 10N - the same as our Project CRS.
• Run and close the tool. You should now see a new temporary layer called Reprojected. It looks the same since our QGIS Project was reprojecting the prior layer on the fly to the same projection. However, we can now buffer this layer. It can help to rename this temporary layer in your Layers panel to something more meaningful such as cultural-spaces-reprojected.

• Now, Buffer 500 meters around the reprojected cultural spaces layer, Reprojected. At first, just run the tool with only the input layer and buffer distance modified. The temporary output layer, Buffer, will look like the image below, with a buffer generated for each point feature.

If we were only interested in the general area within 500 meters of a cultural center, we could scroll down and check the Dissolve result option before running the Buffer tool. The Dissolve option indicates whether or not you want the buffers of individual features to dissolve if they overlap.

• Run 500 meter buffers again on Reprojected cultural spaces, but this time check Dissolve result. The output will look like the image below.

SAVE your project.

Difference

Difference is like a spatial subtraction. Again, it will create a new layer so you don’t have to worry about permanently altering your existing data (the correlate tool in ArcGIS, Erase, does just that).

To Do

• Just to practice, run the Difference tool to find areas that are within 500 meters of a cultural center, but are not parks. Your Input Layer will be Buffered and your Overlay Layer will be van-parks. It may help to rename the Buffered layer which was dissolved prior to running this tool.

Drag the output Difference to the top of your Layers panel, and uncheck extraneous layers.

Dissolve

Dissolve takes multiple features within 1 layer and dissolves the boundaries between them. This is exactly what happened when we checked the Dissolve option on in the Buffer tool.

As it stands, the shapefile for census-tracts has numerous features. When symbolizing the layer for our reference map earlier, we were unable to get rid of these lines. Perhaps you don’t want these lines visible. Dissolve will remove the differentiation; however, as an important caveat, the resulting layer will no longer have distinct features in the attribute table.

To Do

Open the Dissolve tool under Vector geometry

• Dissolve census-tracts.

Merge

Writes QGIS: Merge “Combines multiple vector layers of the same geometry type into a single one.”

Merge can be a useful tool to manage your data. For example, we currently have 2 layers for parks: van-parks and burnaby-parks. To make life easier, we could just merge them all together into one layer. Because we are essentially combining datasets, the attribute table of the resulting layer would include the complete information for all parks.

To Do

• Open the Merge Vector Layers tool under Vector general.

• Click the three dots to choose your Input Layers. Select van-parks and burnaby-parks, then click the < arrow icon to return to the tool parameters.

• Set the Destination CRS to be that of the project. This only matters if your Input layers have different CRSs.

• Run the tool and close it. Return to your Map Canvas and zoom-to the new Merged layer. Open the attribute table as well to confirm the success of your merge.

Count points in polygon

This tool will count the number of points in each polygon and append the total to each feature of the input polygon as a new attribute.

• Count how many public-art features are in each Vancouver neighborhood. Which neighborhood has the most public art?

Centroids

• Centroids will calculate the geometric center of each feature and output a layer consisting of those points. This is useful if you have a polygon layer and want to turn it into a point layer.

• To practice, run Centroids on local-areas-clip.

Select by location

Select by location allows you to select features in 1 layer based on their spatial relationship with those in another layer using various spatial operators.

• Use Select by Location to find all parks from your Merged layer that are within Vancouver. This should successfully highlight just those parks that are within the city limits again.

Cancel your selection from the Selections Toolbar.

Note that you can also locate both the Select by Location and the Select within Distance tools from the Selections toolbar.

Select within Distance

Slightly different than the above tool, Select within distance “creates a selection in a vector layer. Features are selected wherever they are within the specified maximum distance from the features in an additional reference layer” (QGIS).

• Let’s practice by selecting all public-art within 100 meters of Cultural Space. Remember to use your reprojected version of cultural-spaces as your . You may need to run Reproject layer on public-art to assign it the project CRS, NAD83 / UTM zone 10N, as well before you can run this tool.

Designing Workflows

Now it’s time to put everything you learned together by designing workflows to answer spatial questions. Using the tools above, think through how you might solve for the following…

3 Create a layer that visualizes areas of Vancouver that are NOT within 300 meters of a park.

4 Which Vancouver neighborhood has the fewest total parks?