Add Districts & Points
If you haven't already, add the Chicago neighborhoods polygon districts to your map:
- Click the Add Vector layer button
- Click browse
- Locate and open the Neighborhoods shapefile
Also make sure the crime data is added:
- Click the Add Delimited Text Layer
- Locate and open the crimes csv file
- Right click on the crimes layer to save as a shapefile
Note: Confirm the Coordinate Reference System (CRS) of the two layers are a match. If CRS are not a match, spatial join would be impossible.
Look at the attribute table for the crimes data. You should see a field called crimeType containing a categorization of the crime into one of several defined types.
Crimes Count per District (in polygon)
To determine the number of crimes in each neighborhood, we'll spatial join the crimes count (in points) to the district layer (in polygon).
- Click on Vector >>> Data Management Tools >>> Join attributes by location
- Select Neighborhoods as the Target Vector Layer and crime as the Join vector layer
- Select take summary of intersecting features in the attribute summary drop-down
- Type Count in the statistics for summary box
- Keep all records(including non-matching target records) in the join table drop-down
- Create a folder to save the joined layer
- Make sure all boxes are ticked as shown in the image above
The joined layer attribute table should populate the count field as shown below,
The joined layer should be as shown below,
NOTE:
- Go ahead to rename the joined layer in the General tab of the layer properties
- Always check the CRS of each layer especially the joined layer, for any changes
In the next section, you'll determine narcotics count.
