EXPLAINER

How we made the Montreal crime tracker

The driving force behind this project was a question: how can we best show crimes in Montreal in a useful and easy-to-understand way?

We divided island into hexgrids in effort to show tally of criminal events in easy-to-understand way

We split up the island of Montreal into 970 hexagons, and with the help of geospatial software, we counted the number of crimes inside each one. (CBC)

The driving force behind our Montreal crime tracker project was a question: how can we best show crimes in Montreal in a useful and easy-to-understand way?

Visualizing crime data is tricky because of the sheer volume of events.

In any one year, close to 50,000 criminal acts are registered. 

The City of Montreal has a map showing the number of incidents by intersection: the size of the circles indicates the number of crimes near that corner. 

We chose a different way to visualize the situation.

We split up the island into 970 hexagons — what's known as a hexgrid. Each hexgrid is 900 metres in diameter and contains roughly 20 intersections.

With the help of geospatial software, we counted the number of crimes inside each hexagon.

This allows us to present an aggregate, but still detailed, view of crimes. It shows the number of incidents in immediate areas, larger than a street corner but smaller than a borough.

The island of Montreal was divided into a hexagonal grid, each area having a diameter of 900 metres. Crimes were aggregated into each hexgrid to show general trends. (CBC)

To protect the privacy of victims, the exact locations of the crimes are hidden in the police data, and only the nearest street intersection is disclosed.

This presents a challenge. Crimes that were counted in one hexagon may have actually occurred in a neighbouring one.

Here's an example.

In the image below, we see several car thefts (pink points) reported along Bélanger Street. Since we only know the nearest intersection, the actual thefts could have occurred on cross-streets southeast or northwest of Bélanger. But these intersections are inside one hexagon, which gets the tallies for all those incidents.

A drawback of using a hexgrid to aggregate data that's been disclosed to the nearest intersection is that crimes that happened within one hexgrid might have been counted in the adjacent one. (CBC)

Despite its shortcomings, in an aggregate sense, this system is useful for showing general concentrations of different types of crimes.

The analysis is provided automatically with CBC's in-house software. The code and source files can be accessed here.

Changes in crime rates over time

One feature of this site is that we will be able to show you significant increases and decreases in crimes over time.

As the Montreal police service releases more data, we'll have better material to make comparisons between years.

About the Author

Roberto Rocha

Journalist

Roberto Rocha is a data journalist with CBC/Radio-Canada.