In another little mapping experiment with QGIS and open data from the City of Toronto, I visualized the 32,189 locations of [type-of-incident-withheld] that were recorded in Toronto from 1990 to 2013. I put out a little quiz about this map on Twitter, so I will only reveal what the points represent towards the end of this post. However, the dataset is readily available from Toronto’s open data catalog, both in tabular and GIS-ready Shapefile format.
According to a report by Global News, City crews on occasion have to deal with 20-25 of these incidents a day. As part of their data journalism, Global News created a hexagonal heatmap of the 1990-2013 data, see their article [type of incident will be disclosed].
In contrast, I mapped each point individually using lighter shades of blue for more recent years. While it is often recommended to use the darker and/or more saturated end of a colour scheme for the more important values (arguably the more recent incidents), with the ever more popular black map background, this approach is inverted: the lighter symbols will create the greater contrast, and thus appropriately represent the more important, often the larger, values. The boundaries shown in the background are City wards.
As I finish teaching GEO241, our 2nd-year Cartography course in the BA in Geographic Analysis program, I am still having trouble identifying the thematic map type implemented here. It is not a dot density map, as a dot density map uses a unit value (could be seen as 1 dot = 1 incident) and places dots within the area for which the data were collected, but not at the exact location of occurrence. The same reasoning applies to Dr. John Snow’s map of cholera death in London 1854, which is not a dot (density) map either.
Instead, I think this map can be considered a proportional symbol map, where the point symbols at real point locations — not conceptual points such as Census tract centroids — are defined in proportion to a variable (BREAK_YEAR), yet not in terms of their size but in terms of their lightness. Clicking on the above teaser will open the full map with the title Water Main Breaks, City of Toronto, 1990-2013. So yes, there were a whopping 32,189 water main breaks in the City of Toronto during those 24 years! This situation is expected to worsen with the aging municipal infrastructure, see for example the Toronto Star’s 2010 article with a map showing downtown water mains built pre-1900. And it is not a new phenomenon either, as shown by this lovely photograph from the City of Toronto Archives (Fonds 200, Series 372, Subseries 72, Item 31), dated May 3, 1911: