1. Why do some buildings have their actual shapes and some are just dots?
The shapes of the buildings come from OpenStreetMap, a collaborative map service, like a Wikipedia for maps. The data in OpenStreetMap is submitted by volunteers, so it’s incomplete, covering mostly the central boroughs and towns. However, it’s constantly being updated with more building shapes and the map will be updated in the future.
The City of Montreal is also expected to release the data for the shapes of all buildings, which we will use once it’s out.
2. Why are so many buildings grey?
The data for the construction years come from the city’s land evaluation office, which has the information for most, but not all buildings. Addresses missing a construction year may also be abandoned structures, vacant lots, or spaces that saw no construction, like green spaces or parking lots. In some cases, the year of construction is simply not available. Institutional buildings like schools and churches are especially absent.
According to the city’s land evaluation office, lots that have multiple buildings built at different times are prone to having unknown years.
It’s also possible that during the preparation of this map, the year of constructions available for dots were not assigned to the corresponding building. Some dots may represent several addresses, and only got matched to one building in the group.
In total, 32,000 buildings out of the 350,000 in the map (about 9 per cent) don’t have a construction year.
3. Why are the years divided unevenly in the legend?
Colours were assigned to years according to how many buildings were built that year. The vast majority of buildings were built after 1900, so it made more sense to have more colour divisions in those later years for more nuance.
4. Are the construction dates accurate?
Not all of them. The older the building, the more likely there is to be an error in the date. For example, the year of construction for the house at 17013 Gouin Boulevard W. is 1700 in the land evaluation data. But a heritage document pegs that same house at 1830.
Likewise, these condos at 1451 Notre Dame West were probably not built in 1600, before the city was founded. The land evaluation office says that if a piece of a former structure is present in a new building, like a wall or foundation, the original year is maintained.
5. How was this map made?
The map has several data sources:
All data sources were merged to assign years of construction to buildings shapes where possible, and the rest to the address points.
The data was prepared using the Python programming language and QGIS mapping software, and visualized for the web with Carto.
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