Following in Québec’s footprints: Adding new buildings to the Community Map

May 2, 2024

Emma Melis

As soon as the new building footprints dataset was released by the Government of Québec with its increased data quality and coverage extent, we knew it had to be considered for inclusion in the Community Map of Canada.

In 2023, the Province of Québec published a building footprints layer to their provincial open data portal, Données Québec, which hosts hundreds of datasets from municipalities and organizations across the province. This new building footprints layer was created in partnership with the University of Laval and Natural Resources Canada. It was automatically generated from available LIDAR sources and available government imagery and contains building polygons across most of southern and eastern Québec. The dataset includes 2.5 million building features which appear in 972 of Québec’s 1300 census divisions. Given the extent of the coverage and the quality of the data, as soon as we saw this new dataset, we knew it had to be considered for inclusion in the Community Map of Canada.

A topographic map of Québec showing spatial data coverage from a Government of Québec dataset. The coverage region is shown in a light orange polygon that mostly covers southern Québec, whereas the non-covered area (mostly northern Quebec) is shown with a pale beige colour.

A map showing the coverage of the new building polygons dataset released by the Government of Québec. The coverage region (shown in light orange) is mostly in Southern Québec.

Before this new building layer was added to the Community Map of Canada, one of the main sources for building footprints (apart from direct community contributions) in the province was a freely-available open source Microsoft layer created (rather crudely, at times) from orthophoto imagery. It was an excellent surrogate in the absence of a better, arguably more authoritative, data source, but, as with any polygons automatically-generated from large-scale orthophoto imagery, it wasn't necessarily always 100% accurate.

Enter the Province of Québec building polygons dataset.

Where the Microsoft layer sometimes rounded out or cut building corners at an angle, the provincial buildings layer had more finely-detailed building footprints, as can be seen below in a few building polygons in Louiseville and Yamachiche, towns located along the St. Lawrence River in eastern Québec.

A comparison of topographic maps for the regions of Saint-Edouard-de-Maskinongé and Saint-Sauveur. The comparisons before and after changing sources show that the building coverage is more complete and accurate to the underlying imagery after the swap.

Some pockets of communities in Québec previously lacked building footprint coverage due to a lack of data, leaving gaps in some otherwise-filled-in communities, like Saint-Édouard de Maskinongé and Saint-Sauveur. The provincial dataset was able to fill in some of these gaps.

The prioritization of direct community sources and accuracy was maintained in this provincial building swap. Since the objective of the Community Map of Canada is to feature accurate, authoritative and up-to-date community data, as well as to engage and empower communities to contribute their data for autonomous community representation, the GIS analysts who represent Québec on the basemap combed through each of their communities, comparing the new building layer with previous data sources. My colleagues Kira Lazda and Sonja Antic assessed the data coverage community by community, comparing the quality and coverage of the new building polygons to the older imagery-generated buildings layers provided by Microsoft and other sources like Natural Resources Canada in order to identify which layers should be selected and published to the basemap for each municipality and region.

To make the coverage decision for each region, they asked the following questions for each municipality:

Coverage and Completeness - Does the data layer cover all the building features in the area it is supposed to cover?

Positional Accuracy – Are the features in the data layer positioned accurately, compared with the underlying imagery?

Shape Accuracy – Are the building shapes in the data layer accurate compared to the underlying imagery?

Quality of data – What are the attributes and what does it describe? Ex: building classification, name of the buildings, etc.

Four images that each show a comparison between different maps and data sources for various Québecois communities. The four topics of the map comparisons, clockwise from left to right are: Coverage and Completeness, Positional Accuracy, Shape Accuracy, and Quality. The maps all show different levels of building precision and dataset completeness for the map extent.

Four images that each show a comparison between different maps and data sources for various Québecois communities. The four topics of the map comparisons, clockwise from left to right are: Coverage and Completeness, Positional Accuracy, Shape Accuracy and Quality. The maps all show different levels of building precision and dataset completeness for the map extent.

After the building swap, 45% of census subdivisions in Québec now use Province of Québec building footprints. Before the provincial building layer was provided, 92% of Québec’s census subdivisions were represented on the Community Map of Canada using Microsoft and Natural Resources Canada building footprints (in the absence of self-contributed data), and now after the building swap, only 45% of census subdivisions use those sources, now replaced by the provincial source. Building a national basemap, changing building polygons and comparing data sources, municipality by municipality, is hard work but when it leads to a more accurate cartography of Canadian communities, it's all worth it.

Thanks to frequent data updates from our more than 410 contributors (and counting!), the Community Map of Canada provides a good representation of Canada in real-time, especially given that data updates are synced to the basemap daily and a new version of the basemap is published twice per week (every 72 hours). This makes the Community Map the most accurate basemap available for Canadian users with an unparalleled update frequency and direct community data source.

Are you a community or authoritative data source in Québec or Canada and would like to get your data on the map to save on credits and basemap maintenance resources? Email us at communitymaps@esri.ca to get involved. And if you are a GIS user, consider using the Community Map of Canada basemaps as your default basemaps for your mapping endeavours. You can access it on your browser and across the ArcGIS system.

This post was translated to French and can be viewed here.