The Belgian OSM community is importing buildings from governmental data into OSM for some years now. In December I was supposed to present a analysis about this process regarding the import of buildings data from the PICC, the source of data for the Walloon region.

Unfortunately I got sick and I could not present. Anyway, here are some key numbers about this process not only for Wallonia but for Belgium.

The big picture

In Belgium, there are 3 different sources of government data for buildings, each one for the 3 regions of Belgium: Flanders, Wallonia, Brussels. All these sources are integrated in what we call the “building import tool”: the web application buildings.osm.be. People who want to use this tool are encouraged to learn about the import process and to conflate (merge) with existing buildings. In many places indeed, there are already buildings in OSM and integration of every single imported building with existing ones is the preferred way, rather than “delete and replace”. We also ask to not blindly trust official data and to always look if current data in OSM does not bring interesting added value in terms of accuracy and/or local knowledge. After all, it is one of the key force of OpenStreetMap.

What are the lessons

Having imported thousands of buildings myself in the past 3 years using this tool, I found some weird situations in the government data: oddities in house numbering, strange shapes of buildings compared to aerial imagery, etc. Honestly, these are very rare situations, but still it might be interesting to report it to the administration. What is more frequent are update of buildings compared to official data: during the import, by comparing with the aerial imagery or local knowledge, one can find some new buildings, or demolished ones, or some changes in the building outline.

For other opinions, see this thread: https://community.openstreetmap.org/t/feedback-about-the-buildings-import-process-for-the-picc/138241

Some numbers (and a map)

A total of 3,564,874 buildings were imported into OSM in Belgium using the building import tool. It is nearly half of the total number of buildings in OSM in Belgium. Also, before the import tool was set up, a lot of buildings were manually drawn (or semi-manually using some JOSM plugin) from the governmental WMS imageries. So actually even more buildings in OSM somehow come from governmental data. On the other hand, using the import tool, contributors actually replaced/added tags into already existing OSM buildings. So it is hard to tell without further analysis the exact part of buildings that comes from official data.

Without looking at the histories of OSM objects, it is rather difficult to count the number of users who have imported buildings using the tool. By looking at the tag “source:geometry:ref”, 175 contributors have imported (or edited after someone else’s import) at least 100 buildings. The top 10 contributors have imported about 67% of the total imported buildings. This is often observed in OSM (and in other crowdsourcing project): a small amount of users makes the most of the contributions.

Here is a map of the imported buildings colored by the top ten contributors.

Nowadays, there are still some streets with missing buildings, roughly drawn building blocks without addresses in some city centres, but the places with missing buildings tend to disappear. Yet, in the future, adding new buildings, tagging demolished ones and updating building geometries across the country will remain necessary.