Humanitarian OSM Team/Freetown building dissolve 2025
Note concerning significant changes to OSM building data in Freetown, Sierra Leone
Humanitarian OpenStreetMap Team (HOT), OpenStreetMap Sierra Leone (OSM SL), CODOHSAPA (Sierra Leone affiliate of Slum Dwellers International) and FEDURP (the Federation of Urban and Rural Poor in Sierra Leone) and Slum Dwellers International (SDI) have been working together for more than a year developing open geospatial data and skills to support informal settlements communities to plan, implement and advocate for upgrades to their neighbourhoods. This is part of SDI’s Know Your City (KYC) programme.
In three Freetown communities (Cockle Bay, Kroobay and Kolleh Town) we have generated aerial imagery using drones (hosted on OpenAerialMap), digitised buildings using JOSM and developed a workflow for ground truthing building footprints using field mapping tools (Vespucci and SW Maps) and techniques - the reason for this last step being that remote mapping cannot well demarcate the footprint of buildings in dense urban environments, only rooftops, and buildings in these communities are very dense and rooftops overlap.
The ground truthing component involved two person teams, including one mapper (OSM SL) and one resident of the settlement itself, surveying all buildings in each of these communities, using Vespucci to make live changes.
Whilst this was very successful as a workflow, there have been unintended consequences. Instead of limiting the detailed building mapping to disambiguating different buildings under (what looks from the imagery like) one rooftop, mapping teams also spatially disambiguated different households within individual buildings. This is a problem as data at this level of granularity is considered by FEDURP and Slum Dwellers International as ‘household level data’ and, as such, puts families living in those households at risk of exposure (in a scenario where their occupancy is already precarious).
As such, we will be making some significant changes to the building data in three settlements (Cockle Bay, Kroobay and Kolleh Town), dissolving internal partitions of specific buildings so that the building itself retains its geometry and tags, but the household level data is no longer present.
We are making this information available in case there is any suspicion of impropriety or vandalism as a result of these changes. Both Pete Masters (HOT) and Tommy Charles (OSM Sierra Leone) are available if there are any questions.
Workflow/mapper instructions
Detailed mapping instructions can be found directly in one of the dedicated HOT Tasking Manager projects.
Work began on these projects on 4 August 2025, and was completed on 12 August 2025.
All projects can be seen in this filtered list.