Power BI is one of the most widely deployed business intelligence platforms in UK public and private sector organisations, yet its built-in and freely-available map visuals offer poor support for the British National Grid, UK-specific address locators, or ONS administrative boundaries - the everyday currency of UK geospatial analysis. The OS Power BI Map Visual was built to close that gap.

Under the hood, the visual is a TypeScript application built almost entirely on open-source geospatial libraries: Leaflet.js for rendering, Turf.js for spatial operations, proj4 for coordinate transformations (including OSTN15 datum shift between ETRS89 and OSGB36), and shpjs and topojson-client for client-side geometry import and parsing. It geocodes tabular data using UK-native identifiers - postcodes, UPRNs, Eastings/Northings, and GSS statistical codes - the last of these automatically resolving to the corresponding ONS boundary (LSOAs, MSOAs, wards, local authorities, and many more) by querying the ONS Geoportal directly, with no boundary file management required from the user.

The visual supports choropleth and point maps, custom geometry upload (GeoJSON, WKT, TopoJSON, shapefile), and two-way cross-filtering with other Power BI visuals. All of this runs inside Power BI's sandboxed custom visual API - a constrained environment that creates its own engineering challenges.

Ordnance Survey has released the visual as open source under an MIT licence. In this talk we will show what it does, sketch the FOSS architecture that makes it work (as well as the places where non-FOSS tools are used), discuss design decisons we made, present a live demo, and invite the community to use, fork, and contribute.