Cartopolis: an open-source, self-hostable 3D social map built from public data

A project called Cartopolis, posted to Hacker News on 28 September 2026, presents a walkable, globe-sized 3D copy of the real world assembled entirely from open public data, shared as one experience between everyone viewing it. According to the announcement, it runs in the browser, on the Linux and Windows desktop and on Android, and it is free, open source and self-hostable.

A map with property records attached

The level of detail in the Netherlands is what sets the project apart. According to the post, buildings come from the Dutch BAG and 3DBAG registers, terrain elevation from AHN, and street trees, lighting and paving from national and municipal object registers, with aerial photography served through PDOK and live public-transport departures and vehicle positions from the Dutch NDOV feed. Each building carries an information card listing construction year, use, number of units, floor area, status, monument listing and addresses — and walking inside reveals the building's storeys. The world is simulated too: real sun position, live weather built from MET Norway forecasts and NOAA cloud data, and routing on foot, by bike or by car. Movement is unbroken from a doorway all the way to orbital views of Earth.

Social features, with server-side moderation

The social layer is what makes it a social map rather than a viewer. Users appear as avatars, chat in a local channel scoped by distance, use proximity voice or a world-wide chat, and hold private conversations that the announcement says are end-to-end encrypted using the MLS protocol — the server only forwards ciphertext it has no key to decrypt. Sharing is built in: a link reopens the exact same view in another browser or the app, and users can save places with photos and map pins. They can also place and script objects and 3D models, swap a real building for an IFC architectural model, and move content in and out over an HTTP API. Moderation tools — blocking, reporting and account deletion — are enforced on the server, alongside a web console for managing accounts and data pipelines.

Detail is Dutch-first, the globe is generic

According to the post, the map, sky, weather, routing and the shared world work anywhere on Earth, but register-backed detail — measured buildings, relief, street objects, aerial photos, transit — covers the Netherlands, plus whatever a municipality publishes itself. Outside that area the terrain is flat and building footprints come from OpenStreetMap. Each register is implemented as a separate pipeline in the project's atlas component, which suggests other countries' registers could be added the same way.

Rust, Bevy and a floating origin

Cartopolis is written in Rust on the Bevy game engine, split into crates with distinct roles: a core for shared protocol types and coordinate math, a Bevy-free geodata crate that decodes vector tiles and builds the routing graph, the MLS encryption layer, a simulator server built on Tokio and SQLite, the atlas pipelines with a terrain server and operator console, the Bevy client itself, and an Android shell. Planet-scale rendering uses a vendored fork of the big_space floating-origin library to avoid precision problems at global coordinates. The browser version requires WebGPU; released desktop builds need AVX2 and FMA instructions, found in CPUs from roughly 2013 onward; the Android APK targets arm64 on Android 9 and above and is described as the least tested of the three clients. A client can also start offline in a single-user mode with no server. The code is dual-licensed under MIT or Apache-2.0, while data and assets carry their own terms, including OpenStreetMap's ODbL and a mix of CC0 and CC BY 4.0 datasets.

Why it matters

Cartopolis joins two things that usually live apart: authoritative open government data and a shared multiplayer space. The result resembles the metaverse pitch, but built bottom-up from public registers under open licences rather than inside a proprietary platform — and, because it is self-hostable, any community, municipality or planning office can run its own instance. The HTTP API and the per-register pipeline design make it a plausible working tool for planners and architects, the stated audience alongside residents and governments, rather than just a demo. It is also a stress test of the open web stack: a WebGPU browser client rendering planet-scale geometry, with server-enforced moderation and end-to-end encrypted chat, in software anyone can inspect, fork and deploy.