How I built a maps app on open data
- Parchment Maps
- maps
- openstreetmap
- architecture
In this series
- 1How I built a maps app on open dataYou are hereWhat Parchment is, who it is for, and the four commitments it runs on. The rest of the series is how far that has actually got.
- 2Where Parchment startedThe goal was a maps app that is private, self-hostable and free, built on open data. The free public endpoints were not a shortcut around that goal. They were the goal.
- 3Turning map providers into an interfaceParchment stopped asking which service to call and started asking which job needs doing. The idea came from Home Assistant, and it changed how every feature after it got designed.
- 4The subsystem that became a productThe free APIs were not too expensive. They were unchangeable. Building my own geospatial engine split the architecture in two, and the half that holds no personal data turned out to be a business.
- 5Where Parchment stands, and what is leftParchment is in closed alpha behind a waitlist. Here is what works, what is still being built, and what has to be true before I let more people in.
I started Parchment in November 2023 and it has eaten most of my spare time since.
OpenStreetMap is the largest open map of the world, traced and tagged by volunteers down to the bench outside the pub and the step at the door. The data is remarkable. The apps built on it are mostly not.
What it is for#
Every mainstream maps app is a driving app with a transit tab bolted on. That is rational as a business decision and miserable if you do not own a car.
I ride a bike and take the train, so Parchment is arranged around cycling, transit and walking rather than treating them as modes you switch into.
Mostly that means mixed trips. A real journey is a walk to a rack, a ride to a station, a train, and a walk at the far end — one trip, planned once, seams included. It also means the details you only notice when you are outside: hills, unpaved surface, a curb you have to lift a wheel over, which station entrance is step-free, whether the bus stop has a shelter or is a sign in the grass. Volunteers recorded all of that years ago. Almost no consumer app surfaces it.
How it gets there#
Open data. OpenStreetMap underneath, so the map cannot be revoked or repriced. When it is wrong you can fix it at the source, for everyone.
Self-hostable. The only honest version of a privacy promise is letting you keep your data on hardware you own.
You are not the product. No ads, no selling where you go. Companies buy geospatial API access from Barrelman, and privacy should not require a homelab, so there is a hosted option as well. Never you, sold onward.
It should be good. Open source alternatives get graded on a curve, and I would rather not be. The goal is the app you would pick anyway.
I will not sell out. Most software I have loved was eventually acquired and hollowed out by people who did not use it. An acquisition is the single event that breaks all four commitments above at once, so it is not enough to promise it will not happen. The projects that survive intact are the ones that put themselves somewhere they cannot be sold, and that is the bar I want Parchment held to.
The rest of the series chronicles the challenges and revelations that I have had during the development of my silly little maps app.