TopoLine. A climbing app for places phones don’t reach.

I live in the Peak District, and climbing is one of the things I love doing most. Climbers find their routes with topos: a photo or drawing of the rock with each climb marked on it, its grade, its length and a description. They’re in guidebooks, on websites and in apps, and they all share one problem. The crags are where phones don’t work.

So I wanted an app that’s calm and simple, works with no signal at all, and makes it easy for anyone to draw a topo of their own crag. Not a social network or a video app: a quick way to make and use topos.

The brief kept the app to one simple structure, crag, sector, topo, route, “no deeper”, and set the tone in one line: create your own crags, draw your route lines, filter by grade, use it at the crag. “No signal, no clutter, no faff.”

Climbers’ feedback sharpened it. Finding the climb matters as much as the climb itself, “not just the bottoms but sometimes for sea cliffs, the abseil points above”. So TopoLine grew directions from the car park, map pins for parking, abseil points and descents, and maps that work offline.

Mark’s first new route came in September 2026, at Faraid Head, a headland of dunes and sea cliffs near Durness on the north coast of Sutherland, climbed with the mountaineer Mick Fowler and George Bell. It’s called I’m Lemon Barnaby, and it’s in the UKClimbing logbook and the Scottish Mountaineering Club’s database of Scottish climbs.

On the same trip Mark made a Durness crag in TopoLine from a phone photo of the cliffs: the app doing exactly what it was built for.

• I’m Lemon Barnaby on UKClimbing
• Faraid Head on the Scottish Mountaineering Club’s route database
• The Durness crag on TopoLine

AI wrote the code, but the experience was designed. The look started from a detailed design hand-off: colours named after the rock (chalk, limestone, slate, moss, rust and a Route Red), three typefaces, and screen-by-screen specifications. Mark is working on the design in Figma too, still a work in progress, and the logo is Mark’s own: a faceted grey boulder with a red route line up it.

Then came the details only a climber would ask for. A few of Mark’s requests, and what they became:

Everything else in the app serves this screen. The photo fills it, with each route drawn on as a numbered line in its grade colour. The colours are automatic and can’t be changed, because a colour picker would make a rainbow that means nothing:

• Up to Severe
• Up to HVS
• Up to E3
• Up to E6
• Harder
• No grade yet

Pinch to zoom up to eight times: the numbers and markers stay the same size, so they never cover the rock. Filter by type of climb and “up to grade” in seven grading systems. Tap a route and its details open under it in the list: the description, a photo of where it starts (“am I on the right line?”), and buttons to tick it or mark it as a project. Topos can also carry abseil stations, notes, arrows and belay stances.

On a computer, My Crags is a studio for making topos. Press the orange Draw route button, click up the line from the ground, double-click to finish, then drag the white handles to shape it. A small “+” sits on the curve between handles: drag it to add a bend. Drag the routes into order from left to right and the numbers follow; set the grade and the colour follows.

Before anything is published, a checklist makes sure the photo is uploaded, every route has a line and a grade, and the topo has a location. The person publishing confirms the photo is theirs to share.

Mark was clear from early on that the phone matters most:

So a whole crag can be made on a phone, at the crag, with no signal: details, location (“use my current location” works offline), photos from the camera at full quality, sectors, topos and lines, tapped on with a finger. Back in signal, it uploads itself: the words first, then the photos one at a time, with a progress bar. If the signal drops, it carries on where it stopped and never makes a copy twice, and there’s always a Stop, Retry or Delete button, never an endless spinner.

One tap on Save downloads a whole crag to the phone. After that it opens straight from the phone, instantly, and quietly checks for changes when there’s a signal. The brief was firm about it: a saved crag must open with no signal, without asking for a login or checking a subscription.

• Every topo, with its photos at full resolution
• A photo of where each route starts
• A 15 km map around the crag: footpaths, cliffs, parking and landmarks
• A blue “you are here” dot with an arrow for the way you’re facing, from the phone’s GPS, which works without a signal
• Ticks and reports made offline, sent by themselves later
• Pro members get 28 days between check-ins, and crags you made yourself never expire

Maps were the biggest journey. Google Maps was priced up and ruled out on cost on day one. The first maps came from hosted services; by July, TopoLine was drawing its own, from OpenStreetMap data for the whole planet, about 65 GB of it on a disk of its own, with no charge per map tile and no keys to manage.

After feedback from climbers, the maps were restyled the way walkers’ maps are: bold footpaths and bridleways, cliffs as strong dark lines, woods, fields and scree, peaks, parking and campsites. For Great Britain there are 10-metre contours and hill shading, made from Ordnance Survey’s free height data. The same style is used on the website and in the app, so a crag looks the same on both.

Creating is free and unlimited for everyone, and so is browsing the website, because the topos are what makes the app worth having. What Pro pays for is having every crag offline on your phone. People who make crags keep them free on the app for life. In Mark’s words: “this is to reward owners and uploaders.”

Payments are built two ways: Stripe on the website, and Google Play and App Store billing in the app, through RevenueCat. Both write to the same account, and whichever subscription runs longest wins. Both are still in test mode.

TopoLine is three things that share one server: the public website, the My Crags studio, and the Android app. The app’s JavaScript updates over the air, so most fixes reach the phone without a new build.

For location, the app uses the phone’s GPS for the dot and the way you’re facing, and the website the browser’s own location. Parking and access points can also carry a what3words address. Offline, the app keeps its own small database and folders on the phone for the saved crags, photos, map tiles and anything made at the crag, waiting to upload.

Mark set the direction and tested every build on a phone. Claude wrote the code, the documents and the tests and put each change live, over about 30 Claude Code sessions, most of it between 6 June and 7 July 2026. 219 of the 243 saved changes carry Claude’s name. The result is about 32,000 lines: 10,000 for the website, 12,800 for the app, 7,100 for the server and 2,000 of tests.

The rule was that a different session checks the work, often on a different model. Its findings came back, the fixes went in the same day, and each one was written into the project’s notes, so the next session started with everything that had been learned.

The website is live and the Android app runs on Mark’s phone, updating itself over the air. Before it can go on Google Play, there’s a list of jobs the stores rightly insist on: