Why this survey exists
On a steep lane above Aberystwyth there is a road sign that says 32%. For years, the same hill carried a sign that said 25%. Same tarmac, same gravity — a quarter steeper, apparently, by decision of a sign-maker.
Everyone who cares about hills has met this problem. A book says one number, a GPS unit says another, the sign says a third, and the rider halfway up has an opinion of their own. I wanted to know which number was true — and it turned out that was the wrong question.
Britain publishes national laser-scanning surveys as open data — aircraft measuring the height of the ground roughly every metre, free for anyone to use. So instead of adjudicating between the sign and the book, I measured the hill — every metre of it, from the public data, with a method anyone can re-run.
There is a full circle in that for me. Years ago I authored the Parliamentary Office of Science and Technology's report Electronic Government: Information Technologies and the Citizen (POST, 1998), which argued — among other things — that data the public had already paid to create should be made freely available for anyone to use. This site is a small demonstration of why: open the data, and eventually somebody measures every hill in the country with it.
The answer on that lane — Cefn Llan — was that both signs were right. Over its steepest 2 metres the road is 32%. Over its steepest 100 metres it is 25%. Steepness has no single true value: it depends on the distance you measure it over, and almost nobody who publishes a gradient tells you theirs. That one realisation is the whole site. Every number here states its measuring distance. That is the difference between a measurement and a claim.
I have stood under the 32% sign since, and I still can't tell you what the council measured to get it — but I can tell you exactly what the laser survey says at every distance, and you can check every step of the working: the method, the data sources, the exact survey tiles, a machine-readable file per climb. Where a value is less certain — under tree canopy, near a bridge — the page says so instead of pretending.
The survey started with that one Welsh hill and rolls on, region by region, wherever good open elevation data exists: every drivable road is screened, thousands are measured, and only the couple of hundred most notable per region get pages — the rest are measured all the same and live in the open dataset. Today the site carries 257 climbs across Wales and England. Signs, books and local knowledge aren't the enemy of any of this: they are evidence, recorded and tested like everything else. If you know something a laser can't — a name, a gate, a sign we've missed — tell us.
All of this — the pipeline, the measurements, this site — is the work of one human, enhanced and enabled by Claude Code (Fable), building on open data, open tools and open methods. When these pages say “we”, that is who they mean. Corrections are welcome, and so is scepticism — the point of showing the working is that you don't have to take my word for anything.
Peter Hirst
Aberystwyth, August 2026