Why your GPS says you climbed more than you did
GPS elevation gain is easily inflated by 20-40%. Why it happens, what trees, stops and barometric altimeters have to do with it, and how to read the figure.
What ‘total elevation gain’ actually measures
It isn’t the difference between the lowest and highest points of the route – that’s the net height gain. The total is the sum of every climb, one by one, all the way along. If a route climbs 300 m, drops 100 and climbs another 200, its total elevation gain is 500 m, even though the summit is only 400 above the start.
And that’s the problem: it’s a sum. Any measurement error that shows up as a climb gets added to the total and is never cancelled out. A three-hour track can have two thousand points; if each one’s altitude wobbles by a few metres because of noise, those wobbles get added up hundreds of times.
The key number
A phone GPS has a typical vertical error two to three times the horizontal one. Where it places you to within 5 m on the map, it can be 10–15 m out in height. If that wobble repeats along the track, the total is easily inflated by 20–40%, and by a lot more under dense tree cover.
Where the error comes from
Satellite geometry
BUILT INA GPS works out your position by trilateration with the satellites it can see. For horizontal position they’re spread all around you, which is the good scenario. For height, though, they’re all above you: there are none beneath your feet to help pin down the vertical axis. That asymmetry is built in, and no phone fixes it, however expensive.
Trees and ravines
BOUNCED SIGNALUnder a dense beech wood or in a narrow canyon, two things happen at once: you see fewer satellites, and the ones you do see arrive bounced off tree trunks and rock walls. That reflected signal has travelled further than the direct path, so the receiver calculates a longer distance and puts you somewhere you aren’t. In a gorge, the track ‘jumps’ from one side to the other – and every jump adds metres of climbing that never happened.
Stopping doesn’t help
STANDING STILLThis is the one fewest people know about: a stationary GPS keeps measuring. If you sit down for twenty minutes to eat with recording on, the receiver keeps logging points and each one has its own height error. With no filter, that stop can add tens of metres of climbing to a route on which you never moved from the spot.
The barometric altimeter: better, but not infallible
Many mountain watches – and some phones – have a barometer, which works out height from atmospheric pressure. It’s considerably more stable than GPS at measuring changes in height, which is why watches tend to give more believable totals.
But it measures pressure, not height. If the weather changes during the route, the barometer reads it as climbing. A front moving in and lowering the pressure as you walk gets logged as a steady climb you never did. That’s why good watches recalibrate against GPS every so often: each system covers the other’s blind spot.
Why smoothing isn’t cheating (and what it costs)
Almost every platform applies a filter before adding things up: changes in height below a threshold – typically between 3 and 10 m – are thrown out as noise rather than terrain.
That’s the right thing to do, and it’s also why the same route gives you different figures in two different apps. It’s not that one of them is lying: each has chosen its own threshold. Set it low and noise creeps in and the total shoots up; set it high and you lose real short climbs and the total comes out short. There’s no ‘true’ value, only a compromise.
What is a genuine difference between platforms is whether or not they tell you.
What Suviard does about this
We smooth the profile so the altitude chart doesn’t jump about absurdly, but we don’t correct the data towards what it ‘should’ be: we’d rather you saw what was recorded. Every route comes from a track logged by a real person on their own device, and we don’t rewrite it to make it look nice.
For the same reason, every route page shows the estimated time and the actual time together. The estimate comes from a classic mountain formula – Naismith’s rule and its corrections for elevation – and the actual time is how long the person who did it took. Showing only the estimate would be neater and a lot less honest: a ‘three-hour’ route on paper can take you five if the ground is bad.
If you spot a route whose figures don’t add up, let us know. We check it by hand.
How to read the number, in practice
- 01Compare totals from the same source. Two routes measured with the same app and the same threshold can be compared with each other even if both are inflated. Comparing one app’s number with another’s tells you next to nothing.
- 02Be suspicious of big, round totals on short routes. A thousand metres of climbing in eight kilometres does exist, but on flat, wooded ground it’s usually noise.
- 03Pause recording during long stops. It’s the fix that removes the most false metres, and the only one that’s entirely in your hands.
- 04Look at the profile, not just the figure. A profile with continuous sawteeth in an area you know is flat is noise; one with clean ramps is terrain.
- 05For planning, trust the actual time over the estimate, and treat elevation gain only as an order of magnitude. The mountains don’t forgive an optimistic figure.
In short
Total elevation gain is a sum of small differences, and sums amplify error instead of cancelling it out. Your GPS isn’t broken: it’s measuring height on the axis where it’s least accurate, and adding it up two thousand times. Treat it as an estimate with a margin of error, not a measurement – and be wary of any platform that gives it to you to three significant figures without a footnote.
More on how we work with data: about Suviard.