Field notes 6 MIN READ
Why Strava and Garmin disagree about elevation
Same ride, two totals, sometimes a difference of hundreds of metres. Neither is lying. They are answering slightly different questions.
Ride a hilly loop, look at your watch, look at Strava. The totals differ. Sometimes by a little, occasionally by a third.
This is not a bug in either. It is a genuinely hard question with more than one defensible answer.
Elevation gain is a calculation, not a measurement
Your device measures altitude — either from air pressure, or by looking up your position in a terrain database. Altitude is a measurement.
Gain is what you get by walking through that series and adding up every rise. And that is where it stops being physics.
A GPS or barometric altitude series is noisy. Standing still at a traffic light, the recorded altitude drifts up and down by a metre or two. Add up every one of those rises over four hours and you can accumulate hundreds of metres of climbing without leaving the junction.
So every system applies a threshold: a rise only counts once it has been sustained for some minimum. Strava's help documentation describes needing consistent climbing of more than about 10 m for activities without a barometer, and around 2 m for those with one.
Move that threshold and the answer moves with it. On our own reference ride, computing gain at a 3 m threshold gives 1 140 m. At 0 m — counting every wobble — it gives 1 505 m. At 20 m it gives 462 m. Same ride, same file, three answers.
Barometric versus terrain lookup
A watch with a barometric altimeter measures pressure and derives altitude. This catches small undulations that a map lookup never will, which is why Strava treats barometric data as higher quality.
Without a barometer, Strava cross-references your position against its own elevation database. That is smooth and stable and knows nothing about the bridge you rode over.
The awkward case is when a barometric device is not recognised — a missing device identity in the file — and its measurements get replaced by a terrain lookup anyway. This is the usual explanation for a re-upload changing the number.
Pressure is weather
A barometric altimeter cannot tell the difference between climbing and a falling front. Ride six hours into deteriorating weather and pressure drops for both reasons at once. The altimeter reports the sum.
Most devices correct for this at the start, from GPS or a known point, and some correct continuously. None of them can fully separate the two.
What we print
If the file states an elevation gain, we print that.
We can compute one — we do, when there is nothing else — but a number that depends on a threshold nobody can see is a poor thing to put on a wall. The number on your watch at the top of the climb is the number you told people. That is the one that belongs under the line.
Questions people actually ask
- Which number is correct?
- Neither, and both. Elevation gain is not a measurement but a calculation, and it depends on a threshold you cannot see. A barometric device with a clean pressure reading is usually the better source.
- Why does the same ride change when I re-upload it?
- Because the processing is applied on upload. If the device identity is missing from the file, Strava may treat a barometric recording as a GPS one and substitute its own terrain data.
- Does weather affect elevation?
- Yes. A barometric altimeter reads air pressure, and a passing weather front changes pressure independently of your altitude. A long ride through a falling front will over-report climbing.
- Which number do you print?
- Whatever the file states. If the activity carries an elevation gain in its metadata, that is the number on the paper — because it is the number you remember.
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