Why elevation gain changes every time you hike the same trail
Walk the same loop twice in a month and the elevation gain your app reports is unlikely to match exactly, even though the ground under your boots has not moved. That gap unsettles people more than almost any other number a hiking app produces, because distance mostly behaves the way you would expect and elevation, for reasons that have nothing to do with the trail itself, does not.
The short answer
Elevation gain is not a fixed property of a trail the way its length is. It is a measurement made fresh on every walk, from whatever altitude data your phone happened to log that day, filtered by whatever noise threshold the app applies. Two recordings of the same path are two separate measurements, not two readings of one stored number, so a difference between them is the normal outcome rather than a sign that anything went wrong.
A gap of a few percent between visits is ordinary. A gap of tens of percent usually traces back to one of the specific causes below — mostly weather and exactly where your feet went, not the phone misbehaving.
What is actually different between two recordings of the same path
The weather on the day
A phone that leans on a barometric sensor for smooth altitude is reading air pressure, and pressure shifts with the weather independently of how high you actually are. A front moving through partway into one of your two walks, and not the other, shifts the apparent altitude by a real amount on one recording and not the other — purely because the air was doing something different that afternoon.
Satellite geometry at that hour
GPS altitude is inherently noisier than GPS position, and how noisy depends partly on the arrangement of satellites overhead at the moment you were walking, which changes through the day and from one day to the next. A walk recorded in the morning can have a cleaner vertical signal than the identical walk done in the afternoon, for reasons that have nothing to do with the trail.
Where your feet actually went
No two walks of the "same" trail retrace an identical line to the centimetre. Stepping around a puddle, cutting a switchback slightly wide, or pausing somewhere the altitude reading happens to jitter all change exactly which points get logged — and which of them cross the app's noise threshold and get counted as a climb.
Where the recording actually started
A route started from the car park is not the same recording as one started from the trailhead sign further up the road, and the two can differ in both distance and gain before either walk has covered any of the trail most people mean when they name it.
What is not happening
The trail is not changing height between visits, your phone is not degrading, and the app is not making a mistake an update would fix. Elevation gain looks precise because it arrives as a round number, but it is closer to an estimate than to a measurement, built by adding up thousands of small differences that were each individually uncertain. Expecting two visits to match to the metre asks more precision of the process than it was ever built to deliver.
How to actually use the number
Compare a route against itself, inside one app, across several visits, rather than expecting any single reading to be the truth. A range — say, roughly 700 to 780 metres across five recordings of the same loop — tells you more than one figure pretending to be exact, and it is the honest shape of what the measurement actually is.
Distance holds up far better across repeats than elevation does, because horizontal GPS accuracy is simply better than vertical. If you need one number to trust closely from walk to walk, lean on distance and treat elevation gain as the rougher of the two.
TrailCam keeps distance, moving time, pace and elevation gain for every route in its history, so comparing this visit against the last time you walked the same trail is a matter of opening two saved routes side by side rather than trying to remember a figure from a month ago. It is free to download.
Questions
- Is my phone broken if elevation gain is different every time I hike the same trail?
- No. Small differences between visits are the normal outcome of measuring altitude fresh on every walk. Look for a rough range across several recordings rather than expecting one exact figure to repeat.
- Why does distance match better than elevation when I repeat a hike?
- GPS is inherently more accurate horizontally than vertically, so distance holds together across repeats in a way elevation gain does not. Treat distance as the steadier of the two numbers.
- Does weather really change the elevation number, not just the walk itself?
- Yes, if the phone uses a barometric sensor for altitude. That sensor measures air pressure, and pressure shifts with the weather independently of your actual height, so the same trail on two different days with two different weather systems can log two different gain totals.
Recording your own routes
TrailCam is an iPhone app that records your route with GPS, keeps video clips, photos and voice notes pinned to the spot where you took them, and exports the result as GPX 1.1 or CSV. It is free to download.