Does elevation gain come from GPS or a barometer on an iPhone?
Both, usually, and not in fixed proportions. An iPhone's location system can draw on a GPS-derived altitude and on readings from the phone's own barometric pressure sensor, and it leans on whichever one is behaving better at that moment rather than picking one source and sticking with it for the whole hike. The elevation gain calculated afterward reflects whatever mix came out of that, which is part of why the same trail can hand back a different number on two different days.
The short answer
Height is the one part of a GPS fix that satellites are inherently bad at, and pressure is the one thing a barometer measures directly and continuously, so a phone that has both keeps switching between them rather than trusting either one alone. GPS altitude comes from the same satellite signals as your position, just used to solve for a different unknown. Barometric altitude comes from a small pressure sensor built into the phone, converting a change in air pressure into an estimated change in height. Neither one is the elevation gain a hike report shows — that number is calculated afterward, from whichever blend of the two the phone recorded along the way.
Why GPS alone is a weak tool for height
A GPS position comes from triangulating your distance to several satellites at once, and the geometry behind that triangulation is much stronger for position on the ground than it is for height above it. Every satellite involved sits somewhere in the sky above you, never below or off to the side the way it would need to for height to be measured as cleanly as position is. The practical result is that vertical error in a GPS fix is routinely several times larger than horizontal error, even at a moment when the position on the map looks perfectly solid. That mismatch is why a track's route can look accurate while its elevation reads jumpy right on the same walk.
What a barometer adds — and the one thing it cannot tell apart
Air pressure drops in a smooth, predictable way as you gain height, so a barometer turns a small pressure change into a height change far more responsively than GPS can, which is why barometric readings tend to look smoother and catch a short flight of switchbacks that GPS altitude alone might blur past. The catch is that air pressure also drops and rises with the weather, for reasons that have nothing to do with height at all. A front moving through while you are standing still at the same spot changes the pressure your phone is reading, and a barometer on its own has no way to tell that apart from having actually climbed.
How this shows up in your elevation gain number
Because the phone is combining a source that is smooth but weather-sensitive with one that is noisy but weather-independent, the elevation gain a hike shows afterward is a recalculation from that mixture rather than an untouched reading from either sensor. Walking the same trail on a calm, stable day and again on a day when a system is moving through can produce two different totals without a single step of the route actually changing, because the mix of GPS and barometric influence landed differently each time. It is one more reason, alongside ordinary GPS noise, that elevation gain is the figure most likely to disagree between two recordings of an identical route — or between two apps looking at the same one.
Where TrailCam fits in
TrailCam records elevation as part of the GPS track and draws it as a profile underneath the map when you replay a hike, so the climb is something you can look at rather than just a single total. A route keeps that elevation on every point when it is exported as GPX 1.1, which is part of the optional TrailCam Pro upgrade; the app itself is free to download.
Questions
- Can I choose whether my iPhone trusts GPS or the barometer more for elevation?
- No — that blending happens inside the phone's own location system, not in an app built on top of it. A hiking app receives whatever altitude value the phone reports at that moment and has no separate switch to prefer one source over the other.
- Why did the same trail show more elevation gain on a stormy day than on a clear one?
- A barometer reads air pressure, and pressure shifts with the weather independently of height. A change in the weather while you are on the trail can nudge the pressure-based part of the elevation reading even though the ground under your feet never changed.
- Does this same GPS-versus-barometer mix-up affect distance too?
- Not in the same way. Distance is built mainly from your horizontal position, which is exactly what GPS is good at measuring. Elevation is the outlier because it depends on this extra blend with a barometer, which is why the two numbers can behave so differently on the same hike.
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.