Why two apps show a different distance for the same hike
You walk one route with a phone in each pocket, or upload the same file to two services, and the distance comes back different both times. It is a strange thing to see, since the ground you covered was exactly one length. The reason is that no app measures distance directly — every one of them adds up the gaps between a string of recorded points, and how that addition is done varies from app to app more than most people expect.
The short answer
Distance is not something a phone reads off the ground. It is computed afterwards, as the sum of the straight-line gap between each recorded point and the next. That sum depends on exactly which points you have, how far apart they were recorded, and whether anything has smoothed or simplified the line before adding it up. Change any of those three things and the total changes too, even for two recordings of the identical walk.
So there are really two separate questions hiding inside "why is the distance different": did the same file get measured two different ways, or did two different recordings of the same walk produce two different sets of points in the first place? Both happen, and they call for different explanations.
Same file, two different totals
Upload one GPX file to two services and you can still get two numbers, because a service is free to recompute distance from the points however it likes rather than trusting whatever total the file happened to be exported with. Some sum every point-to-point gap exactly as recorded. Others simplify the line first, dropping points that sit almost on the straight line between their neighbours, which very slightly shortens the total. Others apply smoothing that pulls small zigzags back toward a straighter path, which shortens it a little more. None of these approaches is wrong; they are different, reasonable answers to "how long is this slightly wobbly line."
This is exactly why re-uploading the same GPX to a different platform can shift the number by a percent or two without anything about the route itself changing. The file did not change. The arithmetic performed on it did.
Two recordings, two different sets of points
The bigger source of disagreement is usually further upstream, before any file even exists: two apps recording the same walk rarely capture the same points. A GPS fix on a phone lands within a few metres of the true position, not exactly on it, and that few metres of wobble is present in every single point a recorder writes down. Sum up the gaps between thousands of slightly-wobbly points and the wobble does not cancel out the way you might hope — it mostly adds a small amount of extra zigzag length that was never actually walked, because a jagged path between two points is always at least as long as the straight line between them.
How much extra length shows up depends on how often the app records a point and whether it filters what it gets. A recorder that writes a point every second or two, without smoothing, tends to sum up slightly more zigzag than one that records less often or trims the noise before adding. Neither behaviour is a defect — it is a choice about whether to keep the rawest possible track or a slightly tidied one, and different apps land in different places on that trade-off.
A few other things quietly change the number
- Where in your pocket, bag or hand the phone rode changes how clean a view of the sky it had, which changes how much wobble made it into the recording.
- An app that snaps recorded points onto a known trail or road for display can report a slightly different distance than one that plots the raw points as recorded.
- A route that includes real elevation change will read longer if an app measures distance along the slope rather than along the flat map projection, since climbing and descending covers more actual ground than the map line suggests.
- Starting the recording a few seconds early or stopping it a few seconds late adds a small stretch of standing-still wobble that a second app, started and stopped more precisely, will not have.
What to actually do about it
Do not treat any of this as a competition to find the "true" number — for a walk on foot, GPS was never going to deliver a distance accurate to the metre, and the small disagreement between two reasonable measurements is not a sign either one is broken. What matters far more than matching another app exactly is staying consistent with yourself: use the same app to record, and its totals become comparable with each other even if they never quite match a friend's app or a trailhead sign.
TrailCam records distance, moving time, pace and elevation gain for every route, and exports the underlying track as GPX 1.1 or CSV, so you can see for yourself exactly which points a total was built from rather than taking any app's number on faith.
Questions
- Which app is giving the "correct" distance?
- Usually neither is more correct than the other. Both are computed from a string of GPS points that each carry a few metres of ordinary wobble, summed up in whatever way that particular app or service chooses. Small disagreements are the normal result, not a sign of a fault.
- Why did uploading the same GPX file to another service change the distance?
- Because the receiving service usually recalculates distance from the raw points rather than trusting the total it was given, and different services simplify or smooth the line by different amounts before adding it up.
- Does recording more often make the distance more accurate?
- Not necessarily. Recording more often captures the route in finer detail, but it also captures more of the small GPS wobble at every point, which tends to push a raw, unsmoothed total slightly higher rather than closer to the ground truth.
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.