Elevation gain — why every app gives a different number
No two tools agree on how much you climbed, and it is not because one is right — elevation gain is computed three different ways from three different sources.
Open your file
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a .fit file from a watch or bike computer
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Three sources, three answers
A watch with a barometric altimeter derives elevation from air pressure; without one, it uses GPS altitude, which is far noisier vertically than horizontally; and a mapping website throws both away and re-derives your climb from a terrain model along your track. These are three independent methods, and they disagree by design — barometric drifts with weather, GPS altitude wanders, and a terrain model is only as fine as its grid. So a climbing total is the output of whichever method produced it, not a measured truth all three should converge on.
Why the totals can differ a lot
The disagreement is largest exactly where climbing matters most: on steep, rough or wooded terrain, where GPS altitude is worst and a coarse terrain model smooths away real rollers. A smoothing threshold — how small a rise counts as “gain” — changes the total again, and every tool sets it differently. Two apps reading the same file can report climbing totals tens of percent apart, all correct for their method and threshold.
See which source your file used
This page reads your .fit file on your device and shows the elevation fields it actually holds — barometric altitude where the watch recorded it, GPS altitude separately — so you can see which source your climbing total was built from instead of wondering why two apps disagree. It runs locally and uploads nothing.
This is one field among many. Open your .fit file to see every field it holds, or the activity-type list for what each sport records.