Open an open-water swimming .fit file — GPS between strokes
Open water is where swimming gets GPS back and immediately misuses it — every distance in the file leans a little long, for a reason worth understanding.
Open your file
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a .fit file from a watch or bike computer
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A fix only when your wrist surfaces
GPS does not work underwater, so in open water the watch can only grab a position when your arm swings clear of the surface — a fraction of a second per stroke. The resulting track is sparse and jagged, and each recovered fix that lands slightly off adds a small zig-zag the straight swim never made. Summed over a kilometre, those zig-zags make open-water distance run long, which is the normal error direction here rather than a defect.
Why it is noisier than any pool swim
A pool swim counts clean lengths; an open-water swim reconstructs a path from intermittent, surface-snatched fixes. Same sport token, completely different measurement, and completely different reasons for a wrong number: setting-and-turns in the pool, satellite-and-surfacing in open water. A file that mixes both across a season mixes two error profiles under one label.
What you get from this page
This page opens your open-water file on your device and shows the GPS track with its real sparseness, the distance the watch derived, and the stroke data where present — so the overshoot is legible as geometry rather than a mystery.
In the corpus this tool is tested against, 42 activities resolved to this sport. See the full activity-type list for every sport a .fit file can declare, and open any .fit file whatever its sport.