Field notes 5 MIN READ
GPX, FIT or TCX — which file should you send?
Three formats, one job. What each of them actually stores, which one your watch produces, and why it does not matter as much as you would think.
You will meet three file extensions, and the difference matters less than the forums suggest.
GPX is the lowest common denominator
GPX is XML, it is old, and everything reads it. A track point holds a latitude, a longitude, a timestamp and usually an elevation. Heart rate and power can be attached, but through an extension that not every tool respects.
Its advantage is that nothing refuses it. Its cost is size: XML is verbose, so a long ride is a large file for what it contains.
FIT is what your watch actually recorded
FIT is Garmin's binary format and it is what most modern devices write natively — Garmin, Wahoo, Suunto and Coros all produce it. It is compact, and it carries far more than position: cadence, temperature, power, lap structure, device identity.
For a print, most of that is irrelevant. But one field is not: FIT usually carries the barometric altitude the device measured, rather than an elevation looked up from a map. That is the more honest number on a hilly ride.
FIT files often arrive gzipped, as `.fit.gz`. That is normal and needs no action.
TCX sits between them
TCX is Garmin's older XML format. It stores laps and trackpoints and reads like GPX with more structure. You will mostly meet it in exports from older devices or from Training Peaks.
So which one?
Send what you have. The line on the paper is drawn from coordinates, and all three carry coordinates at full precision. A 42 km marathon comes out identically whichever of the three it arrived in.
The only place the choice shows is elevation, and only when the profile is switched on. If you have both a FIT and a GPX of the same activity and the ride was hilly, the FIT is the better source.
What we do with it
Whichever arrives, it is parsed in your browser, thinned to the number of points a sheet of paper can actually resolve, and trimmed at both ends before anything is drawn. The original file never leaves your machine.
Questions people actually ask
- Which format gives the best print?
- None of them. All three carry latitude and longitude at full precision, and the drawing comes from those. Send whichever your device gives you.
- My watch exported a .fit.gz file. Is that a problem?
- No. That is a FIT file that has been compressed, which is what Garmin and Wahoo often do. It is unzipped automatically when you bring it over.
- Does the format change the elevation numbers?
- It can. FIT files usually carry the barometric altitude your watch measured; GPX files sometimes carry a corrected elevation instead. Where an activity has metadata with a stated elevation gain, that number wins over anything we recompute.
- Can I convert between them?
- Yes, but there is rarely a reason to. Converting can only lose information, never add it.
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