FIT

This is a minimal OCaml project to parse FIT files as they are produced by personal fitness devices. FIT is a binary format that groups basic values in records, which typically include a timestamp.

    {
  "msg": "20",
  "13": 11,
  "2": 1900,
  "5": 1732414,
  "6": 0,
  "1": 1669620,
  "0": 622905943,
  "253": 971857351
}

Each record has a global message number (20) which defines the purpose of the record and a number of values in position slots. The meaning of these is defined in the FIT Protocol but this library (so far) only implements the parsing. For example, 20 message is called record in the FIT protocol and slots have these meanings:

• 0 position_lat
• 1 position_long
• 2 altitude
• 5 distance
• 6 speed
• 13 temperature
• 253 timestamp

Values are further scaled and shifted, which is also defined in the protocol, and this transformation is only implemented for a few fields of the "record" message:

    {
  "msg": "record",
  "timestamp": "2020-10-17T06:25:19",
  "0": 622927716,
  "1": 1703145,
  "speed": 2.745,
  "distance": 277.23,
  "altitude": -118.8,
  "temperature": inf
}

CLI

The fit command emits the data to stdout in JSON format. I am using this currently for inspecting FIT files. The FIT file in data/ is from a bike computer.

    $ fit data/xpress-4x-2020-10-17.fit | head -25
[
  {
    "msg": "0",
    "3": 5122,
    "4": 971850094,
    "1": 267,
    "2": 1803,
    "5": 0,
    "0": 4
  },
  {
    "msg": "68",
    "0": 17,
    "1": 1,
    "2": 0,
    "3": 10,
    "4": 255,
    "5": 0,
    "6": 0,
    "7": 0,
    "8": 232,
    "9": 0,
    "10": 60,
    "11": 19693
  },
  ...

Using FIT

This is work in progress and has not been published as an official Opam package. Consider pinning it to make it available in your local Opam installation.

opam pin add -y git+https://github.com/lindig/fit

Now it is available:

    $ utop -require fit -require rresult
utop # open Rresult;;
utop # Fit.read "data/xpress-4x-2020-10-17.fit" >>= fun fit ->
       Fit.to_json |> R.return;;

Resources

• https://developer.garmin.com/fit/protocol/
• https://www.pinns.co.uk/osm/fit-for-dummies.html

Contribute

If you find this useful, please contribute back by raising pull requests for improvements you made.