package fit

FIT

This is a minimal OCaml project to parse FIT files as they are produced by personal fitness devices. The main purpose of FIT is to provide a library for reading such FIT files but in addition it includes a small command line tool to emit FIT files in JSON format.

FIT is a binary format invented by Garmin that groups basic values in records, which typically include a timestamp. Below is such a record in JSON format as emitted by the tool.

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

Each record has a global message number (like 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, message 20 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": "69",
  "0": 4
},
{
  "msg": "record",
  "timestamp": "2020-10-17T06:24:03",
  "latitude": 52.21163995563984,
  "longitude": 0.1410909835249186,
  "speed": 2.767,
  "distance": 65.54000000000001,
  "altitude": -118.8,
  "temperature": 10
},

As can be seen above, FIT does not implement decoding of "69" message blocks. The "record" (20) block is usually the one we are interested in and FIT implements decoding of the most interesing fields.

CLI

This code is primarily intended as a library but it also provides a small binary. 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
  },
  ...

Installing FIT

Fit is published as an Opam package such that it can be installed from Opam:

opam install fit

You can also pin it directly for access to unpublished changes:

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

Once installed, you can use it:

    $ 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;;

The fit binary takes a FIT file as argument:

    $ fit data/xpress-4x-2020-10-17.fit | tail -15
    "58": 11,
    "20": null,
    "21": null
  },
  {
    "msg": "activity",
    "timestamp": "2020-10-17T08:22:35",
    "0": 7263000,
    "5": 971860957,
    "1": 1,
    "2": 0,
    "3": 26,
    "4": 1
  }
$ fit --help

Contribute

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