Fix Strava GPX

README.md

Your GPS device has messed up and the recorded route has a faulty segment?

This code iterates over recorded points, finding biggest distance discrepancy and allowing you to remove it and save an altered XML version leaving out those measurements.

fix_strava_gpx.js

const fs = require('fs').promises;
const convert = require('xml-js'); // https://github.com/nashwaan/xml-js

const π = Math.PI;
const DEG2RAD = π / 180;

const RK = 6373; // mean radius of the earth (km) at 39 degrees from the equator

function distance(a, b) {
  // convert coordinates to radians
  const lat1 = DEG2RAD * a.lat;
  const lon1 = DEG2RAD * a.lon;
  const lat2 = DEG2RAD * b.lat;
  const lon2 = DEG2RAD * b.lon;

  // find the differences between the coordinates
  const dlat = lat2 - lat1;
  const dlon = lon2 - lon1;

  // here's the heavy lifting
  const aa =
    Math.pow(Math.sin(dlat / 2), 2) +
    Math.pow(Math.sin(dlon / 2) * Math.cos(lat1) * Math.cos(lat2), 2);

  const c = 2 * Math.atan2(Math.sqrt(aa), Math.sqrt(1 - aa)); // great circle distance in radians

  return c * RK; // great circle distance in km
}

function pairConsecutives(arr) {
  return arr.slice(0, arr.length - 1).map((el, idx) => [el, arr[idx + 1]]);
}

async function go(gpxFile, removals) {
  const xml = await fs.readFile(gpxFile);
  const o = convert.xml2js(xml);

  const rootEl = o.elements[0];
  const trkEl = rootEl.elements.find((el) => el.name === 'trk');
  const trkSegEl = trkEl.elements.find((el) => el.name === 'trkseg');

  // remove outliers (sort dec):
  removals.sort((a, b) => b - a);
  removals.forEach((idx) => {
    trkSegEl.elements.splice(idx, 1);
  });

  const data = trkSegEl.elements
    .map((el) => {
      const lat = parseFloat(el.attributes.lat);
      const lon = parseFloat(el.attributes.lon);

      const timeEl = el.elements.find((el) => el.name === 'time');
      const time = new Date(timeEl.elements[0].text);

      return { lat, lon, time };
    });

  let maxI;
  let maxDist = 0;
  let maxDt = 0;

  pairConsecutives(data).forEach(([a, b], i) => {
    const dist = distance(a, b); // in km
    const dt = (b.time.valueOf() - a.time.valueOf()) / 1000; // in secs
    const vel = (dist * 3600) / dt; // in km/h
    console.log(
      `#${i} | dist: ${Math.round(
        dist * 1000
      )} m | dt: ${dt} secs | vel: ${vel.toFixed(2)} km/h`
    );
    if (dist > maxDist) {
      maxDist = dist;
      maxDt = dt;
      maxI = i;
    }
  });
  
  console.log(
    `to #${maxI + 1} ~> maxDist: ${Math.round(
      maxDist * 1000
    )} m, with dt of ${maxDt} secs`
  );

  const xml2 = convert.js2xml(o, { spaces: 1 });
  const gpxFile2 = gpxFile.replace('.gpx', '_fixed.gpx');
  await fs.writeFile(gpxFile2, xml2);
  console.log(`saved ${gpxFile2}`);
}

go(
  // file to read
  '/Users/jdi14/Downloads/Afternoon_Run.gpx', 
  [
    // readings to leave out
    348,
    349,
    350,
  ]
);