Normalizes GPX file by replacing track points with exactly same coordinate with interpolated values, removing leaps with inhuman speed.

normalize-gpx.js

var fs = require("fs");
var xml2js = require("xml2js");

var gpx = process.argv[2];

// https://gist.github.com/wteuber/6241786
Math.fmod = function(a, b) {
  return Number((a - (Math.floor(a / b) * b)).toPrecision(8));
};

function clockwiseDiff(from, to) {
  return normalizeLongitude(Math.fmod(to - from - 360, 360) + 360);
};

function normalizeLongitude(lon) {
  return Math.fmod(Math.fmod(lon + 180, 360) + 360, 360) - 180;
};

fs.exists(gpx, function(exists) {
  if (!exists) {
    console.error("File " + gpx + " not found");
    return;
  }

  fs.readFile(gpx, "utf8", function(error, data) {
    if (error) {
      console.error("Reading file " + gpx + " failed: " + error);
      return;
    }

    xml2js.parseString(data, function(error, result) {
      if (error) {
        console.error("Parsing GPX file " + gpx + " failed: " + error);
        return;
      }

      var trkseg = result.gpx.trk[0].trkseg[0];
      var trkpts = trkseg.trkpt;

      var trkptsGroupedByCoordinate = trkpts.reduce(function(res, elem, idx, src) {
        var last = res[res.length - 1];
        if (!last || last[last.length - 1]["$"].lat != elem["$"].lat || last[last.length - 1]["$"].lon != elem["$"].lon) {
          res.push([elem]);
        } else {
          last.push(elem);
        }
        return res;
      }, []);

      var trkptsInterpolated = trkptsGroupedByCoordinate.reduce(function(res, elem, idx, src) {
        if (elem.length == 1) {
          res.push(elem[0]);
        } else {
          if (idx < src.length - 1) {
            var from = elem[0];
            var to = src[idx + 1][0];
            var fromT = new Date(from.time[0]).getTime();
            var toT = new Date(to.time[0]).getTime();
            var tDiff = toT - fromT;
            var latDiff = new Number(to["$"].lat) - new Number(from["$"].lat);
            var lonDiff = clockwiseDiff(new Number(from["$"].lon), new Number(to["$"].lon));
            var eleDiff = new Number(to.ele[0]) - new Number(from.ele[0]);
            elem.forEach(function(item, index) {
              if (index == 0) {
                res.push(item);
              } else {
                var t = new Date(item.time[0]).getTime();
                var factor = (t - fromT) / tDiff;
                // Interpolate latitude
                item["$"].lat = new Number(from["$"].lat) + (factor * latDiff);
                // Interpolate longitude
                item["$"].lon = normalizeLongitude(new Number(from["$"].lon) + (factor * lonDiff));
                // Interpolate elevation
                item.ele[0] = new Number(from.ele[0]) + (factor * eleDiff);
                res.push(item);
              }
            });
          } else {
            elem.forEach(function(item) {
              res.push(item);
            });
          }
        }
        return res;
      }, []);

      result.gpx.trk[0].trkseg[0].trkpt = trkptsInterpolated;

      var builder = new xml2js.Builder();
      var xml = builder.buildObject(result);
      console.log(xml);
    });
  });
});