Don't calculate distances with Pythagoras and simple magic constant.

Simple JS

var Helsinki = {
  lat: 60.1639305,
  lng: 24.9001888
};
var Oulu = {
  lat: 64.4984149,
  lng: 26.1482539
};
var Lappeenranta = {
  lat: 60.9673061,
  lng: 27.9629412
};

function distancePythagora(from, to) {
  var a = Math.abs(from.lat - to.lat);
  var b = Math.abs(from.lng - to.lng);
  var c = Math.sqrt(a^2 + b^2);
  var magicConst = 118;
  return c * magicConst;
}

var HelsinkiOuluPythagora = distancePythagora(Helsinki, Oulu);
var HelsinkiOuluReal = 539.313;
var HelsinkiOuluError = ( HelsinkiOuluReal / HelsinkiOuluPythagora ) * 100;


var HelsinkiLappeenrantaPythagora = distancePythagora(Helsinki, Lappeenranta);
var HelsinkiLappeenrantaReal = 202.753;
var HelsinkiLappeenrantaError = (  HelsinkiLappeenrantaReal / HelsinkiLappeenrantaPythagora ) * 100;

console.log("---");

console.log("Helsinki-Oulu");
console.log("Pythagora: " + HelsinkiLappeenrantaReal);
console.log("Real: " + HelsinkiOuluReal);
console.log("Error: " +  HelsinkiOuluError + " %");

console.log("---");

console.log("Helsinki-Lappeenranta");
console.log("Pythagora: " + HelsinkiLappeenrantaPythagora);
console.log("Real: " + HelsinkiLappeenrantaReal);
console.log("Error: " +  HelsinkiLappeenrantaError + " %");

console.log("---");

console.log("Pythagoras says Lappeenranta is further from Helsinki than Oulu!");
console.log("Pythagora distance difference: " +  (HelsinkiLappeenrantaPythagora - HelsinkiOuluPythagora));
console.log("Really, Oulu quite a bit is further from Helsinki than Lappeenranta!");
console.log("Real distance difference: " + (HelsinkiLappeenrantaReal - HelsinkiOuluReal));