console.log(distVincenty(39.63,-98.78,27.52,2.98));

​

/*!

 * JavaScript function to calculate the geodetic distance between two points specified by latitude/longitude using the Vincenty inverse formula for ellipsoids.

 *

 * Original scripts by Chris Veness

 * Taken from http://movable-type.co.uk/scripts/latlong-vincenty.html and optimized / cleaned up by Mathias Bynens <http://mathiasbynens.be/>

 * Based on the Vincenty direct formula by T. Vincenty, “Direct and Inverse Solutions of Geodesics on the Ellipsoid with application of nested equations”, Survey Review, vol XXII no 176, 1975 <http://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf>

 *

 * @param   {Number} lat1, lon1: first point in decimal degrees

 * @param   {Number} lat2, lon2: second point in decimal degrees

 * @returns {Number} distance in metres between points

 */

function toRad(n) {

 return n * Math.PI / 180;

};

​

/*

​

WGS-84  a = 6 378 137 m (±2 m)  b ≈ 6 356 752.314245 m  f ≈ 1 / 298.257223563

GRS-80  a = 6 378 137 m b ≈ 6 356 752.314140 m  f = 1 / 298.257222101

Airy 1830   a = 6 377 563.396 m b = 6 356 256.909 m f ≈ 1 / 299.3249646

Internat’l 1924 a = 6 378 388 m b ≈ 6 356 911.946 m f = 1 / 297

Clarke mod.1880 a = 6 378 249.145 m b ≈ 6 356 514.86955 m   f = 1 / 293.465

GRS-67  a = 6 378 160 m b ≈ 6 356 774.719 m f = 1 / 298.247167

​

*/

function distVincenty(lat1, lon1, lat2, lon2) {

 var a = 6378137,

     b = 6356752.3142,

     f = 1 / 298.257223563, // WGS-84 ellipsoid params

     L = toRad(lon2-lon1),

     U1 = Math.atan((1 - f) * Math.tan(toRad(lat1))),

     U2 = Math.atan((1 - f) * Math.tan(toRad(lat2))),

     sinU1 = Math.sin(U1),

     cosU1 = Math.cos(U1),

     sinU2 = Math.sin(U2),

     cosU2 = Math.cos(U2),

     lambda = L,

     lambdaP,

     iterLimit = 100;

 do {

  var sinLambda = Math.sin(lambda),

      cosLambda = Math.cos(lambda),

      sinSigma = Math.sqrt((cosU2 * sinLambda) * (cosU2 * sinLambda) + (cosU1 * sinU2 - sinU1 * cosU2 * cosLambda) * (cosU1 * sinU2 - sinU1 * cosU2 * cosLambda));

  if (0 === sinSigma) {

   return 0; // co-incident points

  };

  var cosSigma = sinU1 * sinU2 + cosU1 * cosU2 * cosLambda,

      sigma = Math.atan2(sinSigma, cosSigma),

      sinAlpha = cosU1 * cosU2 * sinLambda / sinSigma,

      cosSqAlpha = 1 - sinAlpha * sinAlpha,

      cos2SigmaM = cosSigma - 2 * sinU1 * sinU2 / cosSqAlpha,

      C = f / 16 * cosSqAlpha * (4 + f * (4 - 3 * cosSqAlpha));

  if (isNaN(cos2SigmaM)) {

   cos2SigmaM = 0; // equatorial line: cosSqAlpha = 0 (§6)

  };

  lambdaP = lambda;

  lambda = L + (1 - C) * f * sinAlpha * (sigma + C * sinSigma * (cos2SigmaM + C * cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM)));

 } while (Math.abs(lambda - lambdaP) > 1e-12 && --iterLimit > 0);

 if (!iterLimit) {

  return NaN; // formula failed to converge

 };

 var uSq = cosSqAlpha * (a * a - b * b) / (b * b),

     A = 1 + uSq / 16384 * (4096 + uSq * (-768 + uSq * (320 - 175 * uSq))),

     B = uSq / 1024 * (256 + uSq * (-128 + uSq * (74 - 47 * uSq))),

     deltaSigma = B * sinSigma * (cos2SigmaM + B / 4 * (cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM) - B / 6 * cos2SigmaM * (-3 + 4 * sinSigma * sinSigma) * (-3 + 4 * cos2SigmaM * cos2SigmaM))),

     s = b * A * (sigma - deltaSigma);

 return s.toFixed(3); // round to 1mm precision

};