Get map midpoint.

map-midpoint.js

function getMidpoint(point1, point2) {
  if (typeof(Number.prototype.toRad) === "undefined") {
    Number.prototype.toRad = function() {
      return this * Math.PI / 180;
    }
  }

  if (typeof(Number.prototype.toDeg) === "undefined") {
    Number.prototype.toDeg = function() {
      return this * 180 / Math.PI;
    }
  }

  var phi1 = point1.lat().toRad();
  var lambda1 = point1.lng().toRad();
  var phi2 = point2.lat().toRad();
  var deltaLambda = (point2.lng()-point1.lng()).toRad();

  var Bx = Math.cos(phi2) * Math.cos(deltaLambda);
  var By = Math.cos(phi2) * Math.sin(deltaLambda);

  var x = Math.sqrt((Math.cos(phi1) + Bx) * (Math.cos(phi1) + Bx) + By * By);
  var y = Math.sin(phi1) + Math.sin(phi2);
  var phi3 = Math.atan2(y, x);

  var lambda3 = lambda1 + Math.atan2(By, Math.cos(phi1) + Bx);

  return new google.maps.LatLng(phi3.toDeg(), (lambda3.toDeg()+540)%360-180);
}