code: function (a, b) {
          var dx = (a[0] - b[0]) * this.kx;
          var dy = (a[1] - b[1]) * this.ky;
          return Math.sqrt(dx * dx + dy * dy);
      }
docs: Given two points of the form [longitude, latitude], returns the distance.
  
  @param {Array<number>} a point [longitude, latitude]
  @param {Array<number>} b point [longitude, latitude]
  @returns {number} distance
  @example
  var distance = ruler.distance([30.5, 50.5], [30.51, 50.49]);
  //=distance