code: function calculateLonOffset(lat, dx) {
    return ((dx / R_EARTH) * (180 / Math.PI)) / Math.cos((lat * Math.PI) / 180);
  }
docs: with a given x-km change, and current latitude
  calculate the increment of longitude
  based on stackoverflow http://stackoverflow.com/questions/7477003
  @param {number} lat - latitude of current location (based on city)
  @param {number} dx - change in km
  @return {number} - increment in longitude