Compressed Lat/Lng Encoding/Decoding

MapQuest Platform Services use a slight variant of the Google Polyline Encoding Format. The MapQuest Platform Services variant of the algorithm allows for arbitrary precision of encoded data. The default precision for encoded data is 5 digits (as with the Google format), but MapQuest line data is often accurate to 6 digits of precision.

note

The precision used during encoding must be the same as the precision used during decoding, or your data will decompress to values which are off by orders of magnitude from the input data.

Javascript Sample Source

function decompress (encoded, precision) {
   precision = Math.pow(10, -precision);
   var len = encoded.length, index=0, lat=0, lng = 0, array = [];
   while (index < len) {
      var b, shift = 0, result = 0;
      do {
         b = encoded.charCodeAt(index++) - 63;
         result |= (b & 0x1f) << shift;
         shift += 5;
      } while (b >= 0x20);
      var dlat = ((result & 1) ? ~(result >> 1) : (result >> 1));
      lat += dlat;
      shift = 0;
      result = 0;
      do {
         b = encoded.charCodeAt(index++) - 63;
         result |= (b & 0x1f) << shift;
         shift += 5;
      } while (b >= 0x20);
      var dlng = ((result & 1) ? ~(result >> 1) : (result >> 1));
      lng += dlng;
      array.push(lat * precision);
      array.push(lng * precision);
   }
   return array;
}

function compress(points, precision) {
   var oldLat = 0, oldLng = 0, len = points.length, index = 0;
   var encoded = '';
   precision = Math.pow(10, precision);
   while (index < len) {
      //  Round to N decimal places
      var lat = Math.round(points[index++] * precision);
      var lng = Math.round(points[index++] * precision);

      //  Encode the differences between the points
      encoded += encodeNumber(lat - oldLat);
      encoded += encodeNumber(lng - oldLng);

      oldLat = lat;
      oldLng = lng;
   }
   return encoded;
}

function encodeNumber(num) {
   var num = num << 1;
   if (num < 0) {
      num = ~(num);
   }
   var encoded = '';
   while (num >= 0x20) {
      encoded += String.fromCharCode((0x20 | (num & 0x1f)) + 63);
      num >>= 5;
   }
   encoded += String.fromCharCode(num + 63);
   return encoded;
}