September 1, 2011

Google Maps API: Polyline encoding/decoding issue

I'm using google's Geometry Library to encode large numbers of GPS location points into a compressed format for storage. You can add this capability to your .js by adding it to the quersystring in the javascript reference:
<script type="text/javascript" src=""></script>
The library provides a very easy way to display a path, add points realtime, store this path as an encoded & compressed string, and feed that string back into a new map when you're ready to display it later. You can see a live example here.

Encoding works great, but I ran into an issue where some of my encoded paths, generated by google.maps.geometry.encoding.encodePath() from my google.maps.Polyline object, would have major errors when using the google.maps.geometry.encoding.decodePath() method. My Polyline would have random right-angle turns that effectively ruined my path. I played around with the encoded string, trying to figure out what was causing the issue, to no avail. I found another implementation of the polyline encoding algorithm, and found an explanation of what was causing the issue.

It turns out that you need to escape the backslashes that may appear in the output string from google.maps.geometry.encoding.encodePath() (and the other library linked to above). So if you're storing the string for later, you want to do something like this:
var encodedPath = google.maps.geometry.encoding.encodePath( _poly.getPath() ).replace(/\\/g,'\\\\');
You can then feed that encoded string into a new Map's Polyline instance like so:
_poly.setPath( google.maps.geometry.encoding.decodePath( encodedPath ) );
It seems like an oversight that this double-backslash issue isn't mentioned in the Google documentation. I spent hours before trying to figure out the problem in my mobile app before coming across the fix.

Finally, after you set a decoded path as the data for a Polyline, use the following code to fit the Map to the bounds of your path:
var bounds = new google.maps.LatLngBounds();
var path = _poly.getPath();
path.forEach(function( latlng ) {
	bounds.extend( latlng );
_map.fitBounds( bounds );	


  1. Hello Justin

    Do you have a demo of this script ? thanks


  2. For a basic Polyline example, look at the source code here: