How to decode the Google Directions API polylines field into lat long points in Objective-C for iPhone?

Question

I want to draw routes on a map corresponding to directions JSON which I am getting through the Google Directions API: https://developers.google.com/maps/documentation/direct

Answer 1

The other answers here seem to be about using Apple Maps, for using Google Maps I found I had to make some modifications to @SedateAlien's great category.

MODIFIED CATEGORY

+ (GMSPolyline *)polylineWithEncodedString:(NSString *)encodedString {
    const char *bytes = [encodedString UTF8String];
    NSUInteger length = [encodedString lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
    NSUInteger idx = 0;

    NSUInteger count = length / 4;
    CLLocationCoordinate2D *coords = calloc(count, sizeof(CLLocationCoordinate2D));
    NSUInteger coordIdx = 0;

    float latitude = 0;
    float longitude = 0;
    while (idx < length) {
        char byte = 0;
        int res = 0;
        char shift = 0;

        do {
            byte = bytes[idx++] - 63;
            res |= (byte & 0x1F) << shift;
            shift += 5;
        } while (byte >= 0x20);

        float deltaLat = ((res & 1) ? ~(res >> 1) : (res >> 1));
        latitude += deltaLat;

        shift = 0;
        res = 0;

        do {
            byte = bytes[idx++] - 0x3F;
            res |= (byte & 0x1F) << shift;
            shift += 5;
        } while (byte >= 0x20);

        float deltaLon = ((res & 1) ? ~(res >> 1) : (res >> 1));
        longitude += deltaLon;

        float finalLat = latitude * 1E-5;
        float finalLon = longitude * 1E-5;

        CLLocationCoordinate2D coord = CLLocationCoordinate2DMake(finalLat, finalLon);
        coords[coordIdx++] = coord;

        if (coordIdx == count) {
            NSUInteger newCount = count + 10;
            coords = realloc(coords, newCount * sizeof(CLLocationCoordinate2D));
            count = newCount;
        }
    }

    GMSMutablePath *path = [[GMSMutablePath alloc] init];

    int i;
    for (i = 0; i < coordIdx; i++)
    {
        [path addCoordinate:coords[i]];
    }

    GMSPolyline *polyline = [GMSPolyline polylineWithPath:path];
    free(coords);

    return polyline;
}

USAGE

// Here I make the call to the Google Maps API to get the routes between two points...

....

// Get the encoded array of points.
NSString *points = routes[@"routes"][0][@"overview_polyline"][@"points"];

// Use the modified category to get a polyline from the points.
GMSPolyline *polyline = [GMSPolyline polylineWithEncodedString:points];

// Add the polyline to the map.
polyline.strokeColor = [UIColor redColor];
polyline.strokeWidth = 10.f;
polyline.map = theMapView;
}

Answer 2

Here's how I do it in my directions app. keyPlace is your destination object

- (void)getDirections {

  CLLocation *newLocation;// = currentUserLocation;
  MKPointAnnotation *annotation = [[[MKPointAnnotation alloc] init] autorelease];
  annotation.coordinate = CLLocationCoordinate2DMake(newLocation.coordinate.latitude, newLocation.coordinate.longitude);
  annotation.title = @"You";
  [mapView addAnnotation:annotation];

  CLLocationCoordinate2D endCoordinate;

  NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"https://maps.googleapis.com/maps/api/directions/json?origin=%f,%f&destination=%f,%f&sensor=false&mode=walking", newLocation.coordinate.latitude, newLocation.coordinate.longitude, keyPlace.lat, keyPlace.lon]];
  ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
  [request startSynchronous];

  if ([[request.responseString.JSONValue valueForKey:@"status"] isEqualToString:@"ZERO_RESULTS"]) {
    [[[[UIAlertView alloc] initWithTitle:@"Error"
                                 message:@"Could not route path from your current location"
                                delegate:nil
                       cancelButtonTitle:@"Close"
                       otherButtonTitles:nil, nil] autorelease] show];
    self.navigationController.navigationBar.userInteractionEnabled = YES;
    return; 
  }

  int points_count = 0;
  if ([[request.responseString.JSONValue objectForKey:@"routes"] count])
    points_count = [[[[[[request.responseString.JSONValue objectForKey:@"routes"] objectAtIndex:0] objectForKey:@"legs"] objectAtIndex:0] objectForKey:@"steps"] count];

  if (!points_count) {
    [[[[UIAlertView alloc] initWithTitle:@"Error"
                                 message:@"Could not route path from your current location"
                                delegate:nil
                       cancelButtonTitle:@"Close"
                       otherButtonTitles:nil, nil] autorelease] show];
    self.navigationController.navigationBar.userInteractionEnabled = YES;
    return;     
  }
  CLLocationCoordinate2D points[points_count * 2];

  int j = 0;
  NSArray *steps = nil;
  if (points_count && [[[[request.responseString.JSONValue objectForKey:@"routes"] objectAtIndex:0] objectForKey:@"legs"] count])
    steps = [[[[[request.responseString.JSONValue objectForKey:@"routes"] objectAtIndex:0] objectForKey:@"legs"] objectAtIndex:0] objectForKey:@"steps"];
  for (int i = 0; i < points_count; i++) {

    double st_lat = [[[[steps objectAtIndex:i] objectForKey:@"start_location"] valueForKey:@"lat"] doubleValue];
    double st_lon = [[[[steps objectAtIndex:i] objectForKey:@"start_location"] valueForKey:@"lng"] doubleValue];
    //NSLog(@"lat lon: %f %f", st_lat, st_lon);
    if (st_lat > 0.0f && st_lon > 0.0f) {
      points[j] = CLLocationCoordinate2DMake(st_lat, st_lon);
      j++;
    }
    double end_lat = [[[[steps objectAtIndex:i] objectForKey:@"end_location"] valueForKey:@"lat"] doubleValue];
    double end_lon = [[[[steps objectAtIndex:i] objectForKey:@"end_location"] valueForKey:@"lng"] doubleValue];

    if (end_lat > 0.0f && end_lon > 0.0f) {
      points[j] = CLLocationCoordinate2DMake(end_lat, end_lon);
      endCoordinate = CLLocationCoordinate2DMake(end_lat, end_lon);
      j++;
    }
  }

  MKPolyline *polyline = [MKPolyline polylineWithCoordinates:points count:points_count * 2];
  [mapView addOverlay:polyline];


}

#pragma mark - MapKit
- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation {
  MKPinAnnotationView *annView = [[[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"currentloc"] autorelease];
  annView.canShowCallout = YES;
  annView.animatesDrop = YES;
  return annView;
}

- (MKOverlayView *)mapView:(MKMapView *)mapView
            viewForOverlay:(id<MKOverlay>)overlay {
  MKPolylineView *overlayView = [[[MKPolylineView alloc] initWithOverlay:overlay] autorelease];
  overlayView.lineWidth = 5;
  overlayView.strokeColor = [UIColor purpleColor];
  overlayView.fillColor = [[UIColor purpleColor] colorWithAlphaComponent:0.5f];
  return overlayView;
}

Answer 3

In case anyone would need the decoding-code in VBA, here is a (working) port:

    Function decodeGeopoints(encoded)
  decodeGeopoints = ""
  ' This code is a port to VBA from code published here:
  ' http://blog.synyx.de/2010/06/routing-driving-directions-on-android-part-1-get-the-route/

  '//decoding
  'List poly = new ArrayList();

  '// replace two backslashes by one (some error from the transmission)
  'encoded = encoded.replace("\\", "\");
  encoded = Replace(encoded, "\\", "\")

  'int index = 0, len = encoded.length();
  Dim index As Long
  index = 0
  Dim leng As Long
  leng = Len(encoded)

  'int lat = 0, lng = 0;
  Dim lat As Long
  lat = 0
  Dim lng As Long
  lng = 0

  'while (index < len) {
  While (index < leng)
     'int b, shift = 0, result = 0;
     Dim b, shift, result As Long
     b = 0
     shift = 0
     result = 0

     'do {
     Do
        'b = encoded.charAt(index++) - 63;
        index = index + 1
        b = Asc(Mid(encoded, index, 1)) - 63
        'result |= (b & 0x1f) << shift;
        result = result Or ((b And 31) * (2 ^ shift))

        'shift += 5;
        shift = shift + 5
     '} while (b >= 0x20);
     Loop While (b >= 32)
     'int dlat = ((result & 1) != 0 ? ~(result >> 1) : (result >> 1));
     Dim dlat As Long
     If (result And 1) <> 0 Then
      dlat = Not Int(result / 2)
     Else
      dlat = Int(result / 2)
     End If

     'lat += dlat;
     lat = lat + dlat

     'shift = 0;
     shift = 0
     'result = 0;
     result = 0
     'do {
     Do
       'b = encoded.charAt(index++) - 63;
       index = index + 1
       b = Asc(Mid(encoded, index, 1)) - 63
       'result |= (b & 0x1f) << shift;
        result = result Or ((b And 31) * (2 ^ shift))
       'shift += 5;
        shift = shift + 5
     '} while (b >= 0x20);
     Loop While (b >= 32)
     'int dlng = ((result & 1) != 0 ? ~(result >> 1) : (result >> 1));
     Dim dlng As Long
     If (result And 1) <> 0 Then
      dlng = Not Int(result / 2)
     Else
      dlng = Int(result / 2)
     End If

     'lng += dlng;
     lng = lng + dlng

     'GeoPoint p = new GeoPoint((int) (((double) lat / 1E5) * 1E6), (int) (((double) lng / 1E5) * 1E6));
     Dim myLat, myLng As Double
     myLat = (lat / 100000)
     'myLat = myLat * 1000000
     myLng = (lng / 100000)
     'myLng = myLng * 1000000

     'poly.add(p);
     decodeGeopoints = decodeGeopoints & Comma2Dot(myLng) & "," & Comma2Dot(myLat) & ",0 "
  '}
  Wend

End Function

Answer 4

The best and lightest answer should be to use the method provided by Google in the framework :

[GMSPolyline polylineWithPath:[GMSPath pathFromEncodedPath:encodedPath]]

Answer 5

Swift 4.2 / Swift 5

let gmsPolyline = GMSPolyline(path: GMSPath(fromEncodedPath: encodedPolyline))
gmsPolyline.map = map

Answer 6

Swift 3.0

let polyline = GMSPolyline(path: GMSPath.init(fromEncodedPath: encodedPolyline))