Find nearest point in polyline/path

Question

I need to find nearest point from given CLLocationCoordinate2D on array of GMSPolyline. I can convert this to GMSPath if that\'s bette

Answer 1

Ok, I've managed to write it. Method nearestPointToPoint:onLineSegmentPointA:pointB:distance: allows you both to find closest coordinate and distance between selected point and segment line (so line with start and end).

- (CLLocationCoordinate2D)nearestPolylineLocationToCoordinate:(CLLocationCoordinate2D)coordinate {
    GMSPolyline *bestPolyline;
    double bestDistance = DBL_MAX;
    CGPoint bestPoint;
    CGPoint originPoint = CGPointMake(coordinate.longitude, coordinate.latitude);

    for (GMSPolyline *polyline in self.polylines) {
        if (polyline.path.count < 2) { // we need at least 2 points: start and end
            return kCLLocationCoordinate2DInvalid;
        }

        for (NSInteger index = 0; index < polyline.path.count - 1; index++) {
            CLLocationCoordinate2D startCoordinate = [polyline.path coordinateAtIndex:index];
            CGPoint startPoint = CGPointMake(startCoordinate.longitude, startCoordinate.latitude);
            CLLocationCoordinate2D endCoordinate = [polyline.path coordinateAtIndex:(index + 1)];
            CGPoint endPoint = CGPointMake(endCoordinate.longitude, endCoordinate.latitude);
            double distance;
            CGPoint point = [self nearestPointToPoint:originPoint onLineSegmentPointA:startPoint pointB:endPoint distance:&distance];

            if (distance < bestDistance) {
                bestDistance = distance;
                bestPolyline = polyline;
                bestPoint = point;
            }
        }
    }

    return CLLocationCoordinate2DMake(bestPoint.y, bestPoint.x);
}

Method nearestPolylineLocationToCoordinate: will browse through all polylines (you just need to supply polylines array == self.polylines) and find the best one.

// taken and modified from: http://stackoverflow.com/questions/849211/shortest-distance-between-a-point-and-a-line-segment
- (CGPoint)nearestPointToPoint:(CGPoint)origin onLineSegmentPointA:(CGPoint)pointA pointB:(CGPoint)pointB distance:(double *)distance {
    CGPoint dAP = CGPointMake(origin.x - pointA.x, origin.y - pointA.y);
    CGPoint dAB = CGPointMake(pointB.x - pointA.x, pointB.y - pointA.y);
    CGFloat dot = dAP.x * dAB.x + dAP.y * dAB.y;
    CGFloat squareLength = dAB.x * dAB.x + dAB.y * dAB.y;
    CGFloat param = dot / squareLength;

    CGPoint nearestPoint;
    if (param < 0 || (pointA.x == pointB.x && pointA.y == pointB.y)) {
        nearestPoint.x = pointA.x;
        nearestPoint.y = pointA.y;
    } else if (param > 1) {
        nearestPoint.x = pointB.x;
        nearestPoint.y = pointB.y;
    } else {
        nearestPoint.x = pointA.x + param * dAB.x;
        nearestPoint.y = pointA.y + param * dAB.y;
    }

    CGFloat dx = origin.x - nearestPoint.x;
    CGFloat dy = origin.y - nearestPoint.y;
    *distance = sqrtf(dx * dx + dy * dy);

    return nearestPoint;
}

You can use it eg in:

- (void)mapView:(GMSMapView *)mapView didEndDraggingMarker:(GMSMarker *)marker {
    marker.position = [self nearestPolylineLocationToCoordinate:marker.position];
}