Find the closest point on polygon to user location

Question

I have an app that find the shortest distance between my user to a polygon.

I want to convert the polygon to Geofence to check the distance between the user to the a

Answer 1

You can use a function like the following to calculate the nearest point from polygon defined by a List<LatLng> and a given LatLng.

It uses the PolyUtil.distanceToLine from the Google Maps Android API Utility Library to compute the distance between the test LatLng and every segment of the list, and a method based on the distanceToLine method from https://github.com/googlemaps/android-maps-utils/blob/master/library/src/com/google/maps/android/PolyUtil.java to compute the projection of a point on a segment.

private LatLng findNearestPoint(LatLng test, List<LatLng> target) {
    double distance = -1;
    LatLng minimumDistancePoint = test;

    if (test == null || target == null) {
        return minimumDistancePoint;
    }

    for (int i = 0; i < target.size(); i++) {
        LatLng point = target.get(i);

        int segmentPoint = i + 1;
        if (segmentPoint >= target.size()) {
            segmentPoint = 0;
        }

        double currentDistance = PolyUtil.distanceToLine(test, point, target.get(segmentPoint));
        if (distance == -1 || currentDistance < distance) {
            distance = currentDistance;
            minimumDistancePoint = findNearestPoint(test, point, target.get(segmentPoint));
        }
    }

    return minimumDistancePoint;
}

/**
 * Based on `distanceToLine` method from
 * https://github.com/googlemaps/android-maps-utils/blob/master/library/src/com/google/maps/android/PolyUtil.java
 */
private LatLng findNearestPoint(final LatLng p, final LatLng start, final LatLng end) {
    if (start.equals(end)) {
        return start;
    }

    final double s0lat = Math.toRadians(p.latitude);
    final double s0lng = Math.toRadians(p.longitude);
    final double s1lat = Math.toRadians(start.latitude);
    final double s1lng = Math.toRadians(start.longitude);
    final double s2lat = Math.toRadians(end.latitude);
    final double s2lng = Math.toRadians(end.longitude);

    double s2s1lat = s2lat - s1lat;
    double s2s1lng = s2lng - s1lng;
    final double u = ((s0lat - s1lat) * s2s1lat + (s0lng - s1lng) * s2s1lng)
            / (s2s1lat * s2s1lat + s2s1lng * s2s1lng);
    if (u <= 0) {
        return start;
    }
    if (u >= 1) {
        return end;
    }

    return new LatLng(start.latitude + (u * (end.latitude - start.latitude)),
            start.longitude + (u * (end.longitude - start.longitude)));


}

You can test it with the following code:

List<LatLng> points = new ArrayList<>();
points.add(new LatLng(2, 2));
points.add(new LatLng(4, 2));
points.add(new LatLng(4, 4));
points.add(new LatLng(2, 4));
points.add(new LatLng(2, 2));

LatLng testPoint = new LatLng(3, 0);

LatLng nearestPoint = findNearestPoint(testPoint, points);
Log.e("NEAREST POINT: ", "" + nearestPoint); // lat/lng: (3.0,2.0)
Log.e("DISTANCE: ", "" + SphericalUtil.computeDistanceBetween(testPoint, nearestPoint)); // 222085.35856591124