Distance always shows less distance than real

Question

I already found some examples how to measure straight line distance between my current GPS location and other locations, but all solutions shows about 100 meters less as real.

First I tried this solution:

private double meterDistanceBetweenPoints(double lat_a, double lng_a, double lat_b, double lng_b) {
    float pk = (float) (180.f/Math.PI);

    double a1 = lat_a / pk;
    double a2 = lng_a / pk;
    double b1 = lat_b / pk;
    double b2 = lng_b / pk;

    double t1 = Math.cos(a1) * Math.cos(a2) * Math.cos(b1) * Math.cos(b2);
    double t2 = Math.cos(a1) * Math.sin(a2) * Math.cos(b1) * Math.sin(b2);
    double t3 = Math.sin(a1) * Math.sin(b1);
    double tt = Math.acos(t1 + t2 + t3);

    return 6366000 * tt;
}

and then another one with DistanceTo:

     double distance;
        Location locationA = new Location("Point A");
        locationA.setLatitude(Lat[position]);
        locationA.setLongitude(Lng[position]);

        Location locationB = new Location("Point B");
        locationB.setLatitude(Act1c);
        locationB.setLongitude(Act2c);

distance = locationA.distanceTo(locationB); 

Everything works fine, but it shows always ca. 100 meters less as the real distance is. And the coordinates are ok.

If I check the same distance on Google map (not road of course, but straight line, with measure tool), it shows me for example 330 m and my android app shows 230m. I tried more locations, alway shows less (instead of Google's 880m android shows me 740m).

Also I checked the distance measure with some online tool http://boulter.com/gps/distance/ and it gives me the same result as android.

But I am measuring the distance in my city, and I know that it is ca. 300m, not 200m, so what can be wrong here? These are short distances, so any Earth elipse can't play role here.

I don't want to use Google Play services, as they need http connection.

I also tried the solution from the link in the answer below:

    double getDistanceFromLatLonInKm(double lat1, double lon1, double lat2, double lon2) {
    int R = 6371; // Radius of the earth in km
    double dLat = deg2rad(lat2-lat1);  // deg2rad below
    double dLon = deg2rad(lon2-lon1);
    double a =
            Math.sin(dLat/2) * Math.sin(dLat/2) +
                    Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) *
                            Math.sin(dLon/2) * Math.sin(dLon/2)
            ;
    double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
    double d = R * c; // Distance in km
    return d;
}

private double deg2rad(double deg) {
    return deg * (Math.PI/180);
}

but the same result as all others.

And the last update - same result:

   static final double EARTH_RADIUS = 6371009;
public static double computeDistanceBetween(double lat_a, double lng_a, double lat_b, double lng_b) {
    return computeAngleBetween(lat_a, lng_a, lat_b,lng_b ) * EARTH_RADIUS;
}

static double computeAngleBetween(double lat_a, double lng_a, double lat_b, double lng_b) {
    return distanceRadians(toRadians(lat_a), toRadians(lng_a),
            toRadians(lat_b), toRadians(lng_b));
}

private static double distanceRadians(double lat1, double lng1, double lat2, double lng2) {
    return arcHav(havDistance(lat1, lat2, lng1 - lng2));
}

static double arcHav(double x) {
    return 2 * asin(sqrt(x));
}

static double havDistance(double lat1, double lat2, double dLng) {
    return hav(lat1 - lat2) + hav(dLng) * cos(lat1) * cos(lat2);
}

static double hav(double x) {
    double sinHalf = sin(x * 0.5);
    return sinHalf * sinHalf;
}