public double calcAngle(final int lon0, final int lat0, final int lon1, final int lat1, final int lon2, final int lat2) {
        final double[] lonlat2m = CheapRulerHelper.getLonLatToMeterScales(lat1);
        final double lon2m = lonlat2m[0];
        final double lat2m = lonlat2m[1];
        final double dx10 = (lon1 - lon0) * lon2m;
        final double dy10 = (lat1 - lat0) * lat2m;
        final double dx21 = (lon2 - lon1) * lon2m;
        final double dy21 = (lat2 - lat1) * lat2m;

        final double dd = Math.sqrt((dx10 * dx10 + dy10 * dy10) * (dx21 * dx21 + dy21 * dy21));
        if (dd == 0.) {
            cosangle = 1.;
            return 0.;
        }
        double sinp = (dy10 * dx21 - dx10 * dy21) / dd;
        final double cosp = (dy10 * dy21 + dx10 * dx21) / dd;
        cosangle = cosp;

        double offset = 0.;
        double s2 = sinp * sinp;
        if (s2 > 0.5) {
            if (sinp > 0.) {
                offset = 90.;
                sinp = -cosp;
            } else {
                offset = -90.;
                sinp = cosp;
            }
            s2 = cosp * cosp;
        } else if (cosp < 0.) {
            sinp = -sinp;
            offset = sinp > 0. ? -180. : 180.;
        }
        return offset + sinp * (57.4539 + s2 * (9.57565 + s2 * (4.30904 + s2 * 2.56491)));
    }
Calculate the angle defined by 3 points
(and deliver it's cosine on the fly)
main/src/main/java/cgeo/geocaching/brouter/util/CheapAngleMeter.java