Boost point in polygon giving erroneous results?

Question

I have the following unit test, with both checks failing:

BOOST_AUTO_TEST_CASE(test_point_in_polygon)
{
    typedef boost::geometry::model::d2::point_xy<double> point_type;
    boost::geometry::model::polygon<point_type> poly;
    boost::geometry::read_wkt("POLYGON((-57.0635 -3.58045, -57.8088 -4.75336, -56.7036 -7.65533, -56.4646 -9.08261, -55.219 -9.42072, -54.6047 -9.54237, -53.868 -9.22164, -52.4139 -8.58345, -49.4691 -6.94159, -49.2295 -6.87687, -47.1079 -6.18013, -45.9159 -5.91411, -43.7897 -5.89581, -42.285 -5.95883, -40.1693 -6.38999, -38.1374 -6.87424, -35.7062 -7.67065, -34.6193 -8.40713, -34.1769 -8.74957, -31.785 -9.31504, -30.574 -9.70903, -26.909 -10.5865, -24.9817 -11.268, -22.6134 -12.2833, -21.0927 -13.3427, -20.006 -13.7029, -16.8407 -15.2466, -13.2203 -16.816, -11.3922 -17.4133, -10.7577 -17.6366, -8.54581 -18.2736, -7.18058 -18.6037, -4.86031 -19.0997, -4.25834 -19.3046, -3.39545 -19.3122, -0.985195 -19.7047, -0.340967 -19.9409, 0.75209 -19.8968, 3.30498 -20.3562, 6.82228 -20.6405, 10.4384 -20.5995, 13.8557 -20.2595, 14.7523 -20.1371, 18.3468 -19.4284, 18.9915 -19.4505, 22.5377 -18.5053, 23.2192 -18.4658, 26.4885 -17.3378, 27.2915 -17.2273, 30.5962 -15.8816, 32.7278 -14.6911, 33.8878 -14.1893, 36.2752 -12.7482, 38.5306 -11.1893, 40.8465 -9.28305, 41.1732 -9.05935, 43.372 -7.07535, 45.2436 -5.2073, 46.9925 -3.21699, 48.616 -1.35438, 50.628 0.592424, 52.158 1.37445, 53.1148 2.07603, 50.9152 5.07491, 50.3177 6.38521, 49.9412 7.3891, 48.8311 9.29466, 47.7653 12.692, 45.6416 16.3693, 43.7106 17.9319, 41.4998 17.9314, 37.4335 19.4761, 34.3477 18.4887, 32.1299 18.0676, 29.2684 18.0666, 26.5269 15.0811, 19.7706 14.5304, 15.8916 12.9664, 14.8124 12.5622, 12.783 10.0368, 12.4488 9.65713, 11.1008 7.47097, 8.9187 5.4486, 5.63935 1.10272, -6.34225 4.45488, -8.60563 7.7494, -16.2454 11.2148, -23.399 18.8388, -25.8497 20.2781, -28.8931 22.7068, -31.4379 22.8942, -32.5636 21.9563, -35.2968 20.2469, -37.3132 19.0133, -37.9703 18.2828, -40.4433 15.553, -40.9543 14.1728, -42.8783 10.3486, -46.3297 7.28714, -48.959 5.88183, -52.1939 2.65934, -54.8196 -0.896786))", poly);

    point_type point (-57.8088, -1.5755);
    BOOST_CHECK_EQUAL(boost::geometry::within(point, poly), false);

    point = point_type(-100, -2);
    BOOST_CHECK_EQUAL(boost::geometry::within(point, poly), false);

}

The data looks like this:

I am using it in a similar way to the docs (http://www.boost.org/doc/libs/1_61_0/libs/geometry/doc/html/geometry/reference/algorithms/within/within_2.html).

As in the figure, it is obvious that both points are outside the polygon. Am I doing anything wrong, or have I found a bug in boost?

Here is the answer: http://stackoverflow.com/a/20081230/2249683 — , Jun 27 '16 at 14:04
@DieterLücking thanks for the comment. I rerun the test with everything converted to int, but I get the same results. — Mihai Bujanca, Jun 27 '16 at 14:23
@DieterLücking Wrong library, that refers to Boost.Polygon, OP is using Boost.Geometry. The example in the page linked from the official documentation uses double as its coordinate type. — llonesmiz, Jun 27 '16 at 14:25

score 1 · Accepted Answer · edited May 23 '17 at 11:58

The problem is caused by a mismatch between the polygon type you use, an the polygon data you provide. By using boost::geometry::model::polygon<point_type> you create a polygon that has point_type as its point type, that has clockwise direction, that is closed and several other things that are configured by the default template parameters. In the data you provide the first point is not equal to the last one (this is the definition of closed in this context). You can either, make your polygon not be closed (polygon<point_type,true,false>) or make sure that your first and last points are equal (by copying the first point at the end).

Edit: Another (probably better) approach could be using boost::geometry::correct(poly); without the need to change anything else as shown in this (pretty interesting) answer.

Boost point in polygon giving erroneous results?

1 Answers1