No Intersection Area for geographic coordinates with Boost Geometry

Question

I need to check whether two points (GPS coordinates) q, m are in the same side or on the opposite side or co linear to a line (great circle segment) (p,t). I know that q is not co linear to (p, t). I didn't find and any direct function to use in the boost.geometry library. So I tried to calculate it in a different way.

I construct two triangles (p, q, t) and (p, m, t). Then I intersect these two and check the area of the intersection polygon. Following is my code.

typedef boost::geometry::model::point<
    double, 2, boost::geometry::cs::spherical_equatorial<boost::geometry::degree>
> geo_point;
typedef boost::geometry::model::polygon<geo_point> geo_polygon;

geo_point p(110.48316, 29.05043);
geo_point q(110.48416, 29.05104);
geo_point t(110.48416, 29.05228);
geo_point m(110.48408, 29.05079);

geo_polygon ut, es;

boost::geometry::append(ut.outer(), p);
boost::geometry::append(ut.outer(), q);
boost::geometry::append(ut.outer(), t);
boost::geometry::append(ut.outer(), p);

boost::geometry::append(es.outer(), p);
boost::geometry::append(es.outer(), m);
boost::geometry::append(es.outer(), t);
boost::geometry::append(es.outer(), p);

std::list<geo_point> intersection_points;
boost::geometry::intersection(ut, es, intersection_points);
std::cout << intersection_points.size() << std::endl;

std::vector<geo_polygon> intersection_polygons;
boost::geometry::intersection(ut, es, intersection_polygons);
std::cout << intersection_polygons.size() << std::endl;

Live at cpp.sh

If we plot these two triangles we can clearly see that they intersects on 3 vertices yielding another triangle in the intersected region.

The above code correctly returns the number of intersected points, but it doesn't return any intersection polygon.

3
0

I have tried using geographic instead of using spherical_equatorial coordinate system. But got the same results. Am I missing something ? or this is a problem in Boost.Geometry

Answer 1

To ensure that polygons are closed and oriented as needed, apply correct. Inserting

boost::geometry::correct(ut);
boost::geometry::correct(es);

gives result 1 polygon for your test