within(Point, Polygon) returns wrong result for geographic coordinates

[tobiass-sdl]

typedef boost::geometry::model::d2::point_xy<double, boost::geometry::cs::geographic<boost::geometry::degree>> Point;
typedef boost::geometry::model::polygon<Point> Polygon;

  Polygon p;
  Point p1(21.0005, 3.00053);
  Point p2(21.0005, 1.99947);
  Point p3(19.9995, 1.99947);
  Point p4(19.9995, 3.00053);
  Point p5(21.0005, 3.00053);

  boost::geometry::append(p.outer(), p1);
  boost::geometry::append(p.outer(), p2);
  boost::geometry::append(p.outer(), p3);
  boost::geometry::append(p.outer(), p4);
  boost::geometry::append(p.outer(), p5);

  Point testPoint(20, -177.5);

  assert(boost::geometry::is_valid(p));
  assert(!boost::geometry::within(testPoint, p));

the second assert fails. As the testPoint is on the other side of the planet this is clearly wrong.

branch: develop.

[tobiass-sdl]

It also seems that within returns always true for geographic coordinates if y == 90 or y == -90:

typedef boost::geometry::model::d2::point_xy<double, boost::geometry::cs::geographic<boost::geometry::degree>> Point;
typedef boost::geometry::model::polygon<Point> Polygon;

Polygon p;
Point p1(21.0005, 93.00053);
Point p2(21.0005, 81.99947);
Point p3(19.9995, 81.99947);
Point p4(19.9995, 93.00053);
Point p5(21.0005, 93.00053);

boost::geometry::append(p.outer(), p1);
boost::geometry::append(p.outer(), p2);
boost::geometry::append(p.outer(), p3);
boost::geometry::append(p.outer(), p4);
boost::geometry::append(p.outer(), p5);

Point testPoint(20, 90.0);

assert(boost::geometry::is_valid(p));
assert(boost::geometry::within(testPoint, p));

testPoint.y() == 89.99 works fine.

[tobiass-sdl]

I just realized that boost geometry uses lon, lat instead the lat, lon known from GPS and other systems. Swapping x and y fixed the issue. It might be a good idea to extend the documentation to clearify that: https://www.boost.org/doc/libs/1_81_0/libs/geometry/doc/html/geometry/reference/cs/cs_geographic.html

[awulkiew]

Thanks for the report and the update.

It's not really a feature of the coordinate system. Rather it's caused by the fact that d2::point_xy takes x before y in the constructor and that the library assumes that the coordinate of id 0 is x or longitude while the coordinate of id 1 is y or latitude. Note that you could write your own point type taking the coordinates in whatever order. But still longitude would have to be returned from get<0>(pt) and latitude from get<1>(pt). A better place to describe it would probably be: https://www.boost.org/libs/geometry/doc/html/geometry/reference/access/get.html and/or https://www.boost.org/libs/geometry/doc/html/geometry/reference/concepts/concept_point.html