# Geometry :

Subject: Re: [geometry] Help regarding line intersection on sphere issue
From: Stephen Hardy (Stephen.Hardy_at_[hidden])
Date: 2015-05-18 22:03:19

On 17 May 2015, at 10:50 pm, Stephen Hardy <stephen.hardy_at_[hidden]> wrote:

> Hi,
>
> I have been working on a code deriving plate tectonic motions on the earth’s surface based on geophysical evidence.
>
> I recently tried replacing my spherical geometry code with boost::geometry, using the spherical_equatorial coordinate system. The code became much smaller - this is great.
>
> However, I had a few regression issues, and I’ve traced them down to some peculiarities around the poles.
>
> The following is as simple as I can get it - intersect two lines, both crossing the pole, intersecting at the pole.
>
> ===

> ===
>
> #include <iostream>
> #include <boost/geometry.hpp>
>
> int main(int argc, const char *argv[])
> {
>
> typedef boost::geometry::model::point<double, 2, boost::geometry::cs::spherical_equatorial<boost::geometry::degree> > spoint;
> typedef boost::geometry::model::linestring<spoint> sline;
>
> sline sline1, sline2;
>
> boost::geometry::append(sline1, spoint(90.0, 80.0));
> boost::geometry::append(sline1, spoint(-90.0, 80.0));
> boost::geometry::append(sline2, spoint(180.0, 80.0));
> boost::geometry::append(sline2, spoint(0.0, 80.0));
>
> typedef std::vector<spoint> pointvec;
> pointvec res3;
> boost::geometry::intersection(sline1, sline2, res3);
> for(pointvec::iterator p = res3.begin(); p != res3.end(); p++)
> std::cout << p->get<0>() << " " << p->get<1>() << "\n";
>
> return 1;
> }
>
> ===
>
> This returns
>
> \$ ./boostgeomtestcase
> 90 80
> 0 80
>
> rather than (0 90) or equivalent, as expected. I’m using boost 1.58.0.
>
> * Is there something that I am doing wrong above that is giving me the incorrect result?
> * Is this a known issue (I could not find a bug for it)?
>
> Many thanks,
> Stephen
>
>

Hi,

Further to the above, I’ve now found Ticket #9162 ( https://svn.boost.org/trac/boost/ticket/9162 ) from Sep ’13 - "intersects returns incorrect result in spherical equatorial". This relates to point in polygon tests failing.

I can confirm that this test still fails on boost 1.58.0.

regards,
Stephen

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