Re: bg::cs::geographic and nearest query

From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2019-10-14 17:23:28

Hi,

W dniu 14.10.2019 o 16:57, vexakul via Boost-users pisze:
> Hi all!
>
> I need to store geographic points in rtree and be able to answer nearest
> query.
>
> Can someone provide an example similar to  this
> <https://www.boost.org/doc/libs/1_65_1/libs/geometry/doc/html/geometry/spatial_indexes/rtree_examples/quick_start.html>
> but with point type bg::model::point<float, 2,
> bg::cs::geographic&lt;bg::degree>>? Is that supported? For me it fails to
> compile.
When you define the geographic point as R-tree element type the 
computation should be done in geographic CS by default using WGS84 
ellipsoid and andoyer formula to calculate distances. So simply:

using boost::geometry;

using point = model::point<double, 2, cs::geographic<degree>>;

using rtree = index::rtree<point, index::rstar<4>>;

rtree rt;

// or

std::vector<point> pts;

rtree rt2{pts.begin(), pts.end()};

If you need different ellipsoid or more accurate formula it is also 
possible (since Boost 1.71) though it is not documented. It's because 
I'm not entirely sure what to do with the fact that with this approach 
the EqualTo is called with two or three arguments depending on the 
parameters passed into the R-tree so it may be confusing. So there is 
slight probability that the interface will change in case I found some 
better way of defining CS-specific parts of the computation. So if you 
e.g. need to pass a different ellipsoid you can wrap the R-tree 
parameters together with Index strategy and pass it into the R-tree like 
that:

using boost::geometry;

using formula = strategy::andoyer;// or more accurate strategy::vincenty
srs::spheroid<double> sph(6378137.0, 6356752.3142451793);

using point = model::point<double, 2, cs::geographic<degree>>;
using parameters = index::parameters
 Â Â Â  <
 Â Â Â Â Â Â Â  index::rstar<4>,
 Â  Â Â  Â Â  strategy::index::geographic<formula>
 Â Â Â  >;

using rtree = index::rtree<point, parameters>;

rtree rt{parameters{index::rstar<4>{},
strategy::index::geographic<formula>{sph}}};

// or

std::vector<point> pts;

rtree rt2{pts.begin(), pts.end(),
parameters{index::rstar<4>{},
strategy::index::geographic<formula>{sph}}};

Have in mind that I didn't test the code above so there may be some 
mistakes.

Adam