Geometry Boost List: Re: [geometry] epsg : 3857

From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2019-09-23 13:56:42

Hi,

W dniu 23.09.2019 o 10:57, Mateusz Loskot via Geometry pisze:
> On Mon, 23 Sep 2019 at 10:38, Ahmet Temiz via Geometry
> <geometry_at_[hidden]> wrote:
>> Hi,
>> I am not sure how I can define epsg : 3857 in boost geometry
> Should be something along these lines:
>
> #include <boost/geometry/srs/epsg.hpp>
> #include <boost/geometry/srs/projection.hpp>
>
> boost::geometry::srs::projection<> prj = boost::geometry::srs::epsg(3857);
> boost::geometry::point_xy<double,  epsg<2040>> my_point(0, 0);
>
> Also, search through the source code of the Boost.Geometry tests
> for more details about how `epsg` and `projection` can be used.

This is not correct. The point type can only take cs::cartesian, 
cs::spherical or cs::geographic. EPSG:3857 is web/spherical mercator 
projection so to represent the points you'll use bg::cs::cartesian. The 
problem with this is that the algorithms will treat the coordinates as 
purly cartesian so the globe won't wrap around behind the antimeridian.

What would you like to do exactly?
Do you want to convert to or from it to some other SRS or do you have 
cartesian coordinates in EPSG:3857 and want to use some Boost.Geometry 
algorithms?

Adam