# Boost C++ Libraries

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#### radian

Unit of plane angle: Radians.

###### Description

Tag defining the unit of plane angle for spherical coordinate systems. This tag specifies that coordinates are defined in radians (-PI .. PI). It has to be specified for some coordinate systems.

###### Synopsis

```struct radian
{
// ...
};
```

###### Header

Either

`#include <boost/geometry/geometry.hpp>`

Or

`#include <boost/geometry/core/cs.hpp>`

###### example

Specify two coordinate systems, one in degrees, one in radians.

```#include <iostream>
#include <boost/geometry.hpp>

using namespace boost::geometry;

int main()
{
typedef model::point<double, 2, cs::spherical_equatorial<degree> > degree_point;
typedef model::point<double, 2, cs::spherical_equatorial<radian> > radian_point;

degree_point d(4.893, 52.373);
radian_point r(0.041, 0.8527);

double dist = distance(d, r);
std::cout
<< "distance:" << std::endl
<< dist << " over unit sphere" << std::endl
<< dist * 3959  << " over a spherical earth, in miles" << std::endl;

return 0;
}
```

Output:

```distance:
0.0675272 over unit sphere
267.34 over a spherical earth, in miles
```
 Copyright © 2009-2013 Barend Gehrels, Bruno Lalande, Mateusz Loskot, Adam Wulkiewicz Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)