Spherical Mercator is a popular map projection.

It projects (lat, lon)-coordinates to a square whose sides each measure ~40'000km (the earths circumference). The units of the returned x/y-coordinates is meters.

          (20'000km, 20'000km
  +-------o
  |       |
  |       |
  o-------+

(-20'000km, -20'000km

class SphericalMercator implements Projection {
 static const double _MAX_LATITUDE = 85.0511287798;
 static const double _DEG_TO_RAD =  PI / 180;
 static const double _RAD_TO_DEG = 180 / PI;
 /// half earth circumference in meters
 static const double _HEC = 20037508.34;

 /**
  * Transforms geographic coordinates [ll] into projected coordinates.
  *
  * The projected coordinates are in degrees with the following ranges:
  *
  * * minx=-20037508.34, maxx = 20037508.34
  * * miny=-20037508.34, maxy = 20037508.34
  */
 Point2D project(LatLon ll) {
   // make sure lat is in the range [-_MAX_LATITUDE, _MAX_LATIDUDE]
   var lat = max(min(_MAX_LATITUDE, ll.lat), -_MAX_LATITUDE);
   var x = ll.lon / 180 * _HEC;
   var y = log(tan((90 + ll.lat) * PI / 360)) / PI;
   y *= _HEC;
   y = max(-_HEC, min(_HEC, y));
   return new Point2D(x, y).truncate(8);
 }

 /**
  * Transforms projected coordinates into geographic coordinates.
  *
  * Both coordinates [p].x and [p].y should be in the
  * range (-20037508.34,+20037508.34).
  */
 LatLon unproject(Point2D p) {
   var lon = p.x * 180 / _HEC ;
   var lat = 180 / PI * (2 * atan(exp(p.y / _HEC * PI)) - PI/2);
   return new LatLon(lat, lon);
 }
}