distance method
Calculate the distance between two points.
Implementation
double distance(GeoData data) {
final y2 = pi * data.longitude / 180.0;
final y1 = pi * longitude / 180.0;
final x2 = pi * data.latitude / 180.0;
final x1 = pi * latitude / 180.0;
return _r * acos(sin(y1) * sin(y2) + cos(y1) * cos(y2) * cos(x2 - x1));
}