protected Tile pickSample() {
    final int range = World.SECTOR_SIZE * 2 ;
    Tile picked = null ;
    float bestRating = 0 ;
    for (int n = 10 ; n-- > 0 ;) {
      final Tile s = Spacing.pickRandomTile(this, range, world) ;
      if (s == null || s.pathType() != Tile.PATH_CLEAR) continue ;
      float rating = s.habitat().minerals() ;
      rating /= 10 + Spacing.distance(s, this) ;
      if (rating > bestRating) { picked = s ; bestRating = rating ; }
    }
    return picked ;
  }