private void summariseFertilities() {
    final int SS = World.SECTOR_SIZE, SR = world.size / World.SECTOR_SIZE ;
    fertilityLevels = new float[SR][SR] ;
    for (Coord c : Visit.grid(0, 0, world.size, world.size, 1)) {
      final Tile t = world.tileAt(c.x, c.y) ;
      final Habitat h = t.habitat() ;
      float f = h.moisture() ;
      if (! h.pathClear) f = -5 ;
      if (h == Habitat.CURSED_EARTH) f = -10 ;
      fertilityLevels[c.x / SS][c.y / SS] += f ;
      totalFertility += f ;
    }
    for (Coord c : Visit.grid(0, 0, SR, SR, 1)) {
      fertilityLevels[c.x][c.y] /= SS * SS * 10 ;
    }
    totalFertility /= world.size * world.size * 10 ;
  }