def neighbour(lat:0, lng:0)
      new_point = Point.new(
        lat: southwest.lat + dlat(lat),
        lng: southwest.lng + dlng(lng))
      Tile.including(new_point, depth: depth)
    end
def neighbour(lat:0, lng:0)
      new_point = Point.new(
        lat: southwest.lat + dlat(lat),
        lng: southwest.lng + dlng(lng))
      Tile.including(new_point, depth: depth)
    end
return a neighouring tile offset in tile increments
 TODO: this could be implemented using bit logic