Every landing area shall have a hard-surfaced or grassed area, shall be provided with good drainage and shall be free of dust, gravel or other loose material. It shall be not less in length, width or diameter than twice the overall length of any helicopter landing or taking off. In no instance shall this area be smaller than one hundred fifty (150) feet in diameter or in length of the shortest dimension of a rectangular area. This area shall be free of trees, wires, or other obstructions.