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wind cone, or another approved indicator, which must be operable and <br />clearly visible from the pattern altitude when within one mile of the <br />airport during daylight hours. If the airport is lighted for night <br />operations, the wind indicator must also be lighted in an approved <br />manner. A bulletin board, a telephone, and a sanitary public toilet <br />' facility shall also be provided at all public airports. <br />' C. Imaginary Surfaces <br />Obstructions are a necessary consideration in the design, layout <br />and operations of an airport. An object is considered an obstruction <br />to a public airport if it is of greater height than any of the following <br />airport imaginary surfaces (illustrated in Figure 1): <br />A. The primar surface extends 200' beyond each end of the hard surfaced <br />' runway, making it 3700' long x 500' wide in this case, with the elevation <br />of any point on that surface being the same as the elevation of the <br />nearest point on the runway centerline; <br />B. The horizontal surface at a height of 150 feet above the established <br />airport elevation, the perimeter of which is established by swinging <br />' arcs of 6,000 feet radii from the center of each end of the primary <br />surface of each runway and connecting the adjacent arcs by lines <br />tangent to those arcs; <br />■ <br />7 <br />