The exact altitude(s) at which the airspace over private land can become subject to "substantial impairment" is often debated. Case law in the past has used the height of 500 ft (150 m) in urban or suburban areas,[5] and 360 ft (110 m) above the surface or tallest structure in rural areas[6] as the demarcation of where impairment of property rights can occur. At those times this constituted the limits of "navigable airspace".