Definition: horizontal surface (Applicable to Article VI - Chapter 1)
The "horizontal surface" is an imaginary horizontal plane, circular or elliptical in shape, which: (a) is located at a height of 150 feet above the established airport elevation for any major airport; and (b) is measured on a horizontal radius from the airport reference point (or points ), and extends for a distance set forth in Section 61-41 (Airport Reference Point, Established Elevation and Specified Radii).