An OIS is an imaginary surface that defines the obstruction area in and around the airfield. The size and length of the surface vary depending on the use of the airfield (civilian, military). The OIS determines which objects are vertical obstructions. If an object penetrates the OIS it is considered a vertical obstruction.

• 3-D imaginary “football stadium” shape
• 7 nautical miles about the runway
• Different zones and surfaces for different governing agencies(FAA, NGA, ICAO)

An Obstruction Identification Surface consists of the following approach surfaces:

• Primary surface
• Inner and outer horizontal surfaces
• Conical surfaces
• Transition surfaces