a) Obstacle

Introduction

According to the ICAO Annex 15 definition, an Obstacle is defined as follows:

"Obstacle. All fixed (whether temporary or permanent) and mobile objects, or parts thereof, that:

a)    are located on an area intended for the surface movement of  aircraft; or
b)    extend above a defined surface intended to protect aircraft in flight; or
c)    stand outside those defined surfaces and that have been assessed as being a hazard to air navigation."

The AIXM model for "obstacles" takes a 3-tier approach:

• at the highest level, there is a VerticalStructure class, which is defined as "All fixed (whether temporary or permanent) and mobile objects, or parts thereof that extend above the surface of the Earth". This includes obstacles, but also buildings and any other structures that are raised above the surface and that are relevant for aircraft operations, without being relevant as obstacles. The definition of VerticalStructure has a second sentence that indicates: "Those vertical structures that are located on an area intended for the surface movement of aircraft or that extend above a defined surface intended to protect aircraft in flight are considered obstacles."
• at the next level, there is an association between VerticalStructure and ObstacleArea, which allows to indicate precisely which vertical structures are considered an obstacle situated in one of the areas defined in the obstacle/terrain data collection surfaces defined by ICAO.
• finally, the obstacles that are considered "obstructions" for a particular instrument flight procedure are modelled with a specific association between VerticalStructure and Obstruction.