Obstacle Clearance. Clearance from obstacles can be obtained by lateral clearance and vertical clearance. Obviously, the area surveyed must have finite limits. It is, however, not acceptable for, say, an area 5 nm wide to be surveyed and then permit aircraft to fly within guidance tolerance, 2.5 nm either side of the desired track. The extremities of the surveyed area must gradually permit higher obstacles until at the limit of reasonable expectations of accuracy (guidance tolerance - both equipment and flight technical), the guaranteed clearance is reduced to zero. This assessment is known as creation of MOC (minimum obstacle clearance areas). MOC is discussed later in this chapter. Obstacle clearance could be provided by assessing the highest obstacle to be flown over and by applying a safety margin to the obstacle height. An obstacle clearance height or altitude (OCH/A) can thus be obtained. This is the method of obtaining MSA and with refinements, minimum descent height/altitude (MDH/A) for non precision procedures. As precision procedures provide height guidance, an obstacle 1 000 ft high at 10 nm from the threshold is not as significant as an obstacle 150 ft high 1 nm from the threshold (assuming a 300 ft per mile glide slope). For precision systems, OCH/A is 'range from threshold' dependant. It should therefore be obvious that OCH/A for precision procedures are less than OCH/A for non precision. It must be stressed that, from an operational point of view, the obstacle clearance applied in the development of each instrument approach procedure is considered to be the minimum required for an acceptable level of safety in operations. If you have your own aeroplane and it is not used for commercial air transport, you may operate to OCH limits. Operators apply higher criteria resulting in aerodrome operating minima for commercial air transport.