Allan, the correction is applied to the height above the aerodrome elevation. I'll give you an example
Aerodrome elevation 700ft, QNH 1003, temp -20degC
Altitude given on plate at OM is 1900ft. Ques. what is your true height at the OM?
Firstly work out the Pressure Alt at the airfield (1000ft)
Get your whiz wheel and put 1000ft against -20deg in the "True Altitude" window On the inner scale read your indicated height ABOVE the aerodrome elevation (1200ft ... not 1900ft) and read off against the true height on the outer scale (I make it about 1060ft) add this to the elevation of 700ft to give you an accurate OM crossing altitude of 1760ft ie 140ft low.
Any ISA deviation colder than ISA will cause the altimeter to overread (=dangerous)
The same technique can be used to calculate true altitude anywhere but remember it is only the bit of air between the ground and your aircraft that you are correcting for, because by the time you reach the runway there will be NO error (assuming the met man got his QNH correct!)
Hope this helps
Apollo
