OK that was kinda stupid...good thing is its mostly a title error!! i now see that pressure/position error is simple, and as Lightning Mate described.
However it is clear that I was unknowingly trying to describe temperature error or true altitude....
Can you confirm that what i described was true in relation to true altitude?
except that altimeters do not show a decrease in pressure at a fixed rate, but at an accurate rate for ISA
And perhaps that the term "parcel of air" is a little misleading being that it is the entire column of air not just a sample of it!
and the PEC table bit!!
And 1 more question....without opening a can of worms; Does QNH = QFF at a particular location regardless of temperature, assuming both were calculated at the same place? or the same question in other words; If you are given a QNH at an A/D at an elevation of 5000ft and the temperature at that A/D is +20 deg, for the alt to read an accurate A/D elevation, wouldn't it have to have taken into account the temperature effect (non ISA) on pressure at that aerodrome?