formulas
PH = Elevation + [(1013-QNH)x30]
If QNH is 1030 & airfield elevation is 1500' then 1030-1013=17, 17x30=510, 1500 + 510= PH 2010'
Note: PH can be a negative value, however performance charts don't usually account for this so a zero PH is used.
DH = [(temperature - ISA)x120] + PH
ISA temp= 15 - twice the thousands of feet. i.e 1000' PH is 2, 2000' PH is 4, 4500' PH is 9 etc... (temp drops at a standard rate of 2deg per thousand feet, and ISA at MSL is 15deg.
If PH is 2010 and the temp is 22deg then ISA is 15-4=11, 22-11= 11, 11 x 120= 1320, 1320+1610=DH 2930'