EPSG Guidance Note #7-2.
Note: These formulas have been transcribed from EPSG Guidance Note #7-2. Users are encouraged to use that document rather than the text which follows as reference because limitations in the transcription will be avoided. If there is a requirement to change the linear unit of a vertical axis, the following formulae can be used: for the forward transformation: X2 = (X1 * U1) / U2 for the reverse transformation: X1 = (X2 * U2) / U1 where U1 and U2 are unit conversion ratios for CRS1 and CRS2 respectively. U = [(factor b) / (factor c)] from the EPSG Dataset Unit of Measure table, populated with respect to the SI base unit, for linear units the metre. Then for the forward transformation: X2 = (X1 * (b1 / c1)) / (b2 / c2) = (X1 * b1 * c2) / (b2 * c1) for the reverse transformation: X1 = [X2 * (b2 / c2)]/ (b1 / c1) = (X2 * b2 * c1) / (b1 * c2)
Conversion of a height of 25 metres into US survey feet: For the metre (EPSG UoM code 9001) factor b = 1 and factor c = 1, so U1 has a value of 1. For the US survey foot (EPSG UoM code 9003) factor b = 12 and factor c = 39.37, so U2 has a value of approximately 0.304800609601219 (one US survey foot ≈ 0.304800609601219 metres). Then H(ftUS) = (25 * (1 / 1)) / (12 / 39.37) = 82.02 ftUS. For the reverse conversion of 82.02 ftUS into metres: U1 and U2 are as for the forward case, with values of 1/1 and 12/39.37 respectively. Then Hm = (82.02 * (12 / 39.37) / (1 / 1)) = 82.02 m.