Change of Vertical Unit

Formula

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)

Example

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)) = 25.00 m.

EPSG:1104

Change of Vertical Unit

Attributes

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Formula

Example