Barometric/Hypsometry Levelling
The altitudes or vertical elevations of objects, e.g. mountains, are acquired through measuring atmospheric pressures through use of barometer. It is based on the fact in which atmospheric pressure at a point depends on the elevation above mean sea level (MSL) decreasing gradually along with the height. While measurement of temperature at boiling point of water is used to acquire the height of station above MSL, the method is known as hypsometry.