Hypsometry
Collier's New Encyclopedia (1921)
that department of geodesy which treats of the measurements of the altitudes or relative heights of various points on the earth's surface. In all cases in which great accuracy is essential, trigonometrical methods must be employed, but in other cases sufficiently accurate results may be obtained by levelling, by the use of the barometer, or by the boiling point of water as given by the thermometer. The trigonometrical method is often the only one available, as the height to be measured may be quite inaccessible. The barometric method is based on the fact that as the mercurial column is supported by the atmospheric pressure, it must fall when conveyed from a lower to a higher level, as in the latter case the pressure is diminished. Were the atmosphere uniform in density throughout, nothing could be simpler than the measurement of heights by the barometer, but gases being very compressible, the lower strata of the atmosphere are denser than the upper strata, being exposed to greater pressure. Moreover, increase of temperature affects the density of the mercury in the barometer, and also that of the air, and further complicates the problem. Hence for the greatest accuracy in determining the difference of levels two mercurial barometers and four thermometers are required. Two of the thermometers are used for determining the temperature of the air at the stations, and barometers for determining the temperatwo are attached to the ture of the mercury. The observations are made simultaneously. The aneroid barometer is in some respects more suitable than the mercurial, being much more portable, and requiring two thermometers only. After the necessary observations are made the required height may be calculated by the use of certain logarithmic formule.
What Has Changed in the Last Century
When Collier's wrote this in 1921 the surveyor's tools were the theodolite, the mercurial barometer with its four thermometers, and a thermometer reading the boiling point of water; today height is fixed by satellite. GPS receivers, first fully operational in 1995, give positions to centimetres, and the Space Shuttle's SRTM radar mission of February 2000 mapped the elevation of nearly all land between 56 u0b0S and 60 u0b0N in eleven days u2014 a task that would have taken Collier's methods centuries. Satellite laser and radar altimeters now measure ice-sheet and ocean surfaces to a few centimetres, and heights are referenced to mathematical geoid models such as EGM2008 rather than to a local sea-level mark. The result: Everest was re-measured jointly by China and Nepal in 2020 at 8,848.86 m, settling a figure that trigonometrical survey had disputed for 170 years. The word 'hypsometry' itself has largely shifted meaning, and now more often denotes the study of the distribution of elevation across a landscape u2014 the hypsometric curve u2014 than the act of measuring a single peak.
Written for this edition, 2026.