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Why are two barometers used when measuring height differences?

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surveying Figure 1: Photogrammetric photographs from two short, overlapping flight strips arranged for supplying mapping details. Photo-control points are shown on only one photograph; shading indicates a typical terrain feature such as a lake (see text).

In surveying, atmospheric pressure is used to determine height differences because pressure decreases as altitude increases. However, because atmospheric pressure is not static and changes constantly over time, a single barometer reading is insufficient for accurate measurement.123

To obtain reliable results, surveyors use at least two barometers. One barometer is kept at a reference point of known height and is read at regular intervals. Simultaneously, the surveyor moves throughout the area with the second barometer, recording locations, times, and pressure readings. By comparing the readings from both instruments taken at the same time, the surveyor can account for fluctuations in atmospheric pressure and accurately calculate the differences in height.3