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How does atmospheric pressure help surveyors estimate elevation?

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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 civil engineering and surveying, elevation is estimated using atmospheric pressure based on the principle that air pressure decreases as altitude increases.12

Surveyors utilize a sensitive aneroid barometer to detect these pressure changes, which can correspond to height differences as small as 0.3 to 0.6 meters. Because atmospheric pressure is not static and fluctuates due to local weather conditions, surveyors must employ at least two barometers to obtain reliable data. One barometer is kept at a reference point of known elevation and read at regular intervals, while the surveyor moves through the area recording pressure, time, and location. By comparing the readings taken at the same time, the surveyor can calculate the differences in height between points.3

While this method is often the fastest way to measure heights at well-separated locations, it is generally considered the least accurate of the primary surveying techniques compared to methods like spirit leveling.3