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How does spirit leveling measure differences in 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).

Spirit leveling is a highly accurate method used in civil engineering to determine the difference in elevation between points on the ground.1

The process utilizes a surveying level—a telescope mounted on a tripod and fitted with a sensitive spirit level bubble. The instrument is adjusted until the telescope is perfectly horizontal. A graduated leveling rod is then placed vertically at the points being measured.1

When the surveyor sights the rod through the leveled telescope, the cross hairs provide a reading representing the height of the line of sight above the ground at that specific point. Because the telescope is held at a constant horizontal plane, the difference between the readings taken at two different points directly corresponds to the difference in their elevations. By repeatedly moving the instrument and the rods along a path, surveyors can accurately measure height differences over long distances.1