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Why is spirit leveling less efficient on steep hills?

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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 less efficient on steep hills because the process requires the instrument to be moved and realigned a large number of times to maintain accuracy.1

In spirit leveling, a surveying level uses a horizontal line of sight to measure height differences between points. Because the telescope must remain truly level, the surveyor must frequently relocate the instrument and the graduated staff along the path. On steep terrain, the significant change in elevation over short horizontal distances necessitates constant repositioning of the equipment, making the process slow and labor-intensive compared to its use on flat ground.1

For faster height determination in hilly areas where lower precision is acceptable, surveyors typically employ trigonometric leveling using a theodolite to measure vertical angles and distances.1