Nivellement

Nivellement is a set of surveying techniques used to determine vertical positions of points and to establish a common vertical reference surface. The main goal is to measure differences in elevation and to build a consistent height framework for construction, mapping, and hydrological studies.

The principal instrument is a level, such as a dumpy level or an automatic/digital level, paired with a graduated staff or rod. In differential or spirit leveling, the surveyor establishes a benchmark, sets up the instrument, and records a back sight on the staff. The instrument is then rotated, and a fore sight is taken at the next point. The height of instrument (HI) is used to compute the elevation difference: height difference = back sight minus fore sight. Repeated readings along a line produce a leveling run, which can be used to derive the elevations of successive points relative to the benchmark. Height values are reduced to a chosen datum.

Corrections and uncertainties: readings are affected by curvature and refraction, temperature, and staff alignment. Quality control includes repeating measurements, closure checks, and differential checks against known benchmarks.

Types: spirit differential leveling, precision or line leveling, differential leveling with digital levels; trigonometric leveling uses vertical angles and distances when line-of-sight is obstructed or long distances are involved.

Applications: leveling underpins construction grading, road and railway design, drainage and flood modeling, topographic surveying, and GIS datum maintenance.

Modern developments: electronic and digital levels, automatic data recording, laser and optical tools, and integration with GNSS to tie height data to national vertical datums.