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Lidar acquired from an aircraft with surveyed trajectory and orientation. Ground-return classification removes canopy and structure returns before constructing a bare-ground digital elevation model. Penetration through forest is sampling through canopy gaps, not unrestricted transmission through leaves. Areas with few or no ground returns require uncertainty assessment rather than treating interpolated heights as observations.^[ref]

Range measurement using transmitted laser light and its returned signal. For a pulse traveling a round trip in air, the elementary range is $r\simeq c\Delta t/2$, where $c$ is the propagation speed and $\Delta t$ the measured travel time. Sensor position, pointing and time synchronization place returns in a three-dimensional point cloud. Scanning lidar can record multiple returns from vegetation and the ground; a return is an observation of a reflecting surface, not automatically bare-earth elevation.

Use low-altitude structure-from-motion photogrammetry, with strongly overlapping aerial images and well-distributed surveyed ground control points. Bundle adjustment estimates camera geometry and a dense surface reconstruction; independent check points test its accuracy. A small unmanned aircraft or carefully controlled close-range aerial platform can cover the $0.05\mathrm{k}{\mathrm{m}}^2$ site with centimetre ground sampling. Surveyed targets and stable camera calibration are essential: centimetre pixels alone do not establish centimetre vertical accuracy.

For exposed trench walls or especially demanding local geometry, terrestrial lidar is an alternative or complement. Remove vegetation where permissible, retain the trench’s sharp topographic breaks and state whether the output is a visible surface model or a bare-ground digital elevation model.