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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]

A numerical representation of surface elevation sampled over a horizontal grid or another spatial mesh. A hillshaded rendering highlights slope geometry but depends on artificial illumination direction. A digital elevation model also supplies the topographic phase needed in interferometric synthetic-aperture radar processing.

For regional coverage, use stereo photogrammetry from overlapping satellite or airborne imagery, or a radar-derived digital elevation model where suitable data exist. Match the same terrain features in images taken from different known viewing positions, infer their three-dimensional positions by triangulation, and georeference and grid the result. Satellite stereo or interferometric radar can cover $10^4\mathrm{k}{\mathrm{m}}^2$ efficiently; choose pixel size and vertical precision fine enough for the fault scarps of interest, rather than assuming every regional model resolves small scarps.

InSAR topographic mapping uses the phase difference from a known viewing baseline; repeat-pass deformation work must separate that height contribution from actual motion. Vegetation, cloud, radar shadow and steep slopes can leave gaps or represent the canopy rather than bare ground.^[ref] Ground control and independent elevation checks are needed before interpreting small relief as active faulting.

Use airborne lidar with sufficient pulse density and ground-return classification. Laser travel times give ranges; the aircraft’s surveyed position and orientation place returns in three dimensions. Multiple returns include canopy and, where gaps permit, the ground. Classify ground points, remove vegetation returns and interpolate a bare-ground digital elevation model over the $20\mathrm{k}{\mathrm{m}}^2$ strip.

This specifically addresses the forest-obscured fault scarp, unlike ordinary optical stereo, which primarily models the visible canopy. Leaf-off acquisition can improve ground sampling where relevant. Dense understorey and a complete lack of ground returns still cause uncertainty; interpolation does not create observed ground heights. Inspect ground-point coverage and compare hillshades under several illumination directions.^[ref]

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.

Use repeat-pass interferometric synthetic-aperture radar. An orbiting radar transmits coherent microwave pulses and records the amplitude and phase of the returning field. Form an interferogram from registered complex images acquired before and after the earthquake, remove the orbital contribution and the topographic phase predicted by a digital elevation model, and correct atmospheric and other long-wavelength phase delays. Where interferometric coherence is adequate, unwrap the residual phase to estimate line-of-sight displacement:with sign set by the instrument's displacement convention. Each $2\pi$ phase cycle represents $\lambda /2$ of range change.

This maps a distributed earthquake displacement field day or night and through cloud, including areas inaccessible on foot. Roughly $10$–$30\mathrm{m}$ ground sampling and a scene spanning many kilometres on both sides of the fault can resolve regional deformation; a narrow surface break benefits from finer pixels or complementary sub-metre optical surveys. Sensitivity to displacement can be millimetres to centimetres in coherent areas, but that does not equal spatial resolution. A single viewing direction measures one component; ascending and descending tracks and GNSS constrain more components. At the rupture itself, large gradients and changed ground cover can destroy interferometric coherence; radar pixel-offset tracking or before/after optical correlation can then recover metre-scale fault offsets.

Official explanation: www.usgs.gov/faqs/what-difference-between-lidar-data-and-a-digital-elevation-model-dem

Cited passages: aircraft laser returns, point-cloud representation and filtering vegetation and structures to construct a bare-earth digital elevation model. No publication date is inferred from the crawl date.