A drone cannot see through vegetation, but a drone-mounted LiDAR can measure the ground through gaps in it. Each laser pulse can return several echoes, and the last echo from a pulse that reached the ground gives a bare-earth point. Photogrammetry cannot do this: it models the canopy surface. The resulting ground model depends on canopy density, season and point density, and is verified with check points on the ground.
This guide expands the short answer on our LiDAR and laser scanning service page and supports the BNG survey and topographic survey services.
Why can’t photogrammetry see the ground under vegetation?
Photogrammetry reconstructs the surface the camera can see. Over a grassed embankment that is the top of the grass; over a hedgerow it is the top of the hedge; over woodland it is the canopy. The software has no way of knowing that the ground lies below. On a grassed or scrubbed embankment this produces an elevated false surface, which is why our reservoir survey guide treats photogrammetry alone as unsuitable for freeboard work.
How does LiDAR record the ground beneath a canopy?
A LiDAR pulse has a physical footprint on the ground. When part of the footprint strikes foliage and part passes through a gap, the sensor can record more than one echo from the same pulse: a first return from the canopy, intermediate returns from lower branches, and a last return from the ground. Full-waveform and multi-return sensors capture this. Ground classification then selects the returns that belong to the terrain and rejects the rest.
The important word is gaps. LiDAR does not pass through leaves or timber. The density of ground returns is governed by how much open sky the ground can “see” from the aircraft, which is why the factors below matter more than any headline sensor figure.
What determines how much ground is recovered?
| Factor | Effect on ground returns | What the surveyor controls |
|---|---|---|
| Canopy density and species | Dense, layered or evergreen canopy leaves fewer gaps | Choice of season; multiple passes from different directions |
| Season | Leaf-off deciduous canopy is far more open than leaf-on | Flying window; winter and early spring preferred for woodland DTMs |
| Point density | More pulses per square metre give more chances of a gap | Pulse rate, flying height, ground speed, overlapping passes |
| Flying height and footprint | Larger footprint at height mixes canopy and ground in one pulse | Flying height chosen for the specification |
| Scan angle | Oblique pulses meet more foliage than near-vertical ones | Overlap so that most ground is seen near nadir from at least one strip |
| Understorey | Bracken, bramble, gorse and reed form a second canopy close to the ground | Reported as a limitation; check points placed within it |
| Terrain slope and roughness | Steep aspects thin out returns on one side | Cross passes on steep catchment boundaries |
| Water | The laser is absorbed at the surface | Bathymetric survey where bed levels are needed |
There is no fixed penetration percentage that can be quoted for a site in advance. What can be stated is the method, the season and the density flown, and then what the check points showed.
What can be expected from different vegetation types?
| Vegetation | Expectation for a bare-earth DTM |
|---|---|
| Short grass, mown verge, stubble | Ground effectively at the surface; DTM behaves like open ground |
| Long grass, crops, rush | Ground returns present but reduced; the DTM under standing crop should be treated as soft detail |
| Hedgerow lines, scattered trees | Ground recovered either side and normally beneath, subject to density; the hedge base and any ditch profile need checking |
| Deciduous woodland, leaf-off | Usually the best woodland case; useful DTM for drainage, flood and archaeological work |
| Deciduous woodland, leaf-on | Reduced returns; the DTM is coarser and should be flagged |
| Conifer plantation | Closed canopy year-round; returns limited to rides, gaps and thinned stands |
| Gorse, bramble, bracken, reedbed | Dense low cover close to the ground; the hardest case and often the widest residuals |
Because these expectations vary so much, a rigorous survey places some independent check points on the soft-detail ground and reports them separately from the check points on hard detail. Our guide on how accurate a drone LiDAR survey is explains why.
How is the ground separated from the vegetation?
- Strip adjustment. Overlapping flight lines are matched so the whole cloud is internally consistent and tied to the ground control.
- Noise removal. Isolated high and low points (birds, multipath, reflections) are removed.
- Automated ground filtering. A progressive algorithm grows a ground surface from the lowest reliable points, accepting points within slope and distance thresholds tuned to the terrain.
- Manual editing. Automated results are reviewed in a hillshaded 3D view and corrected where they fail most often: hedge bases, ditch inverts, embankment toes, cut faces and stream banks.
- Classification to standard classes. Ground, low, medium and high vegetation, buildings, water and overhead lines are coded to the ASPRS LAS classification scheme so that downstream users can filter the cloud.
- Product generation. A DTM from the ground class, a DSM from the full cloud, and a canopy height model (CHM) as the difference between them.
The canopy height model is itself a deliverable. On BNG and habitat work it gives hedgerow heights, woodland structure and tree positions on a dated, georeferenced base; on landscape and visual work it drives screening assessments.
Where are the limits?
- LiDAR does not penetrate water, closed evergreen canopy, or dense low cover with no gaps.
- The DTM under cover is an interpolation between recovered ground points; where they are sparse, subtle features such as shallow ditches or ridge-and-furrow can be lost.
- Leaf-on woodland surveyed for programme reasons should carry a stated limitation.
- Check points under cover are harder to observe by GNSS because the same canopy degrades the satellite signal; our GNSS-degraded environments guide covers how control is carried in.
How Angell Surveys does this
Angell Surveys is regulated by RICS and flies UAV LiDAR from the Wingtra Ray fixed-wing VTOL alongside photogrammetry, so vegetated and open ground are captured on the same control in the same sortie. On a water company’s two reservoir sites in the Fens and south Lincolnshire, mature hedgerow boundaries and small woodland blocks completely obscured the ground from photogrammetry; first, intermediate and last-return classification produced the bare-earth model that the hydraulic modellers and archaeologists worked from, delivered as a tiled, ASPRS-classified point cloud with a canopy height model. For a rivers trust’s upland catchment in Stirlingshire, where the best public terrain data was 30 metre resolution, the LiDAR was specified with overlapping passes across the steep boundaries to keep return density on every aspect and delivered a sub-50 cm DTM. Every ground model we issue carries an accuracy statement against independent check points and states its season and limitations.
Related services
Sources
- RICS, Measured Surveys of Land, Buildings and Utilities, 3rd edition, November 2014: hard and soft detail accuracy bands.
- RICS, Earth Observation and Aerial Surveys, 6th edition, March 2023.
- ASPRS, LAS Specification 1.4: standard point classification classes.
- Angell Surveys case studies: Lincs and Fens reservoirs, Calair Burn catchment, Ballimore Restoration, 250 ha solar farm and BESS.