Seeing the forest in three dimensions
A photograph from above shows what the canopy looks like but flattens its height. Foresters often want to know how tall the trees are, how dense the layers are and where the gaps lie. Lidar and drones supply this. They are the main tools for describing canopy structure between the close-up detail of field plots and the wide but flat view of satellites.
How lidar works
Lidar stands for light detection and ranging. A scanner fires rapid pulses of laser light and times how long each takes to bounce back. Since light travels at a known speed, the time converts to distance. Combined with the scanner’s position and angle, each return becomes a point with a location in three dimensions. Millions of points together form a point cloud.
The key forestry advantage is that a single pulse can produce several returns. Some of the light reflects from the top of the canopy, some from branches and leaves lower down, and some may reach the ground through gaps. This is why lidar can describe both the canopy and the ground below it. Ordinary photographs cannot see through the leaves, and lidar often can, at least partly.
Products made from lidar
From the point cloud, analysts typically create:
- A ground surface model, showing the bare terrain. It is useful for planning forest roads, studying slopes and drainage, and spotting old features hidden by trees.
- A surface model, showing the top of whatever is highest, whether canopy or buildings.
- A canopy height model, found by subtracting the ground surface from the top surface. It maps the height of the vegetation everywhere.
- Structure measures, such as how dense the foliage is at different heights, how much light reaches the ground and how large the gaps are.
These can feed models that estimate wood volume and biomass, once calibrated using field measurements from inventory plots. The conversion from structure to carbon is covered in the article on estimating biomass and carbon in standing trees. Lidar does not measure carbon directly; it measures shape.
Types of lidar platforms
Airborne lidar is carried by aircraft and covers large areas, up to whole regions. Point density depends on flight height and speed. Some countries have national programs that scan their land repeatedly.
Drone lidar uses smaller scanners on unmanned aircraft. It gives denser points over smaller areas, and a team can fly it when they choose.
Terrestrial lidar sits on a tripod or is carried on foot. It scans trunks in detail from below and can measure stem diameter, shape and branching, which complements the simple tape measurements described in the article on measuring tree height and diameter.
Spaceborne lidar exists too, with instruments that sample the surface along lines or in footprints rather than providing complete maps.
What drones add
Drones also carry ordinary or special cameras. By flying a grid of overlapping photos, software can reconstruct a three-dimensional surface in a method called photogrammetry. This is cheaper than lidar and creates detailed color imagery, which helps identify species or spot unhealthy crowns.
Photogrammetry has a key limitation: it sees only what the camera sees. It builds a model of the canopy top, but cannot reliably map the ground beneath a dense canopy, so a canopy height model needs ground elevation from another source, such as a lidar terrain model.
Drones can also carry thermal cameras, which measure surface temperature, and multispectral sensors, which capture bands beyond visible light and can indicate plant vigor. These help in checking stress, supporting the work described under forest health indicators.
Uses in practice
- Mapping stand height and density for forest management plans.
- Planning forest roads and harvest layouts around terrain features.
- Spotting storm damage, as in a windthrow event, to decide where to send crews.
- Measuring regrowth in restoration areas and checking how well planted trees are surviving.
- Surveying canopy gaps and structural complexity that matter for wildlife.
- Assessing fire fuel and the patterns left behind by forest fires.
Limits and costs
Neither technology is a magic eye.
- Calibration. Heights and volumes derived from scans are estimates that depend on field data. Without plots for checking, the numbers can be biased.
- Species. Standard lidar says little about what species a tree is, though detailed scans and extra sensors help.
- Understory. In very dense forest, few pulses reach the ground, so terrain accuracy suffers, and small plants below the canopy are hard to describe.
- Coverage and rules. Drones have limited battery life and range and must follow local aviation rules, which can restrict flights. Weather can also delay them.
- Data handling. Point clouds are large and need specialist software and skilled staff.
- Cost. Airborne lidar is expensive per survey, though cheaper per hectare for large areas. Repeat scans are needed to measure change.
Where they fit
Lidar and drones work best as part of a layered system. Satellites show where change happens, aircraft or drones describe structure in selected places, and plots anchor everything to measured trees. Used this way they help managers and researchers see the forest in more detail than either field crews or satellites could provide alone, while still needing the ground work that gives the numbers their meaning.