Forest covers nearly half of Québec. No one can measure it tree by tree. Yet today we know the height of the canopy over tens of thousands of square kilometres, at one-metre resolution. This feat comes not from an army of surveyors, but from a laser that looks at the treetops — and the ground beneath — in the same pass.
The key data: canopy height
Recall the mechanics seen in the geomatics pipeline: the laser's first return traces the top of the vegetation (the DSM), the last return reaches the ground (the DTM). Their difference gives the canopy height model: CHM = DSM − DTM. Each one-metre cell then carries not an elevation, but the height of the trees that stand there.
A single flight delivers both the relief of the ground and the stature of the forest that covers it. That is what neither aerial photography nor fieldwork could do at this scale.
From height to forest variables
Height is only a start. By combining it with field plots — a few parcels measured the classic way — you calibrate models that estimate, across the whole covered territory:
- timber volume and biomass, and therefore stored carbon;
- stand density and its vertical structure (strata, understorey);
- basal area and indices of maturity.
Enhanced forest inventory thus moves from sparse sampling to continuous coverage: you no longer assume the state of stands between plots, you measure it.
Ecology gains a third dimension
Wildlife responds not only to the presence of trees, but to the structure of the forest: height, gaps, complexity of the strata. LiDAR describes this architecture precisely. It is used to map habitats, spot old forests with irregular canopy, track fragmentation, model connectivity for sensitive species.
Canopy structure also influences snow, light at ground level, humidity — microclimatic variables that can now be estimated at broad scale rather than guessed.
Tracking change over time
When a region is flown a few years apart, comparing two CHMs reveals growth, but also harvesting, windthrow, outbreaks, fires. This multi-temporal monitoring — the same principle as for ground movements — turns a single snapshot into a film of the landscape.
A note on Québec 3D
This tool renders the bare ground (the DTM), not the canopy: the vegetation-height models (CHM) were deliberately left out of the project. You will therefore see the terrain beneath the forest — often more revealing than you would expect.
See the ground beneath the forest →The takeaway
In forestry, LiDAR replaced a question — “how many trees?” — with a direct, continuous measure of canopy structure. Canopy height, derived from a simple difference between two surfaces, opens onto volume, carbon, habitat and change. For a territory as vast and forested as Québec, that is the difference between estimating and knowing.