In 3D on Québec 3D, the “Bare earth” button gives a crisp, sharp relief: every bank, every ravine reads at a glance. Switch to “Surface” and the landscape changes in nature — buildings and forests rise into rounded, grey, edgeless bumps. It looks like a blurry, less precise version of the same map. The impression is misleading: this smoothing is deliberate, and this layer answers questions that bare earth cannot.
First, what this “surface” is. The digital terrain model (DTM) describes the bare ground; the digital surface model (DSM) describes the top of everything on it — ground, buildings and vegetation together. As we saw in the LiDAR geomatics pipeline, both come from the same laser shots: the last echo reaches the ground (DTM), the first traces the canopy top (DSM). On Québec 3D, the DSM is what the Surface button reveals: a continuous elevation of the territory's envelope.
Why it is smooth — and why that is a choice
Bare-earth relief is crisp because the ground is a single, continuous surface. The surface adds the above-ground cover — foliage, roof edges, tree lines — whose laser echoes are naturally noisy. A raw DSM bristles with spikes and pits: every isolated branch, every artefact becomes a needle. By laying the height of that cover onto the DTM and smoothing it, you strip out the high-frequency noise and keep the essential: a clean, continuous volumetric envelope.
Smoothing also has technical virtues. It avoids the aliasing and “staircase” artefacts that would blight elevation tiles, and it produces a lighter 3D mesh — fewer triangles to load, smooth rendering in a browser or on a phone. The softness of the surface is not a lack of resolution: it is what makes it displayable at the scale of a province.
As for the absence of colour: like all the relief here, the surface is a hillshade, not a photo. A satellite image shows what things look like; the surface shows their shape and height. The information is not in the hue; it is in the volume.
Reading the clutter of the land at a glance
The DTM alone presents a curiously “deforested” world: the relief is exact, but nothing stands up. The surface gives it back its real envelope. Even reduced to bumps, buildings and trees recover their mass: a neighbourhood becomes a dense raised zone, a forest a uniform textured plateau, a field a smooth plain. Without reading a single label, the density and concentration of these features map the urban fabric by eye. It is far more intuitive than bare terrain, and it is enough to place the relief: you see at once where you are.
What blocks the view: visibility and coverage
Many analyses hinge on what blocks the view: lines of sight, intervisibility, wave propagation for cellular coverage or microwave links. Here the choice of layer is decisive. Bare earth is too optimistic — it ignores obstacles and lets you “see” through forests and neighbourhoods. A raw DSM, conversely, injects noise that fabricates false screens and false clearings. The smoothed surface is the sweet spot: it accounts for the overall thickness of obstacles — trees and buildings merged under a single height envelope — without the errors of high-frequency noise. For a first pass on coverage or intervisibility, it is exactly the right grain of detail.
Cast shadows and sunlight
The same logic holds for shadow and sun. To estimate the shadow cast by a grove or a city block according to the solar azimuth, at the scale of a neighbourhood or a watershed, the smoothed shape is amply enough. There is no need to first separate buildings from vegetation, nor to rebuild roofs in 3D: the continuous envelope already carries the height needed to cast a plausible shadow. A light analysis, drawn straight from the relief.
Two layers, two jobs
The surface does not replace bare earth; it complements it. For a terrain profile, a slope, a drainage route — everything that belongs to fine topography — the DTM remains king: the above-ground cover would only hide the ground. For context, immersion and clutter, the surface speaks. One gives the exact shape of the ground; the other, the real envelope of the landscape. The right reflex: switch from one to the other depending on the question.
The takeaway
Even without telling forest from concrete, the surface layer delivers a continuous volumetric envelope of the territory. It is the ideal tool to place the relief and run light clutter analyses — visibility, shadow, built density — without carrying the complexity of a full vector 3D model, with its detailed buildings (LOD2) and individual trees. Richer than bare earth, lighter than a 3D city model: a middle ground, precisely where many concrete questions arise.
See the difference
In 3D, switch from “Bare earth” to “Surface”: watch the forest and the neighbourhoods rise from the relief, and the landscape recover its thickness.
Toggle bare earth ↔ surface in 3D →