Hillshading shows where the ground changes. It does not say by how much. To lay out a forest road, site a house, plan drainage or prepare a hike, you need two other readings: the slope, which tells whether the ground is workable, and contour lines, which tell what elevation you are at and how the ground rises.
Québec 3D's Explorer now shows both, computed from the bare-earth LiDAR relief. Like the site's other tools, it is free, needs no account, and everything is computed in your browser.
Turning the layers on
- In the map panel, below the Relief / Map switch, click “Slopes”, “Contours”, or both: they are independent switches.
- Zoom in on the area you are interested in. The layers appear from the home-screen zoom onward; further out, the overview data would be too coarse for a reliable slope or contour.
- Switch to 3D relief if you like: slopes and contours follow the terrain.
Your choice is remembered: on your next visit, the layers will already be on.
Slopes, in six classes
The terrain is coloured by slope, expressed as a percentage — the rise, in metres, per 100 metres travelled horizontally. The classes follow those of Québec's ecoforest map, familiar to foresters:
- A — 0 to 3%: flat ground. Nearly transparent, so the hillshading shows through.
- B — 3 to 8%: gentle slope.
- C — 8 to 15%: moderate slope.
- D — 15 to 30%: strong slope.
- E — 30 to 40%: very strong slope.
- F — over 40%: steep slope, embankment, escarpment.
To convert to degrees: 15% is about 8.5°, 40% about 22°, and 100% equals 45°.
Contour lines
A contour line joins points of equal elevation. Tightly packed lines mean a steep slope; widely spaced ones, a flat; closed loops, a summit or a hollow. One line in five is an index contour, thicker and labelled in metres, so you can find your way without counting.
The elevation difference between two lines, the contour interval, is set in the panel:
- Auto adapts it to the zoom: 20 m at regional scale, then 10 m, 5 m and finally 2 m up close.
- A fixed value — 1, 2, 5, 10, 20 or 50 m — applies the same interval everywhere. To stay readable, a fine interval only applies at zooms where it makes sense: 1 m only appears in close-up views; further out, the tool uses the finest interval that remains readable.
Where does the data come from?
Québec's ministère des Ressources naturelles et des Forêts publishes, sheet by sheet, products derived from its LiDAR surveys, including slope maps and contour lines. These files are computed from the same digital terrain model that Québec 3D displays. Rather than bringing in nearly 900 GB of files, the Explorer recomputes slopes and contours on the fly, from the relief tiles already loaded for the map:
- the slope is measured at each point from the neighbouring elevations (Horn's method), over a distance of at least 2.5 m. This window smooths out the small steps caused by rounding elevations to the decimetre, which would otherwise draw false stripes on gentle slopes;
- the contours are traced by the open-source maplibre-contour library, which follows each target elevation across the relief grid.
The upside: the interval is yours to choose, and the layers follow the relief in 3D. The result is very close to the Ministry's products, without matching them pixel for pixel: resolution and smoothing differ.
What it is good for
Forestry — Spot where machinery can operate, lay out a forest road avoiding classes E and F, delineate embankments along watercourses.
Construction and land planning — Size up a building lot, estimate the earthworks, anticipate stormwater runoff, check the grade of a driveway or access road.
Agriculture and drainage — Read the slopes of a field, the flow paths and the flat areas where water pools.
The outdoors — Plan a hike by reading the contours: a trail that crosses many of them over a short distance promises a good climb. Combined with the elevation profile, they give the route's exact elevation gain.
Telecommunications — Before running a line of sight or a viewshed, contours show at a glance the high points of an area, natural candidates for a site.
Limits worth knowing
Slope depends on scale. It is computed at the displayed zoom: up close, on cells of about 1.6 m; at regional scale, on cells tens of metres wide. A 5 m embankment, bright red up close, fades as you zoom out. To judge a specific site, zoom all the way in.
It is bare earth. Slopes and contours ignore buildings and forest, even when the 3D relief shows the surface. In towns, the ground under buildings is interpolated.
Water bodies. Under lakes whose depth is known, the relief goes down to the bottom: contours there become depth contours. On the St. Lawrence, the water surface recorded by LiDAR varies with the tide and can draw lines with no reality on the ground. In 2D relief, water bodies are masked.
Class F is broad. It covers everything above 40%, from a 45% embankment to a cliff. For avalanche terrain, which starts around 30° (about 58%), the map makes no distinction: it does not replace an avalanche terrain exposure map.
Finally, the relief dates from the LiDAR flight: later fills, quarries or works are not shown.
Read the terrain
Open the Explorer, turn on “Slopes” and “Contours”, then zoom in on a hill, a ravine or a quarry: the relief reads like a topographic map.
Show slopes and contours →The takeaway
With two switches, Québec's LiDAR relief becomes a topographic map: slopes classified as on the ecoforest map, contour lines at the interval of your choice, in 2D and 3D. These layers complete the elevation profile, the line of sight and the viewshed for reading the land with no software and no download.