Skip to main content
Ground Reconstruction builds a ground surface for a LiDAR sequence: a smooth, curved skin that follows the real road, floor, or terrain under your data. Once you have one, annotation gets easier. Cuboids and polyline points can snap down onto the ground, so you stop adjusting height by hand. It works in two stages: the computer makes a first attempt from the point cloud, then you fix the parts it got wrong. Every edit can be undone, and nothing is stored until you submit.

​
Pick 4D or 3D ground

There are two ground tools in the editor. They behave identically — the same Generate, the same clean-up tools, the same shortcuts, the same undo — and differ only in how many surfaces they build. Pick 3D when a sequence’s LiDAR calibration is poor. Combining frames that disagree with each other smears the ground into a blur that is useless for snapping — but each frame on its own is still perfectly good, so a per-frame surface stays accurate exactly where you use it. If a frame has its own surface, snapping uses that; otherwise it falls back to the 4D surface. You can have both on the same sequence.
Not sure? Turn on Ground surface from the Layers menu and look at it. If the road looks like several roads stacked at slightly different heights, the frames don’t agree — use the 3D tool.

​
When to Use

Use Ground Reconstruction when:
  • You are annotating a LiDAR point cloud sequence
  • You want cuboids or polyline points to sit on the ground automatically
  • A downstream tool needs height-above-ground (e.g. separating objects from the road)

​
Generate the surface

  1. Open a LiDAR sequence and pick Edit ground (4D) or Edit ground (3D, per frame).
  2. Choose an Algorithm:
    • Cloth simulation (continuous) — the default. Imagine dropping a cloth over the scene from below: it drapes over the real ground and ignores cars, people, and trees. Best for roads and outdoor scenes.
    • Grid (per-cell, fast) — quicker, and good when your points are dense and the ground is fairly flat.
  3. Press Generate ground.
You will see Aggregating frames and reconstructing…, then Surface ready · N ground cells. With the 3D tool you get two Generate buttons instead of one:
  • Generate visible — rebuilds every frame that is currently visible. It reconstructs one frame at a time, so the sheet counts Reconstructing frame 3 of 40… as it goes.
  • Generate frame — rebuilds only the frame you are looking at. Use it to redo a single frame that came out wrong, without touching the rest.
Frames you didn’t generate keep whatever surface they already had. If the result looks wrong, change a setting and press Generate ground again. Re-running is safe — nothing is committed yet. Start with Cell size: smaller means more detail but more memory. The Ignore points… sliders throw away points too close to the sensor, or too far away in a given direction, which helps when distant noise drags the surface around.
Only Cloth simulation produces the point-support data that Trim unsupported ground needs. If that button is greyed out, you generated with Grid, or you loaded a surface saved before support data existed.

​
Clean it up

Five tools, all undoable. They work the same in both tools — with one difference that matters in 3D:
With the 3D tool, every clean-up tool acts on all visible frames at once. One brush stroke, one erase loop or one Apply edits every visible frame’s surface together, as a single undo step. Hidden frames are never touched.So hide the frames you don’t want to change before you edit — and if you only mean to fix one frame, isolate it first.
Suppress spikes — one click. Pulls cells that stick up or down oddly back onto the surface. A slider appears: Apply to keep, Cancel to discard. Trim unsupported ground — one click. Removes the “skirt”: surface the computer guessed at where there were no actual LiDAR points. The fastest way to delete ground floating out into empty space. Also has a slider, with Apply / Cancel. Erase (X) — draw a loop around ground you do not want; it disappears on release. The tool stays open, so keep erasing. To move the camera, switch to Navigate (or press Space), orbit, then switch back to Lasso. Fill (G) — the opposite. Draw a loop around a hole and it fills in using the real ground nearby. Two settings: Direction (how many directions each hole searches) and distance (how far it looks). Widen them if holes refuse to fill. Brush (B) — sculpt by hand. Drag over the surface to push it around. A shaded disc shows exactly which area the brush will affect. Press and hold without moving and the brush keeps applying to the same spot — useful for one stubborn bump. Strength sets how far each stroke moves the ground; the radius sets how wide the brush is. A flat/feathered option controls the edge: feathered fades out (softer, natural), flat moves the whole circle equally (good for deliberate steps). Each stroke is one undo step.

​
Snap annotations to the ground

Turn on the ground snap toggle and cuboids and 3D/4D polyline points sit down onto the surface as you place and move them. Turn it off any time — it does not change anything already placed.

​
Save

Saving takes two steps, and it is easy to stop after the first.
  1. Press Done to finish with the ground tool. This closes the tool and keeps your work, but nothing has been stored yet.
  2. Press Save and submit. This is what stores the ground.
What you see on screen is exactly what gets stored.
If you built the ground while only part of the sequence was loaded, you will see “Save ground from an incomplete sequence?”. The saved 4D surface covers the whole sequence, so areas you never reconstructed may be missing or wrong, and snapping and height-above-ground inherit that. Either Cancel, load the rest of the frames and re-generate — or press Save anyway if the missing parts do not matter.This warning does not apply to the 3D tool: each frame is stored separately, so a frame you never reconstructed simply has no surface rather than a wrong one.
With the 3D tool the message tells you how many frames were stored, e.g. Ground saved for 12 frame(s). Frames whose surface had no ground cells at all are skipped rather than overwriting good ground with an empty grid. If nothing actually changed — you only opened the tool, or you undid every edit back to where you started — Done stores nothing. That is deliberate.

​
Shortcuts

B, G and X work from anywhere, so you can jump straight from erasing to filling without returning to the toolbar. 7/8/9 only act while the Brush is open and you are not mid-stroke. Fill is G rather than F because F is the browser’s full-screen key. Your usual keys keep working: W A S D Q E fly the camera, 1/2/3 change point-cloud colouring, and anything with Ctrl / ⌘ / Shift / Alt (like undo) is never intercepted. While a Suppress spikes or Trim unsupported ground slider panel is open, all of the above are switched off — finish with Apply or Cancel first.

​
Common Mistakes