Compare airborne lidar surveys

Two airborne lidar surveys of the same area can reveal how the bare earth changed over time: for example, after a storm season, a debris flow, or several years of erosion and deposition. Airborne M3C2 helps locate and measure those changes in the 3D survey, rather than relying on a visual comparison of two dense point sets.

M3C2 compares the surfaces along the direction the ground faces. Around each point it averages nearby ground from both surveys inside a short cylinder perpendicular to the local slope, then reports the distance between them. This averaging is less sensitive to point-by-point noise than direct differencing, and the slope-oriented measurement is more meaningful on steep ground than a simple height difference.

Both surveys need ground classification. Airborne M3C2 uses LAS class 2 ground points. Without that classification, trees, buildings, vehicles, and vegetation can dominate the result, so the apparent change may describe the canopy rather than the terrain. Check Classes if you are unsure whether ground points are present.

1. Import both surveys into one project. Put the tiles for each survey in its own layer group, for example 2016 and 2023, with the older survey above the newer one. The tiles do not need to have matching boundaries.
2. Show each group at least once so its data has loaded.
3. In Change Detection, choose the older survey as Baseline and the newer survey as Compare in Airborne M3C2 (layer groups).
4. Keep the default Box Size, Cylinder Radius, and Cylinder Half Length for an initial comparison. The cylinder defaults are sized from the loaded data.
5. Centre the area of interest in the view and choose Compare Around Crosshair. The status line shows progress; when the comparison finishes, the view switches to Change.

The result is Compare minus Baseline: positive values mean the ground is higher in the newer survey (for example, deposition), while negative values mean it is lower (for example, erosion or removal). No value is calculated where there is no ground to compare, such as under buildings, water, dense canopy, or outside the common coverage. These locations appear as zero by default; Show Invalid Change as Black in Change display settings makes them easier to distinguish. Use the Change legend to interpret the values; its range, deadband, absolute-value, and discrete-bin settings affect how they appear.

Move to another area and run Compare Around Crosshair again to add results. A newer run replaces earlier results where boxes overlap. Results last until you leave the project. Start with the default box size; larger boxes take longer and use more memory. A larger cylinder radius smooths the result but can blur small features, and the half length must exceed the largest change you expect to measure.

Before trusting an apparent change, check stable ground such as roads or bedrock: it should read close to zero. A similar shift across the whole area may indicate misaligned surveys or different vertical references. Classification differences can also create isolated patches that look like movement. Both surveys need compatible coordinates and units.