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Ground classification: the erosion radius parameter

  • Status: unverified
  • Applies to: Metashape Pro 1.7.5 — unchanged through 2.3
  • Edition: Pro
  • Diátaxis: explanation
  • Confidence: high
  • Last reviewed: 2026-06-05

Confidence: high. The parameter is documented in the official manual (§ Editing → Classify Ground Points, p. 146) and confirmed via forum post from Agisoft support with a permalink.

What it does

The Erosion radius parameter in Classify Ground Points (added in Metashape 1.7.5) creates a buffer zone around non-ground points. Any ground-classified point within this radius of a non-ground point is reclassified as unclassified.

"Erosion radius determines the indentation (in meters) from unclassified points, ground points within that area will be unclassified. This new option may be useful when classifying areas with houses and trees to avoid 'stumps' on DTM when building elevation model from the ground points class." — Alexey Pasumansky, 2021-10-13, Metashape 1.7.5 (permalink)

The "stump" problem

When a building or tree is classified as non-ground, the ground points immediately adjacent to its base often remain classified as ground. These edge points typically have slightly elevated Z values — they sit on the building's foundation, root flare, or the transition zone where the classifier is uncertain.

When a DTM is interpolated from ground-class points only, these edge points create small raised "stumps" at the base of every removed structure. The DTM surface rises slightly where each building or tree was, then drops back to true ground level — producing visible artefacts in the elevation model.

How erosion radius fixes it

Setting erosion_radius to a positive value (in metres) removes all ground points within that distance of any non-ground point. This creates a clean buffer zone that the DTM interpolator bridges smoothly, eliminating the stumps.

Typical values:

Scenario Erosion radius Rationale
Urban buildings 0.5–1.0 m Foundation footprint extends ~0.5 m beyond walls
Trees / vegetation 0.3–0.5 m Root flare and trunk base
Power lines / poles 0.1–0.2 m Narrow structures, small buffer needed
No erosion (default) 0.0 m Preserves all ground points

Python API

Ground classification is a method on the PointCloud object:

Demo verified: ✗ — requires point cloud from aligned dataset.

import Metashape

chunk = Metashape.app.document.chunk

chunk.point_cloud.classifyGroundPoints(
    max_angle=15.0,       # max angle (deg): ground-slope assumption
                          # (distinct from the separate 'Max terrain slope')
    max_distance=0.5,     # max distance to classified surface (m)
    cell_size=50.0,       # initial cell size for TIN (m)
    erosion_radius=0.5,   # buffer zone around non-ground (m)
)

The default erosion_radius is 0.0 (disabled). The parameter is only meaningful after classification has already assigned non-ground labels — it operates as a post-processing step on the classification result.

When to use it

  • DTM generation from classified point clouds. If your DTM shows raised artefacts at building/tree bases, add erosion radius.
  • Urban scenes with many structures. Buildings with wide foundations benefit from 0.5–1.0 m erosion.
  • Forested terrain. Tree trunks and root systems create transition zones that the standard classifier often misclassifies.

When NOT to use it

  • Flat terrain with no structures. Erosion removes valid ground points near any non-ground classification. On flat terrain with sparse vegetation, this may create unnecessary gaps.
  • When ground point density is already low. Erosion further reduces ground points. If your point cloud is sparse, the DTM interpolator may not have enough points to bridge the gaps cleanly.
  • Before verifying classification quality. Fix misclassification first — erosion is a refinement step, not a substitute for correct classification parameters.

Key points

  • Added in Metashape 1.7.5. Default: 0.0 (disabled).
  • Removes ground points within N metres of non-ground points.
  • Prevents DTM "stumps" at building/tree bases.
  • Accessed via chunk.point_cloud.classifyGroundPoints().
  • Typical values: 0.3–1.0 m depending on structure type.

References

  • Metashape Pro User Manual (2.3), § Editing → Classify Ground Points, p. 146 — documents all parameters including Erosion radius.
  • Point Cloud Classification (Agisoft KB) — the full ground / multi-class / manual classification workflow and DTM generation; and Parameters for ground point classification (Agisoft KB) — a deep dive on Max angle, Max distance, Max terrain slope, Cell size, Return and Erosion radius with worked examples (rural, low-vegetation, satellite).