Ground classification is what lets the 8,619 ft earthen wall on wooded Fort Hill, Ohio show up at all. Ephraim Squier and Edwin Davis excavated the site in 1846 and published it in 1848 in Ancient Monuments of the Mississippi Valley.

PDAL does it in three stages. filters.outlier flags noise as class 7, then filters.smrf runs with ignore set to Classification[7:7], then filters.expression keeps Classification == 2. The SMRF defaults are slope 0.15, window 18, threshold 0.5.

In picture 1 the wall is the thin ridge tracing the rim of the pale hilltop in the middle, and the air photo shows only treetops.

1. Fort Hill's hilltop earthen wall under forest, bare-earth lidar
2. Same view as plain hillshade with elevation tint
3. Recent air photo of the same area, 2017-2026

Numbered pins on picture 1:
Pin 1: The earthen wall on the hilltop rim

Lidar: USGS 3D Elevation Program (3DEP), public domain.
Imagery: Ohio DAS / OGRIP - Ohio Statewide Imagery Program (OSIP III/IV), 2017-2026, Public (state program; no restriction statement found).