The ground command is used to segment the input point cloud into ground versus non-ground returns.

$ pdal ground [options] <input> <output>
  --input, -i         Input filename
  --output, -o        Output filename
  --max_window_size   Max window size
  --slope             Slope
  --max_distance      Max distance
  --initial_distance  Initial distance
  --cell_size         Cell size
  --classify          Apply classification labels?
  --extract           Extract ground returns?
  --approximate, -a   Use approximate algorithm? (much faster)
  --reset             Reset classification prior to segmentation.
  --denoise           Apply statistical outlier removal prior to segmentation.

For more information, see the full documentation for PDAL at