The ground command is used to segment the input point cloud into ground versus non-ground returns using filters.smrf and filters.outlier.

$ 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
--extract           Extract ground returns?
--reset             Reset classifications prior to segmenting?
--denoise           Apply statistical outlier removal prior to segmenting?
--returns           Include last returns?
--scalar            Elevation scalar?
--threshold         Elevation threshold?
--cut               Cut net size?
--ignore            A range query to ignore when processing