A PDAL pipeline that a) classifies into ground/not-ground, b) adds "Height Above Ground" attribute, and c) exports only above ground points. Adapted from https://lists.osgeo.org/pipermail/pdal/2017-July/001367.html

classify-ground-export-aboveground.json

{
    "pipeline": [
        "Carmacks_01.laz",
    {
        "type": "filters.assign",
        "assignment": "Classification[:]=0"
    },
    {
        "type": "filters.smrf"
    },
    {
        "type": "filters.hag"
    },
    {
        "type": "filters.range",
        "limits": "HeightAboveGround[2:]"
    },
    {
        "type": "filters.approximatecoplanar",
        "knn": 10
    },
    {
        "type": "filters.range",
        "limits": "Coplanar[1:1]"
    },
    {
      "filename":"xx-classified.laz",
    }
    ]
}