Determining bare earth DEM from unclassified LAS file?

Question

I have data in LAS format with RGB values created from aerial photogrammetry using a UAV. I am trying to find a solution to extract the bare earth DEM from the point cloud.

I have tried SAGA, Fusion, MCC-LIDAR, but it is seems they need the LAS file to be already classified (which it naturally isn't). Can anyone point me in the right direction with a brief explanation of the process?

Generally, I would need to process about 100 mill points at a time (can tile them if needed).

Answer 1

Generating LiDAR DEMs from unclassified point clouds with:

• MCC-LIDAR - Multiscale Curvature Classification (MCC) algorithm.
  _{(supports LAS versions 1.1 to 1.3)}

MCC-LIDAR is a command-line tool for processing discrete-return LIDAR data in forested environments (Evans & Hudak, 2007).

Workflow:

• a) unclassified point cloud.
• b) ground returns classified.
• c) bare-earth DEM (raster).

---

Let's create a hypothetical situation to further provide an example with code.

MCC-LIDAR is installed in:

C:\MCC

The unclassified LiDAR point cloud (.las file) is in:

C:\lidar\project\unclassified.las  

The output which are going to be the bare-earth DEM is in:

C:\lidar\project\dem.asc  

The example below classifies ground returns with the MCC algorithm and create a bare-earth DEM with 1 meter resolution.

#MCC syntax: 
#command
#-s (spacing for scale domain)
#-t (curvature threshold)
#input_file (unclassified point cloud) 
#output_file (classified point cloud - ground -> class 2 and not ground -> class 1)
#-c (cell size of ground surface)
#output_DEM (raster surface interpolated from ground points)

C:\MCC\bin\mcc-lidar.exe -s 0.5 -t 0.07 C:\lidar\project\unclassified.las C:\lidar\project\classified.las -c 1 C:\lidar\project\dem.asc

To understand better how the scale (s) and the curvature threshold (t) parameters work, read: How to Run MCC-LiDAR and; Evans and Hudak (2007).

The parameters need to be calibrated to avoid commission/labeling errors (when a point is classified as belonging to the ground but actually it belongs to vegetation or buildings). For example:

The MCC-LIDAR uses Thin Plate Spline (TPS) interpolation method to classify ground points and generate the bare-earth DEM.

---

_References:

• _{Evans, Jeffrey S.; Hudak, Andrew T. 2007. A multiscale curvature algorithm for classifying discrete return LiDAR in forested environments. IEEE Transactions on Geoscience and Remote Sensing. 45(4): 1029-1038.}

_{For more options about ground point classification algorithms, see Meng et al. (2010):}

• _{Meng, X.; Currit, N.; Zhao, K. (2010). Ground Filtering Algorithms for Airborne LiDAR Data: A Review of Critical Issues. Remote Sensing, 2(3), 833–860. doi:10.3390/rs2030833}

Answer 2

I think that LasTools might suit your needs, see LASGround. The license is a bit funny depending on what tools. The tools can be downloaded and evaluated prior to purchase; also the product is relatively inexpensive.

Answer 3

I have had good luck with FUSION's (manual here) GroundFilter command. I've had no problem handling 40 million points (unclassified), so wouldn't expect an issue with 100 million.

Answer 4

This can be done with a pdal filter using either Simple Morphological Filter (SMRF) or Progressive Morphological Filter (PMF) algorithms.

Quick

pdal ground --cell_size=5 --extract input.laz out-bare-earth.laz

Creates a bare earth compressed LAS file with a 5 ground unit cell size using PMF. (docs)

For more explanation see the Identifying ground returns using ProgressiveMorphologicalFilter segmentation tutorial.

More involved, using SMRF

A pipeline example that:

• applies the SMRF filter, enlarges the cell size option to 2.0 (coordinate system units) and a 0.75 threshold
• selects only the newly classified ground points (2 is the LAS standard value for ground)
• writes selection to an uncompressed LAS output file (just change extension to .laz for compressed)

Command: pdal pipeline "classify-ground-smrf.json"

The JSON parameters file:

{
    "pipeline": [
        "inputfile.laz",
        {
            "type":"filters.smrf",
            "cell": "2.0",
            "threshold": "0.75"
        },
        {
            "type":"filters.range",
            "limits":"Classification[2:2]"
        },
        "out/smurf_classifed.las"
    ]
}

Extract above ground only

This example a) classifies into ground/not-ground, b) adds "Height Above Ground" attribute, and c) exports only points 2.0 (coordinate system units) above ground.

{
    "pipeline": [
        "input.laz",
        {
            "type": "filters.assign",
            "assignment": "Classification[:]=0"
        },
        {
            "type": "filters.smrf"
        },
        {
            "type": "filters.hag"
        },
        {
            "type": "filters.range",
            "limits": "HeightAboveGround[2:]"
        },
        {
            "filename":"above-ground.laz"
        }
    ]
}

Adapted from Brad Chambers, https://lists.osgeo.org/pipermail/pdal/2017-July/001367.html