This function is made to be used in classify_ground. It implements an
algorithm for segmentation of ground points base on a Multiscale Curvature
Classification. This method is a strict implementation of the MCC algorithm
made by Evans and Hudak. (2007) (see references) that relies on the authors'
original source code written and exposed to R via the the RMCC package.
mcc(s = 1.5, t = 0.3)numeric. Scale parameter. The optimal scale parameter is a function of 1) the scale of the objects (e.g., trees) on the ground, and 2) the sampling interval (post spacing) of the lidar data.
numeric. Curvature threshold
There are two parameters that the user must define, the scale (s) parameter and
the curvature threshold (t). The optimal scale parameter is a function of
1) the scale of the objects (e.g., trees) on the ground, and 2) the sampling
interval (post spacing) of the lidar data. Current lidar sensors are capable
of collecting high density data (e.g., 8 pulses/m2) that translate to a spatial
sampling frequency (post spacing) of 0.35 m/pulse (1/sqrt(8 pulses/m2) = 0.35 m/pulse),
which is small relative to the size of mature trees and will oversample larger
trees (i.e., nominally multiple returns/tree). Sparser lidar data (e.g., 0.25 pulses/m2)
translate to a post spacing of 2.0 m/pulse (1/sqrt(0.25 pulses/m2) = 2.0 m/pulse),
which would undersample trees and fail to sample some smaller trees (i.e.,
nominally <1 return/tree).
Therefore, a bit of trial and error is warranted to determine the best scale
and curvature parameters to use. Select a las tile containing a good variety
of canopy and terrain conditions, as it's likely the parameters that work best
there will be applicable to the rest of your project area tiles. As a starting
point: if the scale (post spacing) of the lidar survey is 1.5 m, then try 1.5.
Try varying it up or down by 0.5 m increments to see if it produces a more desirable
digital terrain model (DTM) interpolated from the classified ground returns in
the output file. Use units that match the units of the lidar data. For example,
if your lidar data were delivered in units of feet with a post spacing of 3 ft,
set the scale parameter to 3, then try varying it up or down by 1 ft increments
and compare the resulting interpolated DTMs. If the trees are large, then
it's likely that a scale parameter of 1 m (3 ft) will produce a cleaner DTM
than a scale parameter of 0.3 m (1 ft), even if the pulse density is 0.3 m
(1 ft). As for the curvature threshold, a good starting value to try might be
0.3 (if data are in meters; 1 if data are in feet), and then try varying this
up or down by 0.1 m increments (if data are in meters; 0.3 if data are in feet).
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.
Other ground segmentation algorithms:
gnd_csf,
gnd_pmf
LASfile <- system.file("extdata", "Topography.laz", package="lidR")
las <- readLAS(LASfile, select = "xyzrn", filter = "-inside 273450 5274350 273550 5274450")
las <- classify_ground(las, mcc(1.5,0.3))
#plot(las, color = "Classification")
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