Geoscience Reference
In-Depth Information
selective filtering is challenging and has been the subject of many research studies.
Most proposed algorithms, including the progressive morphological filtering, must
be fine tuned with a set of parameters to optimize this selection. A selection of
algorithms are implemented in
pkfilterdem
of pktools and in SPDLib (see also
15.4.2 Canopy Models
In addition to a DTM and DSM, other products can be derived from LiDAR data. For
example, by subtracting a DSM from a DTM in a forested area, tree heights can be
obtained. The resulting product is also referred to as a canopy height model (CHM).
Provided the density of the point cloud is sufficiently high (e.g. ten or more points
per square meter), percentile height values (PHV) can provide a profile of the tree
structure. The profile is based on the heights above which a given percentage of pulse
returns is reached. Such a profile can be obtained with
pklas2img
by selecting
the option
-comp profile
and specifying the number of percentile bins with
the option
-nbin
(the default number is 10). Examples for a CHM and PHV are
provided in the case study in Chap.
18
using SPDLib.
15.4.3 Point Density
In Sect.
15.3.3
we have seen how LAStools can provide information on the average
point cloud density. For instance, the average pulse density for the point cloud over
a forest area has been calculated as 16
25 m. We now create
a spatially explicit map of the pulse density for that same point cloud.
For this example, we use the utility
pklas2img
from pktools. The composite
option is set as
-comp number
to calculate the number of points that are covered
for each grid cell. The result is shown in Fig.
15.4
.
.
36, with a spacing of 0
.
pklas2img -i forest_31370.laz -comp number -fir last -dx 1 -dy 1
→
-ot UInt16 -o forest_pulse_density.tif
-i forest_31370.laz
Name of the input LAZ point cloud.
-comp number
Report the number of overlapping points per grid cell.
-fir last
Filter points: select last returns only to obtain pulse density. To obtain a return
density, replace last with all.
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