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Title
Technical Commission VII

International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
Discrete time system

Processed signal
y(n)
Observed signal
x(n) ———4» hn)



ha) = ast 110,9,16,2124,25242116,9,0,-11]
Figure 3. Block diagram of FIR filter
3. ACCURACY EVALUATION OF LIDAR DERIVED
TERRAIN DATA
We performed a test to evaluate and to compare the vertical
accuracy of LiDAR derived terrain data constructed from the
discrete LiDAR pulses and the proposed full waveform
exploitation technique obtained during autumn and winter. For
the accuracy evaluation, a typical hilly area with varying
topography covered by Japanese cedar (Sugi) trees, Japanese
cypress (Hinoki) trees and other deciduous trees was selected.
Ground truth data were measured by performing total station
survey.
Figure 4 shows the terrain model of the site and total station
surveyed check points. The check points were collected from
the stripe of about 400m x 10 m area at the density of about 1pt/
4.5 sqm. The area was covered with both evergreen and
deciduous tree varieties and the topography of the area varied
from flat plane to rolling hill. The maximum slope of the test
site was about 30 %.
To evaluate the accuracy of LiDAR derived ground elevation,
DTM of the ground terrain with 1 m grid size was constructed
from the discrete LIDAR pulses obtained during September, full
waveform LiDAR pulses obtained during September, discrete
LiDAR pulses obtained during December and full waveform
LiDAR pulses obtained during December respectively. The
elevation of the ground on each DTM that corresponds to the
horizontal location of the total station surveyed check points
were then calculated directly by using ArcGis’s Surface Spot
tool.
The difference between measured point elevation data and
LiDAR derived DTM elevations for the four cases are presented
in Table 1. The RMSE between measured data and DTM data
obtained by using discrete LiDAR pulses was 0.73 m during
September, when deciduous trees were full of leaves. By using
the full waveform exploitation technique, the DTM thus
constructed showed an increase in the terrain data accuracy.
The RMSE for the terrain data obtained by using full waveform
LiDAR pulses was 0.59 m.
During December, when the leaves fall off from the deciduous
trees, RMSE between measured data and DTM constructed by
using discrete LiDAR pulses was 0.22 m. For the same winter
data set, the RMSE of the terrain data obtained by using full
waveform LiDAR pulses was 0.21 m.
507

Figure 4. TIN of terrain model and ground truth surveyed
points (blue dots)






A NSSDA Accuracy; (m)
DTM model UR (NSSDA Accuracy —
*
(RMSE, m) RN)
September 0.73 143
discrete
September
«m 0.59 TIS
waveform
December 0.22 0.43
discrete
December
EIE 0.21 0.41
waveform




Table 1. Accuracy evaluation of LiDAR derived terrain data
4. RESULTS AND DISCUSSION
Point cloud generated from the discrete return pulses and full
waveform return pulses along, a part of, the longitudinal section
of the concerned area are compared and demonstrated in Figure
5 for autumn and winter. The white dots in Figure 5 represent
discrete return pulses and yellow dots represent the full
waveform return pulses. For both seasons, we can see that the
point cloud generated from the full waveform return pulses
increased the vegetation/canopy detection and moreover
increased the ground penetration significantly.
To quantify the increment in the ground penetration, the point
cloud generated from the full waveform return pulses and
discrete return pulses were filtered automatically by using
commercial software, TerraScan, to separate the ground points
and vegetation points. The ground points obtained from the full
waveform point cloud and the discrete return pulses are
compared for autumn and winter respectively and presented
below.
During autumn, the ground penetration by discrete LIDAR was
relatively poor with large gaps or missing ground points in
some of the topographically complex terrains. However, by
using the full waveform exploitation technique, ground point
detection was considerably increased throughout the region.