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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
Distance: 5 ns Distance: 6 ns
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percentage of successful detection (%)
percentage of successful detection (76)
2 3 2 3 4
Ratio (A2 / A1) Ratio (A2 / A1)
. 2 echo ;
Echo with: 5 ns À Se Echo with: 5 ns
Distance: 5 ns |Bilimore than 2 echo Distance: 6 ns
(a) Zero-crossing (b)

Distance: 5 ns Distance: 6 ns

1 2 3 4 5
1 2 3 4
Ratio (A2 / A1) Ralio (A2 / A1)
à M2 echo i
Echo with: 5 ns |Ezi1 echo Echo with: 5 ns
Distance: Sns Mimorethan2echo) | Distance: 6 ns
(c) Wavelet-based (d)
percentage of successful detection (%)
percentage of successful detection (%)

Figure 8. Results of overlapped echo detection under variant
relative ratio cases. CR2: blue, MR2: green, RR2: red, (a)(b) (b)
verasetessitig (Nd) wavelet-tased Figure 9. (a) the study area, (b) the point clouds colored by
35 Real waveiprm data test elevation in the rectangle area of (a).

Figure 9 shows the study area where tall and shrubs plants grow
irregularly. The real waveform data is captured by Leica ALS60 Waveforms = Wavelet-based
system. We chose the waveform data in the rectangle area detector
shown in Figure 9 (a). The objective of this test is to see if the
wavelet-based detector can effectively detect the weak and

overlapped echoes so that the number of points increases
obviously. Export point Remove echoes
xport points where
3.5.1 Set up: The results of simulated experiments can be SNR, 4, « 16 dB
used as reference to set up the strategy of detecting echoes in : s
real waveform data. From section 3.3.2 the wavelet-based Figure 10. Flow chart ofthe wavelet-based detection method
detector can confidently detect the echoes if the SNR value of
an echo is greater than 16. Therefore a threshold is set as a
condition to remove the uncertain echoes which may be
incorrectly detected due to the noises. To obtain the SNR value
of each echo, the noises in the waveform needs to be estimated.
Fortunately that there only exists noises in some interval of each
waveform, the noises can be therefore easily estimated using
these waveform sections without target echo information.
Figure 10 shows the procedure of the wavelet-based echo
detection method applied to a real waveform dataset.
3.5.2 Results:
The total number of waveforms is 459708 and the number of
points provided by the instrument is 499997. Based on the
wavelet-based detector, the number of detected points is 590074.
It means that the detector increasingly found 1896 more of the
original number of echoes. This result shows that the ability of
the proposed detector in finding weak and overlapped returns
from waveforms. Figure 11 shows a side view of a selected
profile drawn by the system points and the points extracted by (b)
our method respectively. One can see that the ground and the
trees can be better described by the point clouds extracted by
our methods.

Figure 11. The side view of a selected profile. (a) points
provided from the instrument, (b) points extracted from
waveforms using the wavelet-based detector. The arrows show
some of the extra-points detected by our method.