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Chiroptera and HawkEyell sensors of Airborne Hydrography AB. The HawkEyell sensor can emit more powerful
pulses with the drawback of a relatively low pulse rate of 4 kHz compared to the Chiroptera sensor. Also, data from the
HawkEye sensor were only available for areas which are too deep for obtaining measurements with the Chiroptera sensor.
To enable a fair comparison between all sensors we therefore excluded the HawkEye data from our study. The third data
set was gathered by Aerodata Surveys, the Netherlands, in mid-May 2014 with the Chiroptera sensor. For both
Chiroptera flights the height was 400 m above ground. 23 strips with an overlap 30% and three cross-strips in the second
campaign as well as 20 strips, again with an overlap 30%, and one cross-strip in the third campaign were obtained. Using
these two flight campaigns a comparison with respect to water turbidity was possible. The Chiroptera sensor is composed
of two lasers, an infrared laser (X = 1064 nm, 200 kHz pulse rate) and a green laser (X = 532 nm, 35 kHz pulse rate). The
green laser of the Riegl VQ-820-G sensor and the Chiroptera sensor are claimed to penetrate water to one and 1.5 Secchi
depths, respectively, under good conditions [16, 17]. Secchi depths were measured at several locations by vessels
simultaneously to the laser data acquisitions. Table 1 shows the parameters of the three flights and the mean Secchi
depths which were observed during each campaign.
Table 1. Parameters of the three flight campaigns.
First Campaign
Second Campaign
Third Campaign
Date
31 Oct. - 14 Nov. 2012
28-29 Sept. 2013
5-12 May 2014
Sensor
Riegl VQ-820-G
AHAB Chiroptera
AHAB Chiroptera
Flight height
500 m
400 m
400 m
Measurement rate
- (X = 1064 nm)
149 kHz (X = 532 nm)
200 kHz (X = 1064 nm)
35 kHz (X = 532 nm)
200 kHz (X = 1064 nm)
35 kHz (X = 532 nm)
Overlap
70%
30%
30%
Reachable depth 1
1 x Secchi depth
1.5 x Secchi depth
1.5 x Secchi depth
Mean Secchi depth
6.8 m
8.0 m
5.8 m
The three data sets were delivered in LAS-format that included the 3D point coordinates along with an intensity value. In
order to compensate for the different speed of light in water and air, and to take into account the change of direction of
the light at the water surface, the coordinates of the seabed points were appropriately corrected by the data providers.
They additionally classified the points into the six classes water surface, seabed, underwater vegetation, underwater
object, onshore, and noise. Some statistics of the classified points for the three data sets are summarized in Table 2. The
number of points classified as water surface in the first campaign is clearly smaller than in the other campaigns, in spite
of the higher strip overlap. Only few pulses result in two or more echoes, thus mainly single echoes are observed. This
effect is also described in [18] for a river site using the same sensor. In addition, due to the higher flight altitude the total
amount of the points is smaller compared to the two flights with the Chiroptera sensor, which result in similar numbers
of points per class. In general the amount of points classified as underwater object is negligible, although a few rocks are
known to be located in this area. As one result of the project it can be stated that the detection of underwater objects
solely in ALB data is still challenging in our test sites.
Table 2. Statistics of the classified points.
First Campaign
Second Campaign
Third Campaign
Class) Sensor
Riegl VQ-820-G
AHAB Chiroptera
AHAB Chiroptera
Water surface
14,637,978
(15.4 %)
118,412,469
(42.0 %)
94,489,927
(33.2 %)
Seabed
57,807,774
(60.9 %)
98,839,172
(35.1 %)
91,636,326
(32.2 %)
Underwater vegetation
41,237
(0.00 %)
15,483,702
(5.50 %)
14,377,054
(5.05%)
Underwater object
0
(0.00 %)
180
(0.00 %)
0
(0.00 %)
Onshore
22,138,957
(23.3 %)
49,037,845
(17.4 %)
84,271,762
(29.6 %)
Sum
94,848,546
281,773,368
284,785,069
1 as specified by the manufacturer
Proc. of SPIE Vol. 9638 96380Z-3
