Geography Reference
In-Depth Information
stations, typically within a 10-km radius, to obtain
accurate aircraft geodetic positioning (see next section).
In bathymetric surveys, additional information may be
required.
As the typical flight height is 250-500 m over the
surface and flight lines must be straight, airplane pilots
need to know the topography around the river to avoid
risks when turning back between flight lines. This con-
sideration leads to problems in mountainous areas and
canyons, with rivers having high longitudinal slopes and
energy. Here, LiDAR may be advantageous because of the
difficulties encountered with sonar surveys. A helicopter
platform can be used to reduce these risks in canyons
(Millar 2008).
As bathymetric LiDAR systems often use equalisers
or signal amplification for photons backscattered by the
water bottom, a pre-calibration of this electronic hard-
ware may be required when water properties change
(Josset, 2009). This pre-calibration can be done by know-
ing before flying the dominant optical properties of the
waters and river bottoms (albedo, Secchi depths, etc.) or
with onboard calibration performed while flying for a
short period over an area with constant water depth. This
latter case, however, can rarely be accomplished for rivers
except by using a helicopter in a static position.
All bathymetric LiDAR systems are class IV lasers, i.e.,
eye-safe in accordance with the international standard
IEC 60825-1. However, as they can emit high-energy laser
pulses in the visible domain when flying at low altitudes,
it is important to inform the public that they should not
look up at the plane with lenses. Even with a reduced
background noise signal, nocturnal surveys are not eye-
safe because the human eye normally collects about 12
times more light at night than during the day.
The usual curvilinear shape of rivers makes bathymetric
LiDAR surveys less efficient economically than more clas-
sical terrestrial LiDAR surveys due to the numerous turns
that are necessary between flight lines (see Figure 7.4).
This factor can increase the flight time up to five times
and, consequently, the LiDAR use time. Once again, the
use of a helicopter platform can reduce this difficulty.
Timing errors from large angle beam propagation can
be minimised when scanner incident angles do not exceed
15 to 20 degrees, whereas smaller angles may over saturate
the laser footprint and distort return waveforms. Semi-
circular, rectangular or elliptical swath patterns scanned
in-line or cross-track from the aircraft render uniform
pulse spacing. It is good practice to overlap flight coverage
and swath patterns to maximise data-point collection and
lessen the potential for data gaps in the survey. Repeated
verification of scanner, beam angle, and time calculations
during the survey assure optimal data accuracy and survey
integrity (Guenther, 2007).
Another important system design consideration for
riverine environments is the footprint size related to laser
beam divergence angle. Large footprint (LFP) systems
prevent the proper representation of high-slope areas on
the river bed, returning the elevation of the shallowest
Flightlines are
100 m wide incl overlap
Survey flightlines
4 km
1 minute
Turning flightlines
10 km
3,5 minute
Long and narrow
flightlines
Figure 7.4
Aircraft flight lines on a simple
shape example survey (light grey boxes in the
middle) with the HawkEye LiDAR system;
Courtesy of Airborne Hydrography AB and
Blom Areofilms.
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