they are easily absorbed by the eye and hence the maximum power of the system is
limited by the need to make them eye-safe.
Terrestrial topographic LiDAR systems typically utilize near-infrared (NIR)
lasers with a wavelength of 1,064 nm, generated using the same Nd:YAG lasers as
previously discussed, but not frequency-doubled into the visible spectrum. As with
the 532 nm blue-green laser, this NIR wavelength is focused by the eye and
similarly the power must be limited for safety. Although less accurate, military
instruments often utilize lasers with infrared wavelengths as long as 1,550 nm.
This holds the dual advantage of being eye-safe at much higher power levels and
the beam is not visible using night-vision goggles.
Both terrestrial and bathymetric lasers suffer null or poor returns from dark
substrates. For the former, typical culprits are surfaces such as water, asphalt and
tar, while for bathymetric lasers, dark patches of dense seagrass and algal meadows may be problematic. Fog and clouds also absorb both NIR and visible
wavelengths. Clouds tend to be more problematic for terrestrial surveys as they are
flown at an altitude of 2,000 m, higher than that adopted for marine work.
Given that they do not penetrate water, NIR topographic lasers cannot be used
to assess bathymetry, though they can be used to detect the water surface in
bathymetric applications. By contrast, blue-green hydrographic lasers do reflect off
of terrestrial targets and can be used to measure emergent terrain elevations. By
employing temporal waveform digitizing, hybrid systems such as the Experimental
Advanced Airborne Research LiDAR (EAARL) have demonstrated the capability
of measuring both topography and bathymetry from sounding by a single bluegreen laser (Bonisteel et al. 2009; McKean et al. 2009; Nayegandhi et al. 2009;
Wright and Brock 2002). Figure 5.3 shows such a dataset. This experimental
instrument may signal a future move towards commercial implementation of
multiple-application, single blue-green or NIR wavelength instruments. The
SHOALS and LADS systems have this capability too, but to a lesser accuracy
because of footprint size. The single laser technique is also implemented in
HawkEyeII and the Coastal Zone Mapping and Imaging LiDAR (CZMIL), with
the addition of a segmented detector approach to increase the density of the
measurements. The CZMIL is a new sensor development effort within the National
Coastal Mapping Program which is based around the CHARTS (Compact
Hydrographic Airborne Rapid Total Survey) sensor suite, the main components of
which are a SHOALS-3000 topo-hydro LiDAR and a CASI 1500 hyperspectral
imager.
Dual-wavelength LiDAR provides both bathymetric and topographic LiDAR
mapping capability by carrying both a NIR and a blue-green laser. Access to a
harmonised and consistent elevation model containing both bathymetry and
topography is valuable in reef research because the architecture of terrestrial
watersheds imparts considerable control over the health of corals (Rogers 1990;
Lapointe and Clark 1992), an idea that will be further investigated later in this
chapter. The NIR laser of a dual-wavelength LiDAR is not redundant over water,
because it reflects from the water surface and can be used to acquire the range to
the air–water interface, as well as to distinguish dry land from water using the
5 LiDAR Overview
121
limited by the need to make them eye-safe.
Terrestrial topographic LiDAR systems typically utilize near-infrared (NIR)
lasers with a wavelength of 1,064 nm, generated using the same Nd:YAG lasers as
previously discussed, but not frequency-doubled into the visible spectrum. As with
the 532 nm blue-green laser, this NIR wavelength is focused by the eye and
similarly the power must be limited for safety. Although less accurate, military
instruments often utilize lasers with infrared wavelengths as long as 1,550 nm.
This holds the dual advantage of being eye-safe at much higher power levels and
the beam is not visible using night-vision goggles.
Both terrestrial and bathymetric lasers suffer null or poor returns from dark
substrates. For the former, typical culprits are surfaces such as water, asphalt and
tar, while for bathymetric lasers, dark patches of dense seagrass and algal meadows may be problematic. Fog and clouds also absorb both NIR and visible
wavelengths. Clouds tend to be more problematic for terrestrial surveys as they are
flown at an altitude of 2,000 m, higher than that adopted for marine work.
Given that they do not penetrate water, NIR topographic lasers cannot be used
to assess bathymetry, though they can be used to detect the water surface in
bathymetric applications. By contrast, blue-green hydrographic lasers do reflect off
of terrestrial targets and can be used to measure emergent terrain elevations. By
employing temporal waveform digitizing, hybrid systems such as the Experimental
Advanced Airborne Research LiDAR (EAARL) have demonstrated the capability
of measuring both topography and bathymetry from sounding by a single bluegreen laser (Bonisteel et al. 2009; McKean et al. 2009; Nayegandhi et al. 2009;
Wright and Brock 2002). Figure 5.3 shows such a dataset. This experimental
instrument may signal a future move towards commercial implementation of
multiple-application, single blue-green or NIR wavelength instruments. The
SHOALS and LADS systems have this capability too, but to a lesser accuracy
because of footprint size. The single laser technique is also implemented in
HawkEyeII and the Coastal Zone Mapping and Imaging LiDAR (CZMIL), with
the addition of a segmented detector approach to increase the density of the
measurements. The CZMIL is a new sensor development effort within the National
Coastal Mapping Program which is based around the CHARTS (Compact
Hydrographic Airborne Rapid Total Survey) sensor suite, the main components of
which are a SHOALS-3000 topo-hydro LiDAR and a CASI 1500 hyperspectral
imager.
Dual-wavelength LiDAR provides both bathymetric and topographic LiDAR
mapping capability by carrying both a NIR and a blue-green laser. Access to a
harmonised and consistent elevation model containing both bathymetry and
topography is valuable in reef research because the architecture of terrestrial
watersheds imparts considerable control over the health of corals (Rogers 1990;
Lapointe and Clark 1992), an idea that will be further investigated later in this
chapter. The NIR laser of a dual-wavelength LiDAR is not redundant over water,
because it reflects from the water surface and can be used to acquire the range to
the air–water interface, as well as to distinguish dry land from water using the
5 LiDAR Overview
121