IEEE Geoscience and Remote Sensing Magazine - September 2023 - 26

geometry [38]; 2) laser pulses experience Fresnel reflectance,
which can be as low as 2% [39]; and 3) atmospheric
turbulence and high near-shore wind speeds further
attenuate the laser energy, leading to the failure of the
laser signal to reach a threshold to be processed [39].
With the combination of these effects, null returns may
be found at swath edges and occasionally appear in the
close-to-nadir region (see Figure 5 for an illustration).
Apart from these,
the backscattered laser pulses may
have a high variance of signal strength in these water regions,
resulting in some form of speckle noise in the intensity
images (discussed in the " Speckle Intensity Noise
on Water Surfaces " section).
The presence of data voids in water regions causes unnatural
triangular facets when generating DEMs. Indeed,
the USGS has emphasized replacing these defects in the
resulting DEM with a flat virtual water surface [13]. This
process is called hydroflattening. Existing solutions mainly
rely on either manual digitization or the incorporation of
existing break lines along coastal regions and river banks
[40], [41], depth sounding data [34], and instantaneous
water surface measurements via boat surveys [42] to locate
water regions, followed by hydroflattening.
The shortcomings of relying on manual intervention or
a semiautomatic process have been addressed lately. Water
data points can be first extracted based on an airborne lidar
ratio index, the scan line intensity-elevation ratio (SLIER)
[36]. The SLIER reaps the benefits of how a laser interacts
with the water body. Along each scan line s, the water surface
usually has a relatively lower variance in terms of elevation
z but a higher degree of fluctuation in terms of intensity
I for a given lidar dataset L. With
6sL ,! the SLIER can be
defined as
SLIER
v
= v
z
I
.
(2)
To further take advantage of laser dropouts found at
large incident angles, (2) can be further revised as
cos
SLIER= v ##v
z
I
i
where i refers to the scan angle, Ns
Ns
ns
number of points found among the scan lines in L, and ns
(3)
equals the maximum
refers
to the number of data points found in the current scan line
s. If the swath of s completely covers a water region, the laser
dropouts cause fewer data points, leading to a small ns
value.
This can further boost the SLIER, causing higher values to be
computed on the water surface and lower SLIER values associated
to the land. Sample water surface data points can be located
with higher SLIER values (e.g., the top 10% within L). Then,
a virtual water surface can be defined based on the elevation
of these sample water data points [36]. These data points can
further serve as training datasets for classifiers to distinguish
between land and water regions [43]. Figure 6 provides an example
of the SLIER and the result of water mapping.
With the SLIER aiding in water identification, those data
voids caused by laser dropouts can be located and compensated.
Recently, [44] proposed two scan line void-filling algorithms
to add artificial data points in the water gaps in the
close-to-nadir region and swath edges. The algorithm handling
the close-to-nadir region aims to add these artificial
data points if the distance between two consecutive points
along the scan line is larger than the mean point spacing (or
ANPS) via interpolation. The second algorithm first estimates
the maximum swath width w within L and then adds
artificial data points to the swath until the length of s reaches
w. Assuming that the elevation of these artificial data points
equals the mean value of the water data points, the entire L
and these artificial data points are combined to generate a
DEM. The resulting process leads to a hydroflattened water
surface, and those unpleasant triangular facets disappear in
the resulting DEM accordingly (see Figure 7 for an example).
1,550
1,064 532 1,550
(nm)
Elevation (m)
138
(a)
146
(b)
1,064
(nm)
Intensity
4096 Low
(c)
532 1,550
1,064 532
(nm)
SLIER
High
1,550
1,064 532
(nm)
Land-Water Classification
Water Land
(d)
FIGURE 6. (a) Laser dropouts found in a water region, particularly at the swath edges. (b) A high variance of the intensity values on the
water surface. (c) High SLIER values computed on the water region, particularly on laser channels of 1,064 and 1,550 nm. (d) The use of the
SLIER for land-water classification [36].
26
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE SEPTEMBER 2023
IEEE Geoscience and Remote Sensing Magazine - September 2023

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