IEEE Geoscience and Remote Sensing Magazine - September 2023 - 27

Edge
Nadir
(d)
378
378
(a)
(b)
(c)
(e)
FIGURE 7. (a) Laser dropouts cause data voids along an inland river. (b) The implementation of a scan line void-filling algorithm implemented
at the close-to-nadir region and swath edges. (c) The resulting hydroflattened DEM. (d) The corresponding 3D view of the original DEM
with unpleasant triangular facets along the river. (e) The hydroflattened DEM [44].
SWATH GAPS
Another explanation of void appearance can be attributed
to the gaps found between swaths. In common practice, a
15%-50% overlap between adjacent data strips is a rule of
thumb during mission planning [13], [14], [45]. Combining
a number of partially overlapping data strips can lead
to a seamless mapping of the study region. Putting aside
the possibility of poor flight planning, adjacent flight lines
may have gaps due to various internal and external factors.
Instantaneous flight conditions, such as air turbulence or a
change of heading direction, likely cause undesired movements
of the aircraft, resulting in a crooked flight trajectory.
It is a common problem not only in an airborne lidar
system but also in a UAS platform. As demonstrated in Figure
8, a portion of the airborne lidar data strip is twisted
with a curvy swath due to possible turbulence. A notable
void is thus found between the swaths, a scenario considered
an unintentional cause of data voids. On the other
hand, a change of system settings or system error/bias may
also lead to the presence of swath gaps. This situation can
be further broken down into the following two scenarios.
First, a significant change of terrain height, particularly
on rugged mountainous terrain, may alert the swath of the
airborne lidar if the flying altitude remains unchanged.
Combining two adjacent strips under this circumstance
may lead to the appearance of a swath gap. Although
topographic airborne lidar systems operated with oscillating
mirrors are capable of maintaining a consistent swath
width via adjusting the degree of angular rotation, this may
SEPTEMBER 2023 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
not be the case for systems equipped with rotating prisms.
The change of swath width also happens if the flight altitude
changes during the flight mission. This may also lead
to the formation of a gap between two swaths. Second, data
gaps may be found between two data strips, which can be
ascribed to the accuracy of the GNSS and inertial measurement
unit (IMU). Nevertheless, data gaps may be particularly
obvious along the vertical direction in the overlapping
region. Such a case also would lead to stripe artifacts in the
resulting DEM. Strip adjustment should therefore be applied
to remove system errors and biases so as to improve
the geometric accuracy. Details can be found in the " Stripe
Artifacts in Digital Elevation Models " section.
0.2 km
243
243
0.2 km
FIGURE 8. A swath gap with an annulus-shaped sector due to a
crooked flight trajectory.
27
Height (m)
Height (m)

IEEE Geoscience and Remote Sensing Magazine - September 2023

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