International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
Figure 8. Laser altimetry data of an extreme low-water level in a
Dutch river. The dry groyne fields are clearly visible in this
hillshade DEM.
between both dykes is now at our disposal to compute the
volumes of the high-water level reservoirs.
Up to now, the floodplains have been measured by aerial
photogrammetry. Whether laser altimetry data is useful as input
for the hydraulic model still needs to be researched, but seems
to give a positive contribution given this extreme low-water
level data. Most important for the hydraulic computations are
the line elements in the floodplain, such as dykes, terrain height
steps, drainage ditches and hedges. The quality of these items
derived from the laser data has also to be further investigated.
6. WATER LEVELS
A new application of the laser altimetry technique is to measure
the water level. Some laser systems do receive signals from
water surfaces, in contrast with the FLI-MAP system which
does not receive any signal from the water surface, see figure 4.
The laser strip width decreases compared to land mainly due to
the reflectivity characteristics of water. These water level
heights, measured during a high water situation in the river
including the flooded floodplains, can be used to verify the
water level forecasting model for the Dutch main rivers. The
high point density, continuous laser data and the flexibility of
the system gives advantages above the single point data from
the water gauges, located every 5-10 kilometres downstream.
In January 2003 a high water wave was expected in the Dutch
main rivers (Rhine, Waal and [Jssel). With an airborne platform
laser data of at least | point per 4 m* was obtained. The quality
was checked on two ways. A control of height differences
between overlapping strips and between laser data and
terrestrial measured ground control fields on dykes was
performed. For the latter one the mean difference was between
-5.1 em and 3.6 cm, and the standard deviation ranged from 2.9
to 6.2 cm for reference areas of approximately 5 x 5 m.
From the laser data of the water surface a contour map was
derived showing the details in acceleration and congestion of
the water, see fig. 9. The red and blue lines are every 5 cm, the
green lines every cm. The water level gradient going down-
stream and the cross track gradient in river bends can be
computed from the laser data. An expected downstream
gradient in this river is approximately 10 cm per km, and for the
cross track gradient 2 cm per 100 m.
Another application is to define the reference surface for
dredging from the laser data. The water level gradient measured
during a specific discharge along the river axis is the zero level
for dredging. Dredging is necessary to keep transport by
shipping optimal. The gradient line attained from laser data is a
more realistic representation than using the single point water
gauge measurements.
Figure 9. A contour map obtained from the low water level laser
data, with an aerial photograph as background.
Finally, river experts denoted that in shallow water areas the
water surface contains information about the structure of the
underlying morphology. This phenomena can be recognized in
case of extreme low-water in the riverbed and in case of high-
water in the floodplains. Especially constructions in the
riverbed and different foreland characteristics result in changes
of the water level. Even the decrease of the water surface
around a ship (squat) is visible in the laser data.
7. WAVE AMPLITUDES
Another application of laser altimetry is the determination of
wave amplitude and wavelength. The Transport Research
Centre of the Ministry of Public Works and Water Management
is concerned with shipping. Fast ferries are restricted to produce
little wash, but were still causing problems in some areas such
as erosion to river shorelines, disturbing waves in recreation
areas and undesirable motions of moored ships. To understand
the wash of fast ferries a continuous spatial wash measurement,
obtained by airborne laser altimetry, will improve this, allowing
a better prediction in wash models, and checking on wash-
limits. Most water level measurements are single point time
series, not giving any information about wave direction and
ncighbouring water level heights.
A pilot was carried out to measure the wash of the Huizen-
Almere fast ferry, moving with constant high speed over
shallow and deep waters. The Toposys II laser scanner of
TopoSys GMBH was used by Aerodata International Surveys in
an airplanc. A challenge in this pilot was tuning the exact time
of measurement by the airplane of the fast ferry moving at
constant speed (see fig. 10 for the course of both platforms).
Communication between cockpit and bridge was essential.
However, the standard communication frequency canals are
different for both platforms. Therefore, communication with
VHF was used.
laser strip surface airplane
ferry
Figure 10. Course track and corresponding positioning numbers
for both platforms.
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