between measured and modelled values at the gauging stations
and the interpolation of these values in the 500 m grid system were
added to the Wadden Model. This is the first step in correcting
the Wadden model (table 3).
Gauging Stations | Measured | Modelled | Correction
Vlieland -79 -35 -44
W. Terschelling -88 -43 -45
Harlingen -100 -40 -60
Kornwerderzand -108 -53 -55
Oude Schild -29 +5 -34
Den Oever -63 -32 -31
Table 3. The example of correction of the Wadden Model
based on gauging records (16-05-1993, in cm)
A second correction was applied further to improve the model
with respect to bottom drag which causes a water surface
curvature over the flat areas. The knowledge about the water
surface curvature was obtained through a measurement
campaign carried out during 7-21 April 1994. During this
campaign, 14 pressure loggers were placed along a transect of
7 km in 500 m spacing over one of the large flats in the study
area. The water levels were recorded during a 14-day full
spring tidal cycle with time interval of 1 minute. This
experiment gave us an insight into the curvature of the water
surface at all stages of the tidal cycle. In general, the bottom
drag over the flat causes a water surface gradient of 1:10.000
during incoming tide. During outgoing tide the slope of the
water surface declines gently towards the channel in an order
of 12 cm over 3000 m, which is not as obvious as in incoming
tide. The actual correction of this surface curvature is further
adjusted according to the time in the tidal cycle and the
distance towards the low tide line. A full description is given
in the special ERSWAD report on this experiment (Wang et al.
1995).
The corrections have been applied in zones of 500 m from the
channel edges (low tide line: -110 cm) towards the central part
of the flats. The DEM of the corrected water surface model
was then densified to a 12.5 m grid size equal to the SAR pixel
size.
130000
600000
159000 160000
Elevation
(em)
0
575000 — +
Figure 4. Water Line DEM of the Inter-tidal Area Using
Water Line Procedure with Multiple ERS-1
SAR Images (1993.3-1993.10)
766
2.3 Evaluation of the accuracy of the water line method.
The delineated water lines from the SAR image was combined
with the corrected water surface model at the time of image
acquisition in order to extract the height value of the pixels
along the water lines.
As an example, the evaluation of this mapping method was
done for the SAR image acquired on 09-08-1992. The height
values of the water lines delineated after the Global
Classification Filtering were compared with the bathymetric
DEM. This DEM was constructed by interpolation of ship-
borne echo sounding data surveyed in the Vakloding Program
conducted in 1992-1993 by Rijkswaterstaat. The TIN method
(Triangulated Irregular Network) in Arc/Info software was
applied followed by the bi-linear interpolation of the
intermediate points between two survey lines which can
improve the interpolation. The comparison between water line
height and the bathymetric DEM shows an average of -0.38 cm
of differences. Seventy-one percent of total pixels along the water
line are within + 30 cm (acceptable errors) which is very good.
3. RESULTS
The merging of the 9 water line data derived from the SAR
images from 1993 with the Corrected Wadden Models (CWM)
at the time of each image acquisition gave the height values for
multiple water lines from different images.
On the flats, a higher density of height points are found at
places where the topographic variation are more apparent. This
distribution of height points can be trusted more and will be
more useful than a ground survey with a large line spacing, in
which the minor depressions and gullies may be easily missed.
Of course no values are available in the channels lower than
the low tide line.
The interpolation was run on the basis of TIN module in
Arc/Info, the same procedure as that used for the bathymetric
DEM creation. The resulting Water Line Digital Elevation
Model (WALDEM) is shown in figure 4.
130000 140000 150000 160000
800000 i
Terschelling etevaf
- evation
{em}
Li
BEREaRaRt as on: pus ui os suaR BRR BREST
ooh
581250 —
878000
|
|
|
|
|
|
|
Figure 5. Bathymetric DEM of the western Dutch Wadden
Sea using sounding data by Rijkswaterstaat
from 1992-1993.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
