308 
Figure 1. The test sites in the Scheldt estuary. 
To develop the best suited policy and to check 
results amongst others regular spatial information 
on sediment grain size and vegetation status are 
needed. Airborne and maybe even space borne 
imaging spectroscopy could be a useful tool for 
perceiving this information. 
In view of the fact that the test sites for both 
sediment and vegetation study were situated in the 
easterly part of the Westerscheldt, as indicated in 
fig. 1. (approximate location 4°,07 E, 52°,23 N) 
the airborne experiments with casi over both sites 
were combined in one flight. 
The studies reported here focus on spectral 
discrimination of different sediment and vege 
tation types. 
The selected flat for the sediment study. Plaat 
van Walsoorden, is the highest one in the area 
with maximum (low tide) dimensions of 4x1.5 
km. It is mainly flat and exhibits parts that are 
predominantly sandy and others with a relatively 
large clay and mud content. 
The studied salt marsh is called the ’Verdron- 
ken Land van Saeftinghe’ (Drowned Land of 
Saeftinghe, Saeftinghe being a former village). 
With its 3000 hectares it represents about 80% of 
the remaining salt marsh area in the south-west of 
the Netherlands (Leemans et al. 1980). The domi 
nant species of Saeftinghe are Phragmites austra 
lis, Elymus pycnanthus, Scirpus maritimus, Sparti- 
na anglica, Aster tripolium and Puccinellia mari 
tima. 
2 - MATERIALS 
2.1 PSII fieldspectrometer 
The PSII spectrometer is a plasma-coupled photodiode array with 512 elements and a contiguous nominal 
spectral range from 350 to 1050 nm with 1.4 nm dispersion and 4nm spectral resolution. Data acquisition, 
real time display and analysis are combined in a single portable unit. The total weight of spectrometer and 
computer is 7 pounds. Photons are captured with a 3 meter long fiber optic cable with an acceptance angle 
of appr. 20° (10 and 5°). The instrument, ideal for field work, is capable to detect rather small absorption 
features within its spectral range. The PSII can also be used for simulation of present and future sensors, for 
modelling and calibration as well as for feasibility studies. By virtue of the capability to sum a number of 
adjacent channels one can simulate bands of existing or future sensors. Following this strategy e.g. best 
suited bands of a remote sensor for a certain goal can be defined and selected. 
2.2 Imaging spectroscopy wk’ casi 
The Compact Airborne Spectrographic Imager (casi, see Babey and Anger, 1993) is a pushbroom imager 
with 288 spectral channels with 1.8 nm dispersion and bandwiths of 2.5 nm. The particular instrument we 
used has a spectral range from 380-891 nm. In the casi a 578x288 pixel CCD is used, resulting in an 
availability of up to 578 spatial bands. The casi offers the possibility to sum bands of adjacent channels and 
is thus capable to detect specific spectral features of the viewed target. The IFOV of casi is 35.4° (swath) x 
0.069° (along track). 
23 Landsat TM 
Landsat 5 Thematic Mapper (track 198/199, scene 24) were used in both experiments.