2. AREA OF FLOOD
Moravia includes mountains in its northern and
eastern areas(the southwestern part is hilly, and the
central part is a lowland). The summits of the
mountains exceed heights of 1000 to 1400 meters.
There are many deep narrow valleys with long steep
slopes which form large collection basins. Moravian
lowlands are wide and long valleys situated within
river systems.
Northwestern, central and southeastern
regions of Moravia drain into the Moravia River
which flows into the Danube and subsequently into
the Black Sea. The northeastern part of this area
Table 1. Applied RADARSAT images
drains into the Odra River and then into the Baltic
Sea. The northwestern region belongs geologically
to the Czech Massif whereas the southeastern region
is part of the Carpathians.
3. RELATED DATA
The area affected by the flood was about 25,000
km2. Satellite images were a source of flood
documentation captured in near-real time. Satellite
data brings an overview of the area at one moment
in time. Table 1 shows briefly the obtained radar
images. Their spatial prelaunch
Data type Date area image size orbit incidence angle
Standard 2 10 July 1997 North Moravia 100 x 100 km' ascending 24 - 31
Standard 7 14.July 1997 South Moravia 100 x 100 km' descending 45 - 49
Standard 5 24 July 1997 South Moravia 100 x 100 km' descending 36 - 42
Wide 1 27 July 1997 — South Moravia 150 x 150 km” descending 20 - 31
Wide 1 27 July 1997 North Moravia 150 x 150 km” descending 20 - 31
122
nominal resolution is 30 meters. This radar sensor
operates at a single microwave frequency, known as
C - band (5.3 GHz frequency and 5.6 cm
wavelength). RADARSAT transmits and receives
its microwave energy in a HH polarization. The
spectral resolution of processed images is 16 bits
per pixel. RADARSAT digital products can be
delivered as six different data types. Path Image
product (used in this case) is aligned parallel to the
satellite orbit path. The product was calibrated
which refers primarily to the electrical stability of
the radar sensor and its ability to provide repeatable
measurements over time. The system was designed
to achieve radiometric accuracy within one scene «
1.0 db, over three days < 2.0 db with global
dynamic range 30.0 db. Absolute radiometric
calibration was required so that the magnitude of
the digital data processed can be related to the radar
backscatter coefficients. To achieve this accuracy,
detailed measurements of the radar and processing
system performance were made on a regular basis
(RADARSAT Illuminated).
The study area was also imaged by aerial
photos. There were hundreds of these photos, and
they will serve for future detailed evaluation.
4, IMAGE PROCESSING, IMAGE
INTERPRETATION
Radar images can be viewed as single channel black
and white images with a characteristic "salt and
pepper" appearance. The pixel values represent the
strength of the returned radar signal from the earth's
surface. For each surface feature, there is a
statistical distribution of the probable strength of
that returned signal. Each pixel representing that
surface is assigned a value randomly selected from
the statistical distribution. Therefore, a seemingly
homogeneous surface area has an irregular
distribution of light and dark pixels, producing a
granular effect. This effect is termed "speckle" and
is an inherent property of radar images
(RADARSAT Illuminated). Original image data
were compressed from 16 bits to 8 bits data. Results
of data calibration control showed that the July 24
image values had a 6 - 9 per cent lower average
values in selected targets. The same control
performed for July 27 showed that average values in
the same targets were for 10 - 15 per cent higher,
both compared to the July 14. To modify speckles,
the images listed in Table 1 were smoothed by a 5 x
5 pixel spatial filter. All images were transformed
into a Czech cartographic projection.
Pixel classification techniques, an often
used method in image processing was performed for
surface water. To delineate flooded forest and areas
which were flooded between image pairs, a visual
interpretation was used. In the lowlands, automated
classification of present water surface could be
reliable for more than 90 per cent of the area
excluding urban and forest regions. Whenever these
two features occur, an important principle of radar
backscatter appears - a corner reflector. Two or
three dimensional corner reflection is caused by the
existence of buildings (two and three dimensional).
Scattering from a forest canopy can present a
complex case of volume scattering.- Double-bounce
scattering between tree trunks and the ground is one
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
