Geoinformation for Disaster and Risk Management - ISPRS

The aforementioned data belongs to two main
families: public-domain data (generally accessible
through the web (i.e.: MODIS, Landsat); or
commercial imagery that should be purchased
through reseller companies (i.e.: Eurimage,
Spotimage, DigitalGlobe). Sometimes it is possible to
freely access satellite data on the basis of scientific
agreements between Space Agencies and Principal
Investigators in charge of a specific research (i.e.:
ESA Category-1 users).
Rapid mapping activities can benefit from the
possible activation of the “International Charter for
Space and Major Disaster” that, since November
2000, aims at providing a unified system of space
data acquisition and delivery to those affected by
natural or man-made disasters through Authorized
Users.
Extraction of water bodies
As widely reported, radar images enable the easy
identification of water bodies; therefore they are the
main input data for flood analyses (Aduah et al.
2007, Henry et al. 2003, Schumann et al. 2007). The
all-weather capability of the radar technology and
the possibility to acquire data also during night time,
are crucial advantages of a radar based approach. On
the other hand, they are affected by geometric distortions
(layover, foreshortening and radar
shadows), which are hard to model, especially in
mountain regions. Radar images acquired by satellite
platforms before and after the event are commonly
used for the definition of flooded areas. The collection
of an archive radar image for identification of
the water levels before the event can be skipped if
reliable and updated water bodies data are available
as defined in the so-called NWED (Normal Water
Extent Database) (Wang et al. 2002). Water areas can
be identified on both before and after images
because of the near specular reflection of electromagnetic
radiation emitted by the radar sensors, by
water bodies which are nearly smooth surfaces when
compared with the wavelength of the radar emission
(similar to mirror reflection). Hence water can be
easily identified by its low radiometric values or
dark appearance on radar images. By using change
detection techniques, it is possible to isolate the
flooded areas, by distinguishing them from the
normal areas covered by the water bodies. Figure 4
shows a detail of the first images acquired by the
Italian Radar Satellite Constellation Cosmo-Sky-MED,
to support the AILA cyclone emergency which
occurred in May 2009 (the cooperation of the Italian
Space Agency and the e-GEOS company is acknowledged).
Figure 5 displays the extracted water
bodies, classified as “Reference Water” and “Flooded
Areas” based on data available before the event.
A different approach is used to define flooded areas
based on multispectral optical data processing. For
several reasons, the MODIS sensor is generally used
for large-scale flood monitoring (Brakenridge et al.
2003, Voigt et al. 2007, Aduah 2007). The MODIS
mission provides daily worldwide coverage; images
and derived products are in the public domain.
Furthermore, low geometric resolution (250-500-
1000 m) MODIS data allow a regional view of the
observed phenomena. Therefore, the use of MODIS
data permits a multi-temporal small scale analysis of
the evolution of the flood event in the areas of
interest. The water bodies and flooded areas are
detected by a classification procedure of MODIS
primary reflectance data (Figure 6), available in near
real-time through the NASA/GSFC MODIS Rapid
Response System. For classification purposes specific
radiometric indexes are defined, such as the NDWI
(Normalised Differential Water Index), which is
useful for detection of water bodies and flooded area
F i g u r e 4 : C o s m o - S k y M E D i m a g e - © A S I 2009 (30
th
m, May, 30 2009) covering coastal areas of
Bangladesh
REFERENCE WATER
FLOODED AREAS
Figure 5: Reference water (blue) and flooded areas
(red) identified trough radar imagery
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