7th Workshop on Forest Fire Management - EARSeL, European ...

278
IV - BURNED LAND MAPPING, FIRE SEVERITY DETERMINATION, AND VEGETATION RECOVERY ASSESSMENT
indeed the most affected area, with a peak in the number of wildfires in
the summer because of climatic factors (e.g. high temperature) and fuel
properties.
In order to preserve and protect woodland areas, Law 353/2000 imposes
the drafting of regional plans for the forecast, prevention and active fight
of fires in addition to formative, informational and educational activities.
It is worth noticing that, even if the number of towns with a cadastre of
forest fires passed from 24% in 2006 to 46% in 2007, only 6% of them
thoroughly comply with the directives of the Outline Law in force. Remote
sensing can help to fulfil the requirements of the law in an economic and
effective way as, along with other applications, it allows to detect and map
fire-affected surfaces over wide areas, in inaccessible and dangerous places
and at a low cost when compared to traditional survey techniques.
2 - Materials and methods
This work is a qualitative evaluation of both potentials and limits of several
algorithms for post-fire mapping of burned areas by means of specially
designed spectral indices such as BAI (Burned Area Index), NBR
(Normalized Burn Ratio) and MIRBI (Mid-Infrared Bispectral Index)
(Chuvieco et al., 2002; Benson, 1999; Trigg & Flasse, 2001). In order to get
over the problems highlighted by the single indexes two different approaches
were used: in a mono-temporal approach we employed a single post-fire
image for computing and combining the indices to be used in classification,
while in a multi-temporal approach (change detection) we assessed
for each pixel the difference between the index values computed for the
pre-fire and post-fire images.
The analysis was performed by using multispectral moderate resolution
images from ASTER sensor, operating on NASA’s Terra platform; in particular
we made use of AST_07XT products (On-Demand L2 Surface Reflectance),
which, apart from geometric and radiometric corrections, are characterized
by a further adjustment taking atmospheric conditions and changing in
earth-sun relative positions into account - which allows to transform radiance
value of image pixels in surface reflectance of the corresponding
object on the earth surface. We used files including both the 3 bands of
VNIR sub-system (spatial resolution 15 m) and the 6 bands of SWIR subsystem
(spatial resolution 30 m).
In the mono-temporal approach a single post-fire image acquired on 28th
June 2007 at 09.53.36 a.m. was processed: the image was chosen for the
absence of clouds, the availability of data about wildfires occurred in the
previous period, and mainly because it is immediately following the peak
temperature period occurred in Sicily between 23rd and 26th June 2007
(Figure 1). For the change-detection approach we made also use of another
image acquired on 9th April 2007 at 09.53.47 a.m., in a period with
scarce wildfire events.