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

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IV - BURNED LAND MAPPING, FIRE SEVERITY DETERMINATION, AND VEGETATION RECOVERY ASSESSMENT
to evaluate the performance of the dNBR method as regards to the positively
detected burnt areas and the related false alarms, when applied at
regional scale to detect and map fires of small dimensions in the
Mediterranean region. The dNBR method has been tested on two Landsat-
TM images covering the whole Tuscany region (Italy), collected respectively
before and after the fire season of summer 2000 (characterized by 18
medium to small fire events occurred in considered area). A modified version
of the dNBR method obtained by the analysis of four simple environmental
conditional tests is then presented and the related results are comparatively
reported.
2 - Methodology
The differential NBR index (dNBR) is the difference among the NBR index
evaluated before (NBR b ) and after (NBR a ) the fire season, where the NBR
index is defined (if Landsat TM sensor data are considered) as:
NBR = (B 4 - B 7 )/(B 4 + B 7 )
being B i the surface spectral reflectances as measured in bands 4 and 7
after a suitable calibration (Thome et al., 1997). The procedure to detect
burnt area based on dNBR index consists in classifying as burned the pixels
whose dNBR exceeds a given threshold K i . A suitable threshold needs
therefore to be defined according to a desired trade-off among the number
of pixels correctly classified as burned (hereafter true positive) and the
number of unburned pixels wrongly classified as burned (hereafter false positive).
The analysis of this trade-off is the main focus of this work, in which
the performance of the dNBR method has been evaluated by varying the
threshold value. The obtained results are than compared with those of four
modified dNBR method, all relying on a second processing step (to be
applied after the dNBR one) based on physical considerations coming from
the literature and resulting in the following thresholding:
a) Pixels of the image acquired after the fire characterized by a value of
B5 – B7 higher than 0.08 are considered unburned.
b) Pixels of the image acquired after the fire with a value of B7 lower than
0.06 are considered unburned.
c) Pixels of the image acquired before the fire with a NDVI lower than 0.2
are considered unburnt (pixel must be vegetated before the fire).
d) Pixel with the value of the NDVI index in the image acquired before the
fire lower than that of the image acquired after are considered unburned
(vegetation after the fire must be diminished with respect to the image
acquired before the fire).