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

Relationships between combustion products and their spectral properties in fire-affected shrublands 117
Figure 1 - Experimental design.
3.2 - Post-treatments of field data
To apply subsequent treatments in a homogeneous manner we built a mosaic
file from the 305 circular-shaped files. To quantify the combustion products
we applied a supervised classification process selecting a maximumlikelihood
method. From this classification we obtained the percentages of
the three combustion products studied in this research: (1) ash, where fuel
had undergone a complete combustion; (2) charcoal, where an unburned
fuel component remains; and (3) vegetation remains, vegetation non
affected by the fire.
From direct fieldwork we obtained reflectance data from the VIS and NIR
regions. Applying hyperspectral techniques to this original information we
obtained derived information: (1) the standard first derivative spectra
(FDS) and (2) the absorption band depth (BD).
The FDS transformation can be defined as the reflectance change rate for a
specific spectral distance along the different wavelengths considered
(Dawson and Curran, 1998). This technique emphasizes the wavelengths
were the spectral curve has rough changes in its form.
FDS λ(j) = (R λ(j+1) – R λ(j) ) / (λ (j+1) - λ (j) )
where FDS λ(j) is the first derivative spectra in the wavelength j; R is the
reflectance value; (j) and (j+1) are the wavelengths.
The continuum removal (CR) is the technique applied to obtain the BD data.
From the continuous reflectance spectrum we identified nine absorption
features, five from a vegetation spectrum and four from a charcoal spectrum.
By applying the CR equation the continuum-removed spectra were calculated
and BD (li) was obtained in each wavelength (Mutanga et al., 2004):
CR (λi) = R (λi) / Rc λ(i)
BD (λi) = 1 – CR (λi)