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

4 - Conclusions
Burned areas mapping by multispectral imagery: a case study in Sicily, summer 2000 283
Figure 5 - From left to right: RGB 8-3-2 ASTER combination; multiple threshold; multi-temporal
approach.
However quick and comparatively easy to process they are, algorithms
based on a single index produced errors with different surfaces and cannot
always assure reliable classifications. The scrutiny of frequency histograms
highlights that burned areas have a smaller variance with BAI than with
other indices; therefore BAI is the most sensitive to the spectral response
of burned vegetation and permits a better analysis as it computes the bispectral
distance from a reference point in the R-NIR domain for every pixel
(Chuvieco et al., 2002; Epting et al., 2005).
The multiple thresholding of all three indices with fixed threshold values
which do not depend on the statistical distribution of image pixels requires
a more complex processing but allows greatly improving accuracy in classification.
In fact a simultaneous use of four spectral bands in three equations
instead of only two bands in a single index equation allows to remove
errors with both water bodies and urban areas; classifications may still be
uncertain in the case of either very heterogeneous fires or of events
occurred long before the image was acquired). Furthermore it should be
noted that this method does not require either qualified operators or
assessed data on previous fires.
Accuracy in classification with this approach might be slightly improved by
means of a meticulous selection of threshold values - which would, however,
require accurate statistical studies of histograms from well trained operators
and ground validation of data. The quality of the results would not
justify such a greater operational complexity.
Finally, the classification based on a multi-temporal approach gives almost
the same results which can be obtained when using the multiple threshold
method in a mono-temporal approach, but requires fairly long and complex
pre-processing procedures and does not succeed in removing every commission
error. Here errors are connected with changes in the spectral sensitivity
of the same area/s in each of the two different images, whereas in
the mono-temporal approach they are due to surfaces having almost the
same reflectance as burned pixels.