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

Improvement of dNBR burnt areas detection procedure by physical considerations based on NDVI index 205
3 - Results
The performances of the dNBR method have been assessed by evaluating
the number of true positives, true negatives and false positives (Story
Congalton, 1986) by varying the threshold value.
Figure 1 - The producer accuracy with the threshold.
The results reported in percentage in Fig. 1 show that the performances of
the dNBR method are not completely satisfactory, from “fire mapping” point
of view, that is in highlighting all the burned pixels. As a matter of fact it
is not possible to find a good trade-off between the required high number
of true positives and a low number of false positives. The dNBR method has
been then analysed in terms of “fire detection”. The fire detection task
needs that only one pixel (at least one!) for each burned area is flagged as
burned, and thus requires a lower number of true positives. It has been
found that the best (higher) possible threshold to have yet all the burn
scars detected by at least a true positive is equal to 0.10. By using this
value, a percentage of 81.5% (2048 of 2513) of the burnt pixel are correctly
classified, but these true positives are countered by more than five millions
(5,378,754) false positives, a number trivially to high to consider the dNBR
method useful for fire area detection at regional scale. In order to increase
the performances, the aforementioned four additional criteria (a – d) have
been introduced and tested. The related performances are reported in Table
1 together with those of the original method. It can be noticed how the
proposed upgrades increase the efficiency of the dNBR methods; nevertheless,
the performances are still not satisfactory, and other additional criteria
are under evaluation in order to obtain a more efficient detection procedure.