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

Correction of topographic effects influencing the differenced normalized burn ratio’s optimality for estimating fire severity 273
2.3 - Correcting for illumination effects
Methods that correct for illumination effects are based on the cosine of the
incidence angle, which is the angle between the normal to the ground and
the sun rays (Teillet et al., 1982):
cos γ i = cos θ p cos θ z + sin θ p sin θ z cos (φ a - φ o ) (3)
where γ i is the incident angle; θ p is the slope angle; θ z is the solar zenith
angle; φ a is the solar azimuth angle; and φ o is the aspect angle.
In the c-correction method terrain corrected reflectance ρ t is defined as:
( cos θ ) z + ck ρt = ρa ---------------cos
γt + ck where c k is a band specific parameter.
We propose a modified c-correction method that corrects reflectance to a
maximum illumination cos γ t of one, instead of normalizing as a function
of the solar zenith angle:
( )
1 + c k
ρ t = ρ a ----------------cos
γ t + c k
Index optimality was compared among eight aspect classes and among different
classes of bi-temporally averaged illumination.
3 - Results and discussion
Figures 2A-C depict the topographically uncorrected, c-corrected and modified
c-correction dNBR optimality maps. The modified c-correction dNBR
optimality (mean = 0.66) outperformed c-corrected and uncorrected optimality
(means of respectively 0.59 and 0.57), whereas c-corrected optimality
provided slightly better results than uncorrected optimality. This is also
reflected when the respective histograms are inspected (see figures 2D-F).
(4)
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