7th Workshop on Forest Fire Management - EARSeL, European ...
7th Workshop on Forest Fire Management - EARSeL, European ...
7th Workshop on Forest Fire Management - EARSeL, European ...
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CORRECTION OF TOPOGRAPHIC EFFECTS INFLUENCING<br />
THE DIFFERENCED NORMALIZED BURN RATIO’S OPTIMALITY<br />
FOR ESTIMATING FIRE SEVERITY<br />
Abstract: The influence of illuminati<strong>on</strong> effects <strong>on</strong> the optimality of the<br />
dNBR (differenced Normalized Burn Ratio) was evaluated for the case of the<br />
2007 Pelop<strong>on</strong>nese (Greece) wildfires using Landsat TM (Thematic Mapper)<br />
imagery. Well illuminated pixels exhibited more optimal displacements in<br />
the bi-spectral feature space than more shaded pixels. To correct for illuminati<strong>on</strong><br />
effects, the c-correcti<strong>on</strong> method and a modified c-correcti<strong>on</strong><br />
technique were applied. The resulting mean dNBR optimality of uncorrected,<br />
c-corrected and modified c-correcti<strong>on</strong> data was respectively 0.57, 0.59<br />
and 0.66. Applying a topographic correcti<strong>on</strong> significantly improves the reliability<br />
of change detecti<strong>on</strong> especially in rugged terrain and when low sun<br />
angle images are used.<br />
1 - Introducti<strong>on</strong><br />
S. Veraverbeke, R. Goossens<br />
Department of Geography, Ghent University, Ghent, Belgium<br />
sander.veraverbeke@ugent.be<br />
W. Verstraeten<br />
Geomatics Engineering, Katholieke Universiteit Leuven, Leuven, Belgium<br />
S. Lhermitte<br />
Centro de Estudios Avanzados en Z<strong>on</strong>as Aridas (CEAZA), La Serena, Chile<br />
<strong>Fire</strong> severity is an important factor in post-fire assessment. The Normalized<br />
Burn Ratio (NBR) is the standard spectral index to estimate fire severity<br />
(Key et al., 2005):<br />
TM4 - TM7<br />
NBR = --------------------- (1)<br />
TM4 + TM7<br />
where TM4 and TM7 are respectively the NIR and MIR reflectance of Landsat<br />
TM imagery. Bi-temporal image results in the differenced Normalized Burn<br />
Ratio (dNBR). A pixel-based optimality measure which evaluates a pixel’s<br />
movement in the bi-spectral feature space has been developed (Roy et al.,<br />
2006). An optimal fire severity spectral index needs to be sensitive to fireinduced<br />
vegetati<strong>on</strong> changes and insensitive to perturbing factors such illuminati<strong>on</strong><br />
effects.<br />
These illuminati<strong>on</strong> effects are initiated by both topography and solar positi<strong>on</strong><br />
at the moment of image acquisiti<strong>on</strong>. Topographic effects in ratio-<br />
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