The Terrain Difficulty Index (TDI) describes the characteristics of the landscape which predict the difficulty of extinction in both the initial and extended attack phases. TDI quantifies the availability of access for the arrival of terrestrial means, the ability to penetrate the area where the fire originates, and the complexity of extinguishing fuels.
TDI is represented using five qualitative categories:
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Class 1 (Very low): No accessibility limitations to the firefighting resources, allowing quick deployment of wildfire suppression ground means.
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Class 2 (Low): High density of tracks and paths. Terrain conditions allow the deployment of wildfire suppression ground resources.
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Class 3 (Intermediate): Roads and tracks are slightly more difficult to access and terrain is mildly difficult with increasing slopes.
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Class 4 (High): Low density of roads/tracks in the area. Difficult terrain access with limitations to ground travel.
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Class 5 (Extreme): Very low density of tracks/roads to support strategies. Highly complex terrain conditions including high-slope areas limit the use of heavy equipment.