Huygens Wavelet Propagation Grows a Perimeter by Spreading an Ellipse from Every Vertex
Landscape simulators of the FARSITE family represent the fire perimeter as a closed chain of vertices and advance it in steps. At each vertex, the local fuel, slope, wind and moisture are read from the landscape grid, a local spread rate and length-to-breadth ratio are computed, and a small ellipse — the wavelet — is grown from that vertex for the length of the time step. The new perimeter is the outer envelope of all those wavelets, which is Huygens' construction borrowed from wave propagation: every point on a front is treated as a source of the next front. Because each vertex carries its own conditions, the fire naturally speeds up in grass, slows in a wet draw and turns with the wind without any of that being coded as a special case. The practical difficulties are bookkeeping, not physics: vertices bunch together on the fast head and must be resampled, and when a perimeter folds back on itself the simulator must detect and remove the resulting loop, or the fire will appear to burn ground twice. After this Concept you can explain how a vector fire simulator turns a spread rate into a moving perimeter.
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Landscape simulators of the FARSITE family represent the fire perimeter as a closed chain of vertices and advance it in steps. At each vertex, the local fuel, slope, wind and moisture are read from the landscape grid, a local spread rate and length-to-breadth ratio are computed, and a small ellipse — the wavelet — is grown from that vertex for the length of the time step. The new perimeter is the outer envelope of all those wavelets, which is Huygens' construction borrowed from wave propagation: every point on a front is treated as a source of the next front. Because each vertex carries its own conditions, the fire naturally speeds up in grass, slows in a wet draw and turns with the wind without any of that being coded as a special case. The practical difficulties are bookkeeping, not physics: vertices bunch together on the fast head and must be resampled, and when a perimeter folds back on itself the simulator must detect and remove the resulting loop, or the fire will appear to burn ground twice. After this Concept you can explain how a vector fire simulator turns a spread rate into a moving perimeter.
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