Because the fire front and its characteristics are computed as a vector, raster representation of fire growth must be obtained by interpolation. FARSITE contains a routine for generating variable resolution raster information from the fire perimeters. The routine is tightly integrated with the control structure of the simulation; rasterizing is performed between distance checks (sub-time steps). This increases the proximity of vector information used for interpolating the fire behavior and arrival time at each raster. The raster algorithm interpolates the value for arrival time and fire behavior characteristics for the center point of each raster cell falling within two successive fire perimeters.
Method for interpolating fire behavior and time of arrival for raster centroids.
The interpolation for fire arrival time is a simple inverse-distance weighted average of the radial distance between a given raster centroid and the corresponding fire perimeter segments. The fire behavior interpolation uses an inverse-area weighted average to obtain a value for fireline intensity, flame length, spread rate, or heat per unit area for each raster. Each raster centroid is contained within a quadrilateral defined by four perimeter points, 2 successive points on perimeters at each time step. Each point has values for fireline intensity and spread rate in their respective spread directions. The interpolation computes the inverse proportional areas of 4 component quadrilaterals formed between each corner and the raster centroid. The fire behavior value for the raster centroid is calculated as the weighted average of fire behavior at each corner.