Crown bulk density is an optional raster theme that is critical to determining the spread characteristics of crown fires. The values in the raster theme can be integers or decimal values in either kg/m3 or lb/ft3. The precision of crown bulk density values are used to the nearest 100th kg/m3 or 1000 lb/ft3. Thus, if you have integer values in your crown bulk density theme, you must use the calculator features of your GIS to multiply the original crown bulk density units by 100 (kg/m3) or 1000 (lb/ft3). This conversion is optional for decimal bulk density values.
Thus, if the actual crown bulk density were in metric units (say 0.25 Kg/m3), the value in the raster map would be multiplied by 100 to give an index of 25 for those cells. If the actual crown bulk density were in English units (say .015 lb/ft3), the value in the raster map would be multiplied by 1000 to give an index of 15 for those cells.
IMPORTANT NOTE: You have two options for producing crown bulk density information:
Crown Bulk Density can be linked to the canopy cover theme (see Model > Fire Behavior Options). Using this method, you would enter only the maximum crown bulk density for each forest type assuming 100% canopy cover. During the simulation, your crown bulk density data will be multiplied by the canopy cover fraction to obtain the actual crown bulk density for each raster cell. You must select the Link Crown Density & Cover option in the Model > Fire Behavior Options dialog box.
Crown Bulk Density can be independent of the canopy cover. This option would apply if you have already considered canopy cover during development of the crown bulk density theme on the GIS. If your crown bulk density data are like this, DO NOT select the Link Crown Density & Cover option in the Model > Fire Behavior Options dialog box.