For each fire event the system searches the geo-database for images that will include the specific region. The numbers of images that are retrieved depend on the temporal resolution. Each of these images is processed independently and the Normalized Difference Vegetation Index (NVDI) , Normalized Difference Moisture Index (NDMI) and Normalized Burned Ratio Index (NBRI) are calculated. Thus, if n images are retrieved then that means 3n index images are calculated.
NDVI = ([band 4] – [band 3])/([band 4] + [band 3]) (1)
NDMI = ([band 4] – [band 5])/([band 4] + [band 5]) (2)
NBRI = ([band 4] – [band 7])/([band 4] + [band 7]) (3)
Healthy vegetation like forests has a high content of chlorophyll and that always show higher NDVI values. Whereas burnt areas will show very low values with negative or zero values. For studying forest fire areas near-infrared, mid-infrared and thermal bands are found to be sensitive to burn magnitude changes and are frequently used. NDMI contrasts the near-infrared (NIR) band 4, which is sensitive to the reflectance of leaf chlorophyll content to the mid-infrared (MIR) band 5, which is sensitive to the absorbance of leaf moisture. NBRI is very useful in discriminating the burned areas from the unburned areas.
Index images are used in producing post fire maps and its statistics and the mean index temporal variation graphs. The post fire maps are produced using a master image and the slave images. A master image is the nearest image in time before the fire event, and all images after the fire event are called slave images. Each post fire map is produced by thresholding the difference between the master NBRI image and the slave NBRI image. The result is a binary mask indicating the spatial extent of the burned area. The mean temporal variations are calculated using the earliest post fire map. Based on the master NBRI image and the first slave NBRI image, the burned area of first fire event is determined. In the similar manner the post fire maps for subsequent images are produced. For each subsequent slave image the mean temporal variation values are calculated for the burnt area. Thus, after the first fire event, how forest recovery progresses can be monitored using subsequent post-fire maps.