Comparison of fire detection in savannas using AVHRR's channel 3 and TM images

Detection of active fires in NOAA-11 AVHRR channel 3 (3·75 μm) afternoon images for an area of savannas in central Brazil was studied in relation to fire scars in Landsat TM images. Three consecutive TM images of the same area thus covering two periods of 16 days provided the 'ground truth' for the comparison of fire detection in AVHRR images of 15 and 14 days in each period. Using TM data as reference, 57 per cent of the new fire scars were associated with fires in channel 3 at the same places, in the first period. Seventy-four per cent of the active fires detected by AVHRR were verified in the TM imagery while the remainder was associated mainly with agricultural areas burned before the first period. For the second period, percentages were 53 per cent and 36 per cent respectively; the high percentage of unverified fires occurred mainly in grassland areas previously burned. Reasons for these results are discussed, namely, reflective soils in AVHRR's channel 3, fire scars not detected by TM because of precipitation, wind and fast changes in soil surfaces, and fires not active, or covered by clouds during AVHRR overpasses. A regression equation between the areas of the AVHRR fire pixels and of TM fire scars is also presented for the study area.