Advantages and drawbacks of NOAA-AVHRR and SPOT-VGT for burnt area mapping in a tropical savanna ecosystem

In this paper we compare and contrast the performances of two low spatial resolution Earth observing systems, National Oceanographic and Atmospheric Administration - advanced very high resolution radiometer (NOAA-AVHRR) and Systeme pour l'Observation de la Terre - VEGETATION (SPOT-VGT), for burnt area mapping in a tropical savanna ecosystem of northern Australia. A multi-temporal composite image was derived from each of the two data sets acquired at the beginning of the 1999 fire season, and a set of rules for extracting burnt areas was compiled using a classification and regression trees (CART) methodology. In the case of the VGT data, the rules were used to segment the entire composite image; for the AVHRR data, a seed-growing approach was tested, with the aim of reducing overestimation of burnt areas. Since burnt areas are often confused with other land cover types, active fires were first identified (the seeds), and then the mapping of burnt areas was restricted to the neighbourhood of the pixels labelled as active fires. For the accuracy assessment, a reference map was derived by visual interpretation of two Landsat thematic mapper (TM) frames. First, the proportions of burnt areas were calculated for 14 x 14 km cells on a regular grid overlaid on AVHRR, VGT, and TM maps and the low-resolution areas were regressed against the TM estimates. Second, the spatial agreement between burnt area maps derived using high and low spatial resolution data was assessed using confusion matrices. We conclude that NOAA-AVHRR and SPOT-VGT show complementary characteristics. The better geometry of the VGT sensor, compared to the AVHRR, allows a higher accuracy in burnt area estimation, but the lack of a thermal channel on VGT is certainly a drawback for both the compositing and the burnt area mapping processes. Both systems tend to overestimate burnt areas when these areas are very fragmented but clearly underestimate when the burn scars are too small to be detected. Lastly, the seed-growing method is promising and could form the basis for a combined use of NOAA-AVHRR and SPOT-VGT systems.