Hotspots are defined as image pixels whose brightness temperatures exceed a pre-defined threshold value. The NOAA satellites carry a thermal detector that measures the average surface temperature of each square kilometre in a strip below the satellites. This temperature information is transmitted to a ground receiving station, and the computer’s ground station will then view a thermal image of the area scanned by the satellites. All the sampled 1 km2 areas with temperatures above the threshold value are selected. The location of each area is then plotted to produce a ‘hotspot map’. ‘False’ fires contaminated by solar reflectance from clouds or water are eliminated.
Threshold values of 316-320 K have been reported in scientific literature. A threshold digital count of 40 (corresponding to a brightness temperature of 321.3 K) is adopted by the ASEAN Specialised Meteorological Centre (ASMC) for hotspot detection. This threshold is a more conservative value compared to some of those reportedly used by other centres, which ranged from 316 K to 317 K. Different temperature threshold will result in different hotspot counts. The lower the threshold, the higher the number of hotspots detected.
NOAA-series satellites has become the most widely used satellite data set for regional fire detection and monitoring because of its availability, spatial resolution, spectral characteristics, and low costs. Its original intention, however, was not for fire detection and land studies. With NOAA, the number, the size and the intensity of the fires and the size of a burned area cannot be determined precisely. NOAA detection analysis is also limited to relatively cloud-free areas. Cloud presence may block the satellite’s view of ground fires. It can cause an underestimation in the extent and frequency of burning, and limits the ability to track vegetation parameters. Another limitation is that forest canopy fires cannot be detected.
Ground and aerial surveillance and other environmental satellites using higher resolution will complement information received from NOAA-series satellites. Environmental satellites such as SPOT and LANDSAT offer much more detailed images (up to 10 X 10 meters resolution for SPOT images) but lower frequency of repeat coverage and narrower swath path than those from the NOAA-series satellites. Terra and Aqua MODIS allow daily-cycle monitoring and observation of rapid, time-varying phenomena like clouds, water vapour, and fire, with Terra descending across the equator at 10:30 am and Aqua ascending across the equator at 1:30 pm. The sensor resolution of MODIS is up to 250 metres compared to NOAA’s, which is only 1.1 km. With these instruments, scientists will be able to better sample fire activity, improve chances of obtaining cloud-free observations, and gather data on how fast and in which direction fires are spreading as well as information on how severely a fire may affect air quality pf downwind urban areas.
Sources and For Further Reading
- Hotspot Interpretation Manual, ASMC’s Intranet
- MODIS Land Rapid Response System
- Fire, Smoke, and Haze: The ASEAN Response Strategy (Chapter 1)
- Forest Fire Prevention and Control Project’s article “A Guide to Hotspots and Forest Fires in Sumatra”
- Integrated Forest Fire Management Project’s webpage “NOAA-AVHRR fire detection within the IFFM Project”