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Active fires are located on the basis of the so-called thermal anomalies produced by them. The algorithms compare the temperature of a potential fire with the temperature of the land cover around it; if the difference in temperature is above a given threshold, the potential fire is confirmed as an active fire or "hot spot."
GWIS uses the active fire detections provided by the NASA FIRMS (Fire Information for Resource Management System).
MODIS Active fires
The MODIS sensor, on board of the TERRA and ACQUA satellites, identifies areas on the ground that distinctly hotter than their surroundings and flags them as active fires. The difference in temperature between the areas that are actively burning with respect to neighbor areas allows the identification and mapping of active fires. The spatial resolution of the active fire detection pixel from MODIS is 1 km.
Additional information on the MODIS active fire product is available at https://earthdata.nasa.gov/earth-observation-data/near-real-time/firms/c6-mcd14dl
VIIRS Active fires
The VIIRS (Visible Infrared Imaging Radiometer Suite) on board of the NASA/NOAA Suomi National Polar-orbiting Partnership (SNPP) uses similar algorithms to those used by MODIS to detect active fires. The VIIRS active fire products complement the MODIS active fire detection and provides an improved spatial resolution, as compared to MODIS. The spatial resolution of the active fire detection pixel for VIIRS is 375 m. Additionally, VIIRS is able to detect smaller fires and can help delineate perimeters of ongoing large fires.
Additional information on VIIRS active fire products can be found at https://earthdata.nasa.gov/earth-observation-data/near-real-time/firms/viirs-i-band-active-fire-data
The mapping of active fires is performed to provide a synoptic view of current fires worldwide and as a means to help the subsequent mapping of burnt fire perimeters. Information on active fires is normally updated daily and made available in GWIS within 1 day the acquisition of the MODIS/VIIRS images. There are some well-known limitations due to the time of satellite overpass that could miss a considerable fraction of the burning activity (fires actively burning are detected only at the time of satellite overpass) or due to cloud cover and undetectable low temperature of the flaming front.
When interpreting the hotspots displayed in the viewer the following must be considered:
1) Hotspot location on the map is only accurate within the spatial accuracy of the sensor
2) Some fires may be small or obscured by smoke or cloud and remain undetected
3) The satellites also detect other heat sources (not all hotspots are fires).