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."
EFFIS uses the active fire detection 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 complements 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 in Europe and as a means to help the subsequent mapping of burnt fire perimeters. Information on active fires is normally updated 6 times daily and made available in EFFIS within 2-3 hours of the acquisition of the MODIS/VIIRS images.
When interpreting the hotspots displayed in the map, the following must be considered:
To minimize false alarms and filter out active fires not qualified as wildfires (e.g. agricultural burnings), the system only displays a filtered subset of the hotspots detected by FIRMS. To this end a knowledge based algorithm is applied that takes into account the extent of surrounding land cover categories, the distance to urban areas and artificial surfaces, the confidence level of the hotspot.
With the identify feature tool, key information attached to each active fire is provided, such as geographic coordinates, administrative district (commune and province) and the main land cover category affected.
Another source for identification of active fires is built through firenews: the news items are selected from a large set of RSS feeds published by various forest fires related sites, and from news feeds filtered out with appropriate keywords. Items from selected feeds are then geolocated and published on the EFFIS web site, in the FireNews section.