Data Sources and Methods
Fire Monitoring, Mapping, and Modeling (Fire M3)
Data Sources and Methods for Daily Maps
The Fire M3 hotspots are obtained from multiple sources:
- Advanced Very High Resolution Radiometer (AVHRR) imagery, courtesy of the U.S. National Oceanic and Atmospheric Administration (NOAA) National Environmental Satellite, Data and Information Service (NESDIS).
- Moderate Resolution Imaging Spectroradiometer (MODIS) imagery, courtesy of the National Aeronautics and Space Administration (NASA) Land, Atmosphere Near real-time Capability for EOS (LANCE) Fire Information for Resource Management System (FIRMS), and from the Active Fire Mapping Program, Remote Sensing Applications Center (RSAC), USDA Forest Service. (https://fsapps.nwcg.gov/afm/)
- Visible Infrared Imaging Radiometer Suite (VIIRS) imagery, courtesy of NASA LANCE FIRMS, University of Maryland and RSAC.
How satellite sensors detect forest fires:
Satellite sensors record the intensity of electromagnetic radiation from Earth in various spectral wavelengths or channels. These wavelengths are found in the visible (0.4–0.7 µm) and infrared (0.7–100 µm) portions of the spectrum. Fires and other sources of intense heat can be detected if a sensor includes a channel near the 4-µm range, which is highly sensitive to radiation emitted from objects hotter than about 200°C. Flame temperatures range from 800 to 1200°C, and fires emit far more radiation in the infrared than in the visible wavelengths. As a result, fires can be detected that cover only a fraction (less than 0.1%) of an image pixel. Other types of objects, such as cloud edges and exposed soil, also produce a large response in the 4-µm channel; consequently, information from other channels is required to filter out extraneous data ("false alarms").
Satellite sensors provide information about forest fires in Canada:
- Thermal infrared scanners flown on board aircraft are often used to map fire hotspots and fire intensity over individual fires or small regions. This information allows fire management agencies to effectively target fire suppression efforts by water bombers and ground attack crews.
- The National Aeronautics and Space Administration's Moderate Resolution Imaging Spectroradiometer (MODIS), with channels specifically designed for fire detection, is the most commonly used satellite sensor for detecting fires over large regions. Fire detection is conducted primarily by the 1-km resolution channels at 4 µm and 11 µm, which have high saturation temperatures of about 450 K (177°C) and 400 K (127°C), respectively. There are two satellites carrying the MODIS sensor, which together provide complete coverage of Canada approximately 4 times daily.
- The US National Oceanic and Atmospheric Administrations's Advanced Very High Resolution Radiometer (AVHRR) also has 1-km resolution channels at 4 µm and 11 µm, but with lower saturation temperatures. There are five AVHRR satellites providing hotspots for Fire M3 via NOAA's Fire Identification, Mapping and Monitoring Algorithm (FIMMA).
- The Visible Infrared Imaging Radiometer Suite (VIIRS), operated by the US National Aeronautics and Space Administration (NASA), was designed to be the successor to both AVHRR and MODIS. There are two satellites carrying the VIIRS sensor, which has fire detection channels at both 750-m and 375-m resolution. The resolution of the VIIRS sensor is finer than MODIS and AVHRR, and it can detect smaller fires.
- The Satellite Pour l'Observation de la Terre (SPOT) Vegetation (VGT) sensor has four channels that measure reflected energy from the Earth. Because it lacks thermal channels, the sensor is not well suited to detecting active fires. However, it does include near-infrared and short-wave infrared channels that are highly effective for mapping the extent of burned forest after a fire has stopped burning. VGT imagery is currently being used for annual mapping of burned forest across Canada.
- Landsat's Thematic Mapper (TM) sensor allows areas to be observed at a 30-m resolution in seven channels. This high resolution comes at the expense of observing a given location only once every 16 days. TM is best suited to providing detailed maps of areas burned by individual fires or fire complexes. These maps can be used to plan salvage logging operations and to verify the extent of the burned areas mapped using coarser-resolution VGT imagery.
Benefits and limitations of using satellites for fire detection
The main benefit of using satellites for detecting fires is that they can cover all of Canada multiple times daily at low cost. This makes them effective for detecting fires in remote, unpopulated regions, where conventional fire monitoring is less intensive. Thick smoke plumes from forest fires, often extending several hundred kilometers, can also be identified by means of satellite imagery.
Satellite fire detection has some limitations that must be kept in mind when examining the daily fire images:
- The tests used by the fire algorithms to remove "false alarms" sometimes fail, leading to false records of fires. However, considering the large number of satellite pixels examined each day across Canada (about 9.5 million), the error rate is extremely low. A fire hotspot can be confirmed when a conical smoke plume is observed emanating from it. However, it is sometimes impossible to see a plume from a small fire or a fire obstructed from view by nearby clouds.
- The algorithms cannot detect fires through thick cloud or smoke. A large fire may therefore go undetected for several days and then appear or reappear later; a small fire may burn and die out without ever being detected.
- The time lapse between satellite image acquisition and image distribution on the CWFIS site is between 1 and 7 hours, depending on the sensor and processing time. This delay, along with the coarse resolution, limits the utility of satellite detection for tactical fire operations.
- The actual size of the actively burning area cannot be determined from satellite imagery. A 1-km² hotspot pixel may represent a fire as small as 100 m². In addition, an intense fire covering an area less than 1 km² may actually show up as a cluster of several hotspot pixels. This is the result of the varying size and spatial overlap of the raw, unprojected pixels.
Modeling activities are the third "M" in Fire M3
For each hotspot location, various fire attributes are modeled with the Canadian Forest Fire Weather Index System and Forest Fire Behavior Prediction System developed by the Canadian Forest Service. Fuel moisture codes and fire weather indices are calculated on the basis of weather observations. The Fire Behavior Prediction System uses fire weather, fuel type, and topography to predict spread rate, fire intensity, fire type, and fuel consumption. This array of fire attributes can be queried for each hotspot by clicking on the hotspot icon on the interactive map.
Hotspot locations and fire behavior attributes are also used as inputs to smoke forecasting models, which are designed to predict concentrations of surface-level particulate matter. For more information, visit FireSmoke Canada.
Using satellite sensors to map burned areas
Since forest fires exceeding 10 km² account for more than 95% of the annual burned area in Canada, 1-km resolution satellite imagery is effective for mapping the vast majority of burned areas. A technique has been developed at the Canada Centre for Remote Sensing (CCRS) that maps burned forest at annual intervals across Canada (Fraser and Cihlar 2000). The method works by combining an annual hotspot map with observed annual changes in a vegetation index from SPOT VGT. The vegetation index is compared for each pixel from one year to the next (e.g., September 2011 and September 2012). Pixels with a significant drop in the index that are spatially coupled to hotspots are mapped as being burned. This technique is used to provide a coarse-resolution burned area product of Canadian forests at the end of each forest fire season.
A separate algorithm has been developed to map burned areas at 30-m spatial resolution using single Landsat TM images. The procedure has been used to automatically map individual burns, which provides a more accurate burned area map and can aid in planning salvage logging operations after a forest fire.
Finally, satellite detected hotspots can also be used to produce approximate burned area perimeters in near-real time. The Fire Perimeter Estimates layer on the CWFIS Interactive map shows the estimated extent of area burned to date and is generated by combining and processing the season-to-date hotspots. Due to the limited resolution and spatial accuracy of the hotspots, the results produced from this method should be considered as very rough estimates, and are best suited for large fires. However, they can be used as a good indicator of burned area when no other fire mapping options are available.
Fraser, R.H.; Cihlar, J. 2000. Hotspot and NDVI differencing synergy (HANDS): a new technique for burned area mapping over boreal forest. International Journal of Remote Sensing 74(3):362–376.