To minimize the occurrence of false alarms over bright/reflective surfaces (e.g., metallic factory rooftops), the VIIRS and MODIS algorithms use slightly more conservative tests to avoid the effects of Sun glint.
Super-Heated Smoke Plumes
A few VIIRS active fire pixels are occasionally located outside the reported perimeter of a large wildfire. The parallax effect causes the tall/superheated plume detection pixel(s) to be displaced laterally when projected onto the surface. Displaced pixels will be located on the fire perimeter’s side further away from the image center and closer to the swath’s edge. If this happens, users may need to look for additional clues to see if it is a data artifact. There have been a few instances when tall, superheated plumes carrying large volumes of hot material into the air are formed over large and intense wildfires; when these occur the VIIRS instrument detects the surface fire along with part of the plume and categorizes them as active fires. Those occurrences typically share the following set of conditions:
(i) Nighttime detection. This is the period during which the VIIRS active fire product is particularly responsive to heat sources thereby favoring plume detection. However, this can also occur during the daytime with;
(ii) Very large wildfires undergoing explosive growth and accompanied by rapid/vertically elongated plume development. Enough hot material must entrain the plume creating a distinguishable thermal signal (i.e., one that significantly exceeds the fire-free surface background);
(iii) High scan angle. This is what will ultimately produce the detections extending beyond the actual fire perimeter.
If those conditions apply, look for alternative observations (previous/next observation) acquired closer to nadir, and try to prioritize the use of the fire detection data accordingly. Unfortunately, the VIIRS active fire algorithm isn’t currently able to distinguish nighttime surface fire pixels from the isolated plume detections due to strong similarities between their radiometric signatures.
Related content:
- Is the VIIRS 375 m product still being refined?
- What is the detection confidence?
- False fires due to super heated smoke plume in nighttime detections (3 August 2018)
- Note on False Fires in VIIRS/NOAA-20 (17 July 2020)
An indication of cloud cover or missing data can be obtained by viewing the co-incident MODIS or VIIRS imagery in FIRMS Fire Map or Worldview. To take cloud and missing data in to account, it may be more appropriate to use one of the 1 km Level 3 or Climate Modeling Grid (CMG) fire products (see the discussion in the MODIS Fire User Guide).
How appropriate are the hotspot/fire locations for my research?
The MODIS and VIIRS fire locations are good for determining the location of active fires, providing information on the spatial and temporal distribution of fires, and comparing data between years. The active fire pixel locations may not always be the most appropriate source of fire related information, and, the data do not provide any information on cloud cover or missing data. Depending on the analysis you are performing, misleading or even incorrect results may result, if the other types of pixels are ignored. In some cases, it is more appropriate to use one of the Level 3 or CMG fire products. For more discussion on this, see FAQ "What is the Climate Modeling Grid (CMD) fire product? and "What is the difference between NRT and Standard quality fire data?"
What is the Climate Modeling Grid (CMG) fire product?
The CMG fire products are gridded statistical summaries of fire pixel information intended for use in regional and global modeling. The products are currently generated at 0.5 degree spatial resolution for time periods of 1 calendar month (MOD14CMH/MYD14CMH) and 8 days (MOD14C8H/MYD14C8H). Higher resolution 0.25 degree CMG fire products will eventually be produced as well. More information can be found in the MODIS Collection 6 Active Fire Product User's Guide.
Can active fires be detected below the forest canopy?
The likelihood of detecting a fire beneath the tree canopy is unknown, but likely to be very low. Understory fires are typically small, and with the tree canopy obstructing the view of the fire, detection will be very unlikely.
Are all active fire hotspots vegetation fires?
No. An active fire represents the center of a pixel flagged as containing 1 or more actively burning hotspots /fires. In most cases hotspots/fires are vegetation fires, but sometimes a hotspot/fire is a volcanic eruption or the flare from a gas well. Different types of thermal anomalies are not currently attributed in the NRT MODIS or VIIRS data.
Can I estimate burned area from active fire data?
It is not recommended to use active fire locations to estimate burned area as determining burned area to an acceptable degree of accuracy is not possible due to nontrivial spatial and temporal sampling issues. For some applications, however, acceptable accuracy can be achieved, although the effective area burned per fire pixel is not simply a constant, but rather varies with respect to several different vegetation and fire-related variables. See Giglio et al. (2006) for more information. See "Where can I get the MODIS burned area product?"
Most recently, Harmonized Landsat Sentinel (HLS) false color composite imagery was included in FIRMS and these data can be helpful in estimating burned area (see FIRMS Blog post on adding HLS).
Can you use the active fire data for detecting volcanoes or volcanic eruptions?
While both the MODIS and VIIRS algorithms routinely detect active volcanoes, the active fire products have not been validated against independent data for their ability to detect volcanoes. There is a separate near-real time MODIS product (MODVOLC) specifically for volcanoes—see http://modis.higp.hawaii.edu/.
What is FRP and what factors should be considered across sensors?
Fire Radiative Power (FRP) is the rate of radiative energy emission per time unit from all fires within a pixel. FRP has been extensively used as a proxy of fire intensity to characterize fire types and is related to the rate of biomass combustion and rate of emissions. FRP estimates are provided for active fire data from MODIS, VIIRS and geostationary sensors used in FIRMS. Sensor spatial resolution, saturation temperature of thermal bands, satellite view zenith angle, time of satellite observation, etc. can affect the accuracy and variability of FRP estimates. Users should consider these factors when comparing FRP information from a specific sensor or multiple sensors.