EOM June 2005 > Departments > News Story

Mapping Fires From the Air

Matteo Luccio

Figure 2: Single frame, thermal wideband image of small boat harbor from 2600 feet (image courtesy of USFS)

Accurate calibration is the key to high resolution thermal imaging. Unlike aerial images in the visible spectrum, until recently the quality of thermal images was typically not sharp and uniform. Space Instrument's sensor, however, is calibrated to the point that it can detect and map even very small temperature differences in, say, a body of water. This makes it very useful for environmental surveys, such as the monitoring of coastlines, rivers, and lakes, because the images generated by the device show very clearly both natural phenomena, such as currents, as well as any human input, such as chemicals, polluted water, fertilizer runoff, etc. (see Figure 2). This improved temperature resolution is due in large part to the fact that the device uses 16 bit encoding, rather than only the 8 bit to 12 bit encoding used by the previous generation of sensors. Also, unlike commercial off-the-shelf, single band, microbolometer cameras, FireMapper�s two narrow spectral bands are optimized to measure high radiance IR radiation from fires.

While FireMapper's technology is applicable to a wide range of Earth monitoring activities, the device owes its name to its particular usefulness to firefighters. While most current sensors saturate even on smoldering ash or sunlit rocks, so that everything warm looks the same, Firemapper provides a full color image that depicts heat intensity up to 1200° C without saturation and can resolve temperatures down to about two 10^ths of a degree in real time. This allows the operator to identify and map flaming fronts, fire lines, and hot spots and to read the temperature in a location on the ground by touching the corresponding pixel on the display. It also makes it possible to detect small spot fires from the air long before they grow into devastating forest fires.

Figure 3: 3-D ortho-rectified image mosaic of Jack White Range, Alaska wildfire (images taken by BLM lead aircraft on 2004 July 15) Click on image to see enlarged.

FireMapper 2.0 features three selectable thermal infrared spectral bands: two narrowband channels for high intensity wildfires and one wideband channel for maximum sensitivity. The wideband channel is typically used for environmental mapping. The sensor features a through-the-lens, absolute calibration system and automatic drift correction to obtain stable, highly-calibrated radiometric measurements.

Because the images created by Firemapper are digital, they can be downloaded in near real time. Additionally, the images can then be georeferenced and orthorectified, so they can be projected onto flat topographic maps or draped over digital elevation models (DEMs)(see Figure 3). Individual images can also be mosaicked to depict the full scope of a large wildfire.

Figure 4: Low altitude image mosaic of retardant line placement at Arizona wildfire (images taken by BLM lead aircraft on 2004 June 3) Click on image to see enlarged.

For low altitude operation FireMapper has a special "Tactical" mode, which allows imaging at 6 six images per second to provide contiguous or overlapping images at low altitude for tactical situations. According to Space Instruments, at 120-knot speeds and an altitude of only 50 feet, the device can attain a 30 percent image overlap (see Figure 4). This capability can be used in real- time to guide aerial tankers over fires. It is also very useful for helicopter operations at low altitude.

Some existing line scanners weigh several hundred pounds because they need to be cooled with liquid nitrogen and they need to mechanically scan across the scene to generate an image. The FireMapper, instead, utilizes a single uncooled microbolometer detector array and operates without mechanical scanning. This allowed the company to reduce the sensor's weight to only 9 pounds — small enough to be flown even in an unmanned aerial vehicle (UAV). Additionally, like charge coupled device (CCD) cameras and unlike traditional line scanners, FireMapper has no high speed moving parts, which makes it more rugged and long lasting (see Figure 5).

Figure 5: FireMapper 2.0 thermal sensor and touchscreen tablet.

Various remote sensing satellites have been used over the years to monitor and study wildfires — including the NOAA, Landsat, and, most recently, the MODIS (Moderate Resolution Imaging Spectroradiometer) satellites. The problem, however, is that none of them were designed to measure very high infrared radiance — a key characteristic of large fires. If the sensors on these satellites do not saturate when looking at fires it is because they use a very large pixel size — thereby greatly reducing their usefulness for mapping the fires.

Forest wildfires are complex environments, in which burned and unburned swaths are mixed, as are different temperature ranges and types of vegetation. Data on all of these aspects are important to understand the behavior of fires and their environmental impacts. Real- time fire mapping is further complicated by the fact that fires create uncertainty as they go. For these reasons, it is not very helpful to just "see" a fire from a satellite. Instead, fire researchers need good estimates of a fire's intensity, its speed (in meters per second), and the amount of energy it imparts to a cell (because this determines the percentage of nutrients lost and the amount of soil erosion likely to occur after the fire).

Figure 6: Old Fire, San Bernadino, California, 3-D ortho-rectified image mosaic (images taken by USFS PSW aircraft on 2003 October 25) Click on image to see enlarged.

Dr. Philip Riggan, a fire scientist with the United States Department of Agriculture Forest Service, first began researching remote sensing techniques for studying wildfires in 1983, in partnership with NASA's Ames Research Center. This partnership led to the development of the Airborne Infrared Disaster Assessment System (AIRDAS), which was used extensively to fight fires in Brazil.

In 1997 Dr. Riggan met James Hoffman, founder and technical director of Space Instruments, who had just developed the spaceborne microbolometer sensor flown on shuttle mission STS-85. In partnership, Dr. Riggan and James Hoffman improved on the latter�s invention and developed the prototype of the FireMapper.

Figure 7: Copper Fire, Los Angeles County, 3-D ortho-rectified image mosaic (images taken by USFS PSW aircraft on 2002 June 7) Click on image to see enlarged.

The Forest Service typically flies its fire reconnaissance flights at an altitude of 17,500 feet. At that altitude, FireMapper gives them a resolution of about 5-7 meters — sufficient to provide good detail in and around fire fronts (see Figure 6 and Figure 7). In the long-wave IR, the atmospheric window in the IR band in which FireMapper works, most of the energy detected comes from the ground surface, because flames are optically thin. However, unsaturated measurements give researchers an indication of the amount of energy going into the ground.

Firefighters don't need as much detail — but they need the information very quickly. For this reason, Dr. Riggan developed FireImager, a system to convey the data collected by FireMapper in the air to the firefighters on the ground in near-real time. The system transmits the data via the Globalstar satellite phone service and an FTP server. Individual images can also be combined into a mosaic in a geographic information system (GIS) to depict the full scope of a large wildfire. An analyst then combines the images into a mosaic, estimates the temperature within each pixel, prepares easily-understood color fire maps for use by firefighters, and posts them to Fireimager.com. While FireMapper's tactical operating mode allows for animation, analysts can derive the speed of a fire even from single images, on the basis of the ground temperature behind a fire front.

The system's refresh rate is currently about 90 minutes, but Dr. Riggan expects to be able to reduce that to about 30 minutes later this year, thanks in part to better satellite communications and in part to automating some of the process. Even 90 minutes, however, is a huge improvement over the Forest Service's current system, which it operates jointly with the Department of Interior: a plane that flies at night and provides fire location information for a 6AM briefing by the incident command. FireImager, instead, will allow firefighters to access fresh fire maps via a wireless Internet connection around the clock.

My sources for this article were James Hoffman and Dr. Philip Riggan. Hoffman, Space Instruments' founder and technical director, is an electrical engineer with a BEE degree from Cornell University and a master's degree from the University of Hawaii. After working for many years at Hughes Aircraft Corporation, now part of Raytheon, and consulting to companies such as Dornier System, TRW, JPL, Rockwell International, and Orbital Sciences, he set up Space Instruments, Inc. in the early 1980s. Dr. Riggan is a GS-14 scientist with the USDA Forest Service, Pacific Southwest Research Station, stationed at the Forest Fire Laboratory in Riverside, California. He received a B.S. in chemistry from San Diego State University in 1973 and a Ph.D. from the College of Forest Resources of the University of Washington in 1979. Dr. Riggan has conducted research on remote measurement of wildfire properties; the biogeochemistry, primary production, plant community development, and effects of fire in Mediterranean ecosystems of California; and the global consequences of wildland fire in tropical ecosystems.

Figure 8: Thermal image of Jim Hoffman.