Airborne/ GPS
Digital Imaging and GPS Technology for Resource Management
The U.S. Forest Service integrates aerial digital imaging technologies and GPS to determine fire damage in the West.
By A. Victor Goodpasture

Introduction
Combining color infrared digital aerial imaging technology and the Global Positioning System (GPS) is helping federal and state resource managers collect data to better manage environmentally threatened lands.
      In California and the West, the technology allows the USDA Forest Service to quickly distribute limited resources to the most heavily damaged and environmentally fragile areas ravaged by fire. It also allows the agency to map and monitor general forest health.
      Digital imaging with small format cameras is a relatively new technology, but is quickly growing in popularity as its imaging speed and resolution increase and its cost decreases. The key to the digital camera is the CCD (charge-coupled device), which actually captures the image data. No less important is the compact storage technology, normally an internal hard drive or a PCMCIA card, which can be removed from the camera and read by a laptop computer or workstation. Professional portable digital cameras can capture 24-bit images in file sizes from 4.5 megabytes to as large as 18 megabytes. The cameras vary in frame capture but normally itÕs around two images per second in a burst of five seconds. Then the camera needs several seconds to clear its DRAM (dynamic random access memory) while the data is written to the hard drive. The most popular professional digital cameras operate almost identically to conventional 35 mm cameras, making them easy to use for anyone who has had practical photographic experience.
      Color infrared (CIR) is an even newer aspect of digital imaging. While CIR films provide greater resolution than the digital camera, the spectral response of the digital camera is very similar to that of film. That means CIR digital imagery can be used with existing CIR aerial photography to interpret and identify changes through time. One of the advantages the CIR digital camera has over CIR film is exposure latitude. CIR film has to be exposed within a 1/2 f-stop of the proper exposure for best results. The CIR digital camera has greater dynamic range. Changing lighting conditions and errors in exposure settings are more easily corrected in a digital imaging processing environment.
      By far, the digital cameraÕs greatest asset is the speed in which captured data can be used. The data can be downloaded to a laptop computer and analyzed while still in flight and decisions regarding further imaging can be made immediately. And once on the ground, the images can be added to an image processor and GIS (geographic information system) environment for quick analysis and comparison.
      GPS technology now gives civilians similar capabilities as well. GPS is already a $2 billion a year commercial industry and is expected to grow to $8 billion by the year 2000, according to the federal government. That increase is expected to come as the government loosens its restrictions on degrading the accuracy of GPS signals. Presently, the Department of Defense only guarantees accuracy for civilian users to about 100 meters. But the system is capable of accuracy to less than 1-meter using advanced processing techniques.
      This past March, the White House announced the governmentÕs intention to provide more accurate signals for commercial and civilian uses. Those applications range from backpacking and pleasure boating to car navigation, emergency rescue, maritime shipping and international air traffic management.
      Now both digital imaging and GPS have found a home with resource managers who are combining the technologies to quickly and cost-effectively gather data on critical resources. The Kodak Digital Science 420 GPS camera captures CIR imagery as well as accepts GPS data simultaneously. The digital camera has a serial port connection that links it to a GPS receiver. As a digital image is taken, the GPS data is recorded and attached to the image as a data block. With the aid of the GPS information, the images can then be linked to a GIS database to provide an updated status of land resources.

California: Determining Fire Damage
The summer of 1996 has not been a good season for American forests in the West. Arson, carelessness and lightning have again set much of Western states ablaze. But after the fire crews mop up and the television cameras have gone home, state and federal land managers are left with the task of protecting the land from further damage. Soil erosion, disease and insects are the subsequent results of forest fires. For land managers, the challenge is quickly assessing where the most heavily damaged and fragile areas are so mitigation measures can begin. This is especially true for soil erosion where heavy rains can have a devastating impact on watersheds after much of the vegetation has been consumed by fire.
      ÒWeÕve had tens of thousands of acres of forests burned this season,Ó says Tom Bobbe, manager of the Remote Sensing Application Center for the USDA. Forest Service in Salt Lake City. ÒWe need to collect information as quickly as possible so resource managers know what areas need immediate attention. There are roads to repair, soil erosion to control, areas to be re-vegetated and fish habitat to protectÑto name a few of our many responsibilities.Ó
      BobbeÕs team is responsible for evaluating new remote sensing technologies. For almost two years, the team has been testing and implementing CIR digital cameras as another data-gathering tool. Not only are they used to track forest fire damage, the digital cameras are also used to provide information on mapping and monitoring noxious weeds, infestations, insects and diseases, mining sites and riparian habitats.
      ÒThe data we collect from our CIR digital cameras allows us to react in days rather than weeks,Ó Bobbe says. ÒThatÕs extremely important because the most critical period is the days following a fire. With that data, we can strategize and prioritize the distribution of mitigating resources.Ó
      The Forest Service contracted Eastman Kodak Company to develop a CIR CCD for a digital camera. In addition to the special CCD, a band pass filter attached to the digital cameraÕs lens limits the spectral response to green, red and near infrared. The camera can use different band pass filters depending on the imaging requirements.
      ÒCIR imagery is preferred for natural resource applications because it helps highlight differences in vegetative health and vigor. We can also more easily separate plant species because of the different spectral responses.Ó
      But managing the large number of images taken by the digital camera proved difficult because the images still had to be geographically referenced. That meant that a manual log had to be kept of the approximate location of each photograph so an image analyst could match the CIR image to a base map or digital orthoimage. Manual logging took time and was prone to errors.
      So the Forest Service once again contracted with Kodak, this time to develop a GPS interface so the location of the camera would be logged when the image was taken. Now attached to each digital image is a data block detailing exposure information, longitude, latitude and altitude. The data is also stored as a text file so it can be easily incorporated into reports and databases.
      ÒHaving GPS is not an end-all solution,Ó Bobbe says. ÒThe GPS coordinates are not located precisely in the center of the image but give the approximate position of the aircraft at the time of the exposure. So you still have to do some calculations to account for distortion in the lens, aircraft attitude, terrain displacement and other sources of error. The real intent of this GPS interface is to give us very good referencing and a general index of the exposure station.Ó
      In the recent Fork Fire near Clear Lake, Calif., for example, the Remote Sensing Application Center, photographed a portion of the burn area to record the various levels of burn intensity. The aircraft flew a series of adjacent flight lines at 15,000 feet. The camera was mounted vertically. Using an intervelometer, an image was taken every five seconds, giving 60 percent overlap along the flight line and 20 percent between adjacent flight lines.
      Back on the ground, the GPS data helped index the imagery, providing approximate coordinates of each image in the burn area. The separate images are digitally mosaicked and georeferenced to an existing digital orthoimage using features common to both images for establishing control.
      ÒOnce the CIR imagery is georeferenced, you have a reasonably accurate raster product,Ó Bobbe says. ÒThen you can delineate the categories of burn intensity. The entire process for mapping 25,000 acres took less than week. It would have taken several weeks had we done it with a conventional film camera. While film does provide more resolution, the digital camera gives us the accuracy we need and does it in a fraction of the time.Ó
      Presently, several U.S.D.A. Forest Service field offices are evaluating the digital camera system. The speed and low operational cost of the digital camera system makes it ideal for small projects, Bobbe adds.
      The mission of the Forest Service is multi-use management of the natural resources. That means balancing environmental protection with commercial and recreational interests. As demands for resources increase, it heightens the importance of monitoring the condition of the those resources.
      The Forest Service currently utilizes a variety of remote sensing systems to help monitor changes in the forest ecosystem. The CIR digital camera complements the use of satellite imagery and aerial photography, and provides a new capability to acquire imagery quickly and inexpensively. The cost of obtaining conventional aerial photography for monitoring projects can be high, especially if the project requires new imagery on a frequent basis.
      ÒThe trade-off is in the level of detail. No single remote sensing system will solve your problems. You need a combination of tools,Ó Bobbe says.

Closing Summary
For the U.S. Forest Service, GPS and digital imaging technology have proven to be flexible and powerful enough to provide real time and near real time data gathering and interpretation. The ability to obtain CIR aerial photography in minutes rather than hours or days is proving its value. Digital imagingÕs portability, ease of use and cost effectiveness make it as invaluable now for aerial surveying as a transit theodolite is for land surveying. However, the key to successful data gathering is not the technologies themselves, but how they are implemented.

About the Author:
Writer/photographer A. Victor Goodpasture is based in Carlsbad, Calif. Throughout his career, Goodpasture has documented innovative applications of medical imaging, printing and publishing, computer-aided drafting and design and document management.

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