| EOM Airborne: Evaluation of a Digital Camera System for Natural Resource Management
By Tom Bobbe and Jim McKean Introduction
Resource managers are continually looking for more efficient methods to acquire accurate and up-to-date information describing the land under their jurisdiction. The U.S.D.A. Forest Service is currently using remote sensing systems, including satellites, airborne multispectral scanners, thermal imagers, airborne video and aerial photography, to support a variety of information needs, such as updating GIS databases, delineating forest ecosystems, and monitoring forest health trends.
A new development in electronic imaging technology has provided another system to acquire remote sensing data. Recently introduced professional quality color digital camera systems have features especially useful for conducting natural resource aerial surveys. Digital camera systems can provide a high resolution digital image that can be easily integrated into an image processing and GIS environment. Since film does not have to be processed or scanned, digital cameras are well suited for small projects which require imagery within a short time frame.
During August 1994, a Kodak DSC 420 digital camera system was used, in combination with airborne video and a 70 mm film camera, as a remote sensing tool to support a riparian mapping and monitoring project. The objectives of this project were to acquire remote sensing imagery to document the current condition of riparian vegetation and to map the location of stream channels in the Golden Trout Wilderness Area of the Inyo National Forest. The project also provided an opportunity to examine and compare features of the digital camera, airborne video, and small format aerial photography. The Digital Camera System
Instead of film, digital cameras use a charge coupled device (CCD) to capture color images. The Kodak DCS 420 utilizes a high resolution CCD which consists of an array of photosites (pixels) to convert photons into electrons and produce a signal whose magnitude can be digitized, processed and stored electronically. The physical shape and size of the digital camera is similar to a conventional 35 mm film camera. In fact, the Kodak DCS 420 uses a Nikon N90 camera body without the standard film back. A modified camera back mounts a 1,524 by 1,012 pixel resolution CCD where film would normally be located. The remainder of the digital camera back contains the microprocessor, image memory, hard drive and batteries. Images are stored on removable PCMCIA hard drives which are commercially available in different capacities.
Since the physical size of the CCD is smaller than a 35 mm frame, the effective focal length is approximately 2.6 times the normal focal length. For example, a 50 mm focal length lens provides the same field of view as a 130 mm focal length when used with the digital camera.
Before starting the aerial surveys in the Golden Trout Wilderness, a test flight was completed to determine the spatial resolution of the digital camera for remote sensing applications. Both the digital camera and a 35 mm film camera with Kodak Ektachrome 100 color slide film were flown at an altitude of 1,000 feet above the ground to acquire vertical photographs of a Tri-Bar aerial photography resolution target. Both cameras were operated with similar camera settings and equivalent focal length lenses to provide approximately the same field-of-view. As expected, film has better spatial resolution than the digital imagery. The 35 mm camera provided a film transparency with three-inch ground resolved distance as interpreted from the resolution target, while the Kodak DCS 420 provided an image file with a six-inch ground resolved distance. Project Goals and Methodology
The Golden Trout Wilderness is a high elevation ecosystem of alpine meadows and small meandering streams in the southern portion of the California Sierra Nevada mountains. These streams are an important part of the native habitat for the golden trout, the official California state fish. Concern has been raised in recent years about the condition of the meadows and possible degradation of the riparian habitat. Meadow vegetation is evolving from grasses and sedges to drier species, such as sagebrush, and stream channels are widening and becoming so shallow that they will not support mature golden trout. Numerous abandoned channels are evident in the meadows, indicating past channel instabilities. There has been much speculation about the causes of these changes. Possible causes include long-term climate fluctuations, natural evolution of meadow hydrologic systems, and overgrazing. Remote sensing data will provide information to help define current meadow vegetation types and channel condition, locate and map abandoned channels, understand the causes of habitat change, and monitor changes in habitat condition over time.
Stream channels in four meadows within the wilderness area, where intensive field data has been collected, were investigated in this study. On three consecutive days, aerial surveys were conducted using a Cessna 206 airplane at altitudes of 1,500 and 3,000 feet above the ground. The Cessna 206 was equipped with a camera bay which allowed the Kodak DCS 420, a Hasselblad 70 mm film camera with Kodak Ektachrome 64 color slide film and a Panasonic 3-CCD color video camera to be used simultaneously.
After each day of flying, the video and digital camera data were reviewed to assure that the coverage and image quality were acceptable. The digital camera images were saved in a TIFF file format using a 486 PC and Aldus Photostyler software. The image data was then exported to the MicroImages TNT Map and Image Processing System. Color and contrast enhancements were applied as necessary to the image data to highlight vegetation types. Interpreting vegetation types is an important part of mapping riparian ecosystems. Figure 1 displays a digital camera image covering a portion of an alpine meadow and riparian area. Vegetation types, such as sagebrush, conifers, willows, and grasses can be identified and interpreted from the digital image.
Individual digital camera frames were georeferenced using control provided by an orthophotograph. Although the most recent orthophotograph was created using 1976 aerial photography, we were able to locate enough features, such as rock outcrops and trees, common to both the orthophotograph and digital image to use as control points. After georeferencing, a sequence of overlapping digital camera frames were resampled and mosaicked to create a composite raster image. The composite raster image can be used to digitize GIS vector data and show where vegetation types or stream channel locations have changed. Figure 2 displays a georeferenced raster image composed from three overlapping digital frames. The vectors from the existing GIS data base overlaid onto the composite raster image show the location of the road in blue and active stream channel in red. The GIS data was digitized from a 7.5 minute quadrangle USGS map which was updated in 1987. A significant difference in the location of the current active stream channel and the GIS data can be seen. Overlaying periodic GIS information over georeferenced digital image data can help fisheries biologists and hydrologists determine trends in hydrologic changes and identify potential mitigation measures. Results and Conclusions
The combination of cameras used in this project provided valuable, complimentary imagery for riparian mapping and also highlighted some interesting advantages and disadvantages of each system. The digital camera proved to have very good sensitivity, dynamic range and linear color response. The Kodak DSC 420 has 36 bit color depth which is especially useful for high contrast scenes such as an overhead view of a forested area with brightly illuminated tree canopies and ground vegetation in deep shadows.
The digital camera provided good quality imagery for both the bright and dark areas while photographic film and video had a tendency to either over or under expose these same areas.
The ability to process and view the digital camera data using a PC and image processing software soon after a flight was very helpful. After reviewing digital image data, camera settings and flight line positions could be adjusted to improve image quality and area coverage. The digital format also makes it relatively easy to construct linear composite mosaics necessary for stream channel studies.
A current limitation of the Kodak DCS 420 digital camera for remote sensing is data storage. A 130 MB PCMCIA hard drive stores 78 digital images. When flown at an altitude of 3,000 feet above the ground with a 35 mm focal length lens, the 78 frames with 20 percent overlap cover approximately 9.5 linear miles. The Hasselblad 70 mm camera also shares this limitation. A large film back holds enough film for approximately 70 frames. The Hasselblad with a 80 mm focal length lens can cover 22 linear miles at an altitude of 3,000 feet above the ground with 20 percent overlap. It is possible to change a Kodak DCS 420 PCMCIA hard drive or Hasselblad film back while mounted in the camera bay during a flight, but making the switch can be difficult, especially when flying during turbulent conditions.
Video cameras have several features useful for conducting aerial surveys. Video tapes are inexpensive and can provide continuous coverage of the all the flight lines. When flying at 3,000 feet above the ground at a speed of 90 miles per hour, a two hour S-VHS video tape has the potential of recording approximately 180 linear miles.
Another important feature is the ability to view the video imagery as it is being recorded during a flight. This allows the operator to make adjustments to camera and lens settings, and advise the pilot if a course correction is needed to maintain position over a flight line. The audio portion of a video tape allows the operator to record flight information and position. It is also easy to annotate video imagery with GPS coordinates to document the position of the aircraft along the flight lines.
Both video and digital cameras perform well under a variety of lighting conditions and can provide useful imagery under conditions which would normally preclude the use of film cameras. The video camera did not perform as well as the digital camera when imaging high contrast scenes which require a wide dynamic range.
Video provides much less spatial resolution than small format aerial photography or the Kodak DCS 420 digital camera. The S-VHS video format records approximately 410 lines of the interlaced video signal. A typical video capture system can digitize and resample a single frame to a 480 x 512 pixel resolution. In comparison, a single Kodak DCS 420 digital image, which is non- interlaced, has more than six times the pixel resolution of a digitized video frame.
There are many potential remote sensing applications for digital camera systems. A major advantage of digital cameras is the ability to quickly acquire a high quality image in a digital format compatible with commercial image processing and GIS systems. It is not likely that current digital cameras, which are best suited for small projects due to limited storage capability, will replace other remote sensing systems such as conventional aerial photography or airborne video. However, they can provide valuable imagery to complement other remote sensing data for resource applications such as environmental monitoring and updating existing GIS data bases. About the Authors:
Tom Bobbe is a program leader with the USDA Forest Service Nationwide Forestry Applications Program located in Salt Lake City, Utah.
Jim McKean is a geomorphologist and geotechnical engineer with the USDA Forest Service in Pleasant Hill, Calif.
Note: The mention of commercial firms or products is for clarity and identification of procedures and methods only, and no endorsement is implied by the authors or the USDA Forest Service. Back |
