Introduction
Color infrared (CIR) aerial photography is widely used for a variety of natural resource interpretation and mapping applications. Although CIR film was developed primarily for camouflage detection during World War II, the combination of green, red and near infrared spectral information has proven to be valuable for distinguishing subtle differences in vegetation species and condition. A new development in electronic imaging has provided another method for collecting CIR imagery. Recently the USDA Forest Service, Nationwide Forestry Applications Program evaluated a CIR digital camera system for remote sensing applications.
      Digital camera systems are an emerging electronic imaging technology that has excellent potential for natural resource remote sensing applications. Digital cameras can provide high quality imagery in near real-time to complement the use of other remote sensing imagery such as conventional aerial photography, airborne video and satellite imagery to collect resource information. Currently several panchromatic and natural color digital camera systems are commercially available. The Nationwide Forestry Applications Program has evaluated the Kodak DCS 200 and DCS 420 natural color digital cameras for resource interpretation and GIS data collection. Based on the success of these preliminary tests, the Nationwide Forestry Applications Program asked the Eastman Kodak Company to develop a CIR version of the digital camera. Features considered important for remote sensing applications include: (1) the camera system should consist of a single camera body and lens assembly, (2) the camera needs to be rugged, easy to use and relatively inexpensive, and (3) the camera should include a GPS interface to reference imagery with position data.

System Description
During 1994, Kodak developed a CIR digital camera system for the Forest Service by modifying the camera firmware, sensor, and software driver of a natural color DCS digital camera. A DCS 200 camera was modified to create the first CIR digital camera system. Subsequent versions of the CIR digital camera will use the newer DCS 420 digital camera. Both the DCS 200 and 420 digital cameras use a high resolution 1,012(h) by 1,524(v) charge coupled device (CCD) for imaging. The primary difference between the DCS 200 and 420 camera systems is dynamic range and image storage. The DCS 420 has 12 bit dynamic range while the DCS 200 has 8 bit dynamic range for each spectral band. The DCS 200 uses an 80 MB internal hard drive that stores 50 image files. The DCS 420 uses removable PCMCIA hard drives that are available with different data storage capacities. A 200 MB PCMCIA hard drive can store 133 image files. The DCS 420 can also obtain images in a rapid, continuous sequence, which is an important feature for stereo overlap.
      The CIR digital camera system obtains image data in the green, red and near infrared spectral range by using a modified CCD and a band pass filter attached to the camera lens. A band pass filter ranging from 500 - 800 nanometers is used to limit the spectral response to green, red and near infrared reflected light. Although the CCD can be used to collect image data up to 1.0 microns in the near infrared, we found that for many resource applications it is preferable to limit the near infrared band to 800 nanometers. Different band pass filters can be defined for specific imaging requirements.
      The CIR digital camera system provides a 24 bit color composite image through a PC or Macintosh computer with image processing software. A TWAIN driver combined with image processing software such as Adobe Photoshop or Aldus Photostyler accesses image data and does the color conversion. Each spectral band is assigned to a primary color and displayed on a computer monitor. The color conversion process assigns the near infrared spectral band to red, the red spectral band is assigned to green and the green spectral band is assigned to blue. This spectral assignment creates a composite color display similar to CIR photography. GPS position is stored with the image data for general position referencing. A serial data port receives GPS data in a standard NMEA GPS data format. The TWAIN driver stores GPS data with the image file. GPS data is displayed in the camera information block along with the date, and camera exposure data.

Preliminary Test Results
The camera was evaluated by collecting imagery on the ground and from airplanes. Figure 1 displays a natural color digital image taken of a forested area in Utah during late fall. The scene includes a combination of white fir (Abies concolor) and quaking aspen (Populus tremuloides) exhibiting their fall coloration. Figure 2 displays a CIR digital image while figure 3 shows a CIR photograph taken with a 70 mm Pentax camera using Kodak 2443 film. CIR imagery is considered more useful than natural color imagery for highlighting the spectral differences between healthy and dying vegetation. The timber stand of white fir includes several dead trees that are less distinct in the natural color image. In the CIR imagery the older dead trees appear gray, the recently killed trees are yellow-green, and the healthy coniferous trees are dark red. Healthy deciduous vegetation normally appears bright red in CIR imagery. However, since the quaking aspen trees are beginning to turn color they appear white or light pink in the CIR imagery.
      An aerial CIR digital image of a riparian site in northern California is displayed in figure 4. The right side of the image includes a recent burn area. Although this image was taken under a dark, overcast sky, the digital camera performed quite well and provided an image useful for delineating burned areas. The burned vegetation appears as blue-gray color while the healthy deciduous vegetation appears bright red. The digital CIR camera can support fire recovery projects that need aerial imagery quickly to access fire damage and identify areas requiring erosion control or other mitigation measures.
      CIR film provides greater spatial resolution than the digital camera. However, the spectral response of the CIR digital camera system closely matches CIR film. Image analysts who have many years of experience interpreting color and CIR aerial photography reviewed the digital CIR imagery. The consensus was that the digital camera provided imagery spectrally very similar to CIR photography. They also made the observation that the CIR digital camera provided imagery with better interpretability under adverse lighting conditions.
      CIR film normally requires special handling and processing to obtain reliable and consistent results. CIR film has very narrow exposure latitude. If the camera exposure is not within 1/2 f-stop of the optimum setting, normal film processing will produce less than desirable results. The digital camera however, provides more latitude in exposure settings. Digital cameras have greater dynamic range and provide more flexibility in dealing with poor lighting conditions and incorrect exposure settings. An individual digital camera file can be adjusted with image processing software to correct color balance and brightness.
      A major advantage of digital camera systems is the ability to use the data directly with image processing and GIS systems. Image data can be accurately georeferenced by transferring geodetic control from orthophotographs, maps or previously controlled aerial photography. Image data can also be rectified using softcopy photogrammetry systems.

Conclusions
Digital camera systems will play an important role in providing resource managers with timely and accurate remote sensing data. A color infrared digital camera system is a unique remote sensing capability that can support a variety of ecosystem management applications such as monitoring forest health, riparian condition, and fire recovery. In addition, a color infrared digital camera system will complement the use of other remote sensor systems for updating GIS data bases, and performing sample surveys and accuracy assessments.

About the Authors:
Tom Bobbe is a program leader with the USDA Forest Service Nationwide Forestry Applications Program located in Salt Lake City, Utah. He can be reached at 801-975-3663. Joe Zigadlo is an engineer working in business development and custom imaging applications with the Eastman Kodak Company located in Rochester, New York.
Note: The indication of commercial firms or products in this article is for the convenience of the reader. No endorsement is implied by the USDA Forest Service.

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