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Airborne: The Airborne Advantage
Digitally captured aerial photography's value and utility meet new application challenges in GIS and remote sensing.
By Bruce Burger Ambitious project goals requiring increased information from mature GIS environments now target efficient, cost-effective use of digital image data. Complementing the availability of accurate film-based digital orthophotos, high resolution color, color infrared and multispectral images acquired with todays commercial digital aerial photography systems competitively meet these growing needs.
The increased interpretability offered by color imagery often dictates its use within diverse decision making arenas. The prospect of widely available, easily obtainable 1 to 4 meter panchromatic and multispectral imagery from orbiting satellite platforms continues to arouse the imagination of those hungry to include multiple image layers within their GIS applications. However, in keeping the glamour and intrigue associated with space-based digital sensors in perspective, the GIS and remote sensing user communities should not forget the quiet industry that has evolved and has consistently provided high resolution digital imagery for many years. Although not marketing with quite the zeal of the high resolution satellite companies, professional aerial mapping firms have continually met a steady demand for digitized aerial photographs captured with high precision film cameras.
Today, digital framing cameras requiring no film or film processing offer new capabilities to cost-effectively acquire black and white, color, color infrared and multispectral aerial photographs directly in digital format for use within GIS and image processing systems. Commercially available systems such as the ADAR Digital Aerial Photography System from Positive Systems currently provide reliably engineered, innovative solutions for capturing hundreds of digital photographs at .50 to 3.0 meters per pixel over large project areas. Without waiting for technology planned for tomorrow, these systems now deliver high resolution, GIS-ready, digitally captured imagery on a daily basis. This imagery is meeting the needs of emerging real world applications...today. Combining New Tools with Proven Platforms
State of the art digital cameras utilize light-sensitive computer chip (CCD) technology to record photographic images digitally through an array of pixels rather than conventional film emulsion. Todays cameras such as the DCS series manufactured by Eastman Kodak Corp. provide from four to 16 times the ground area coverage of digital video sensors in airborne applications. Ground coverage, or footprint, translates directly into the economies of flight which very often determine an applications ongoing commercial viability.
Camera sensors incorporating from 1.5 to 6 million pixels create a digital image similar in visual acuity to that offered by film. Spatial resolution, or ground sample distance (GSD) for each individual pixel is based upon the cameras distance above ground, with typical altitudes ranging from 3,500 to 22,000 feet when utilizing a 20mm focal length lens. When integrated within a comprehensive computerized acquisition system incorporating software and other technologies such as GPS, digital cameras combine with the dependability of proven aircraft platforms to address numerous imaging applications.
The characteristic features differentiating film and digital aerial photography define the inherent benefits of each. While both may be suitable for some applications, today they are more typically complimentary technologies rather than competing alternatives. Continued advances in proven photogrammetric processes developed for, and best suited to scanned film and film-based cameras still make them the undisputed choice for high resolution, high accuracy digital orthoimages that meet todays traditional engineering and precision mapping needs. However, digital cameras unique characteristics and capabilities make them the emerging choice for many other well known and newly evolving applications Ñ especially those where flexibility, turnaround time, key information extraction, and rapid delivery are critical. Advantages for Specific Application Needs
Digital aerial photography systems include from one to four separate cameras. Distinctive capabilities associated with direct digital capture provide several critical benefits including reduced operational costs, rapid delivery, expanded informational content, and greater flexibility in meeting specific application needs.
Eliminating and Empowering. The absence of film eliminates problems associated with film emulsion batches and the potentially troublesome effects of temperature and humidity that can plague color and especially CIR film operations. The elimination of film processing also provides CIR capability that may otherwise be cost prohibitive due to bulk chemical purchase and costly disposal requirements imposed by environmental regulation in many states. In international locales where color or CIR film processing alternatives are non-existent, digital acquisition provides security and control over proprietary photography data, as there is no need to ship film beyond the countrys borders for developing. In Sabah, Malaysia for example, the only in-country processing alternatives available are for black and white film. Though this film is utilized to the extent possible, critical application areas exist in agriculture, forestry and fisheries. Digital aerial photography provides an easy to implement, secure, cost-effective mechanism for the required color, CIR and multispectral components necessary to address these applications.
Rapid Delivery. Digital aerial photographs, depending on the design of the computing system surrounding the camera sensors, are available for viewing moments after exposure. This near real time feature allows in-flight quality assurance of exposure and focus parameters, verification of accurate target coverage, and analysis of overall mission success before leaving the project site. Costly re-flights and re-mobilization are eliminated, and capture of sites offering one-time only acquisition opportunities is more assured.
Systems such as ADAR capture images directly to hard disk, which can be removed hot from the aircraft upon landing to meet immediate needs for further processing in a ground station environment. This is specifically suited to newly emerging precision farming and site specific agriculture applications. Like the crops being harvested, the information content of the imagery itself is perishable. Photographs that are more than 24 to 48 hours old lose most of their value to the cash crop grower. Far beyond the conceptual promise of 24-hour turnaround touted by other technologies still under development, frame-based digital aerial photography systems have already anchored ongoing for profit production remote sensing operations. A commercial example has been repeated by an agriculture service bureau over numerous growing cycles utilizing an ADAR System 5500 multispectral sensor and standard aerial photography aircraft. Operational objectives have included acquisition, processing and delivery of crop status imagery covering over 1 million acres throughout California, Arizona, and other western states. Delivery of images highlighting crop anomalies to growers, including hardcopy, is achieved within 48 hours, at annual costs as low as $10 per acre.
Information Content. Digitally captured CIR and multispectral images typically offer superior spectral fidelity when compared to images scanned from CIR film. Scanned images often exhibit critical loss of dynamic color range as a byproduct of the scanning process. This advantage makes digital aerial photography better suited to environmental and natural resource applications where the imagery is classified through image processing software for wetlands analysis, resource inventory, land use/land cover analysis and site regulatory compliance.
Advantages of Airborne. Whether utilizing film or digitally-based cameras, it seems that despite decades of dependable service as a stable, reliable, recoverable remote sensing platform, the allure of more glamorous possibilities leaves workhorse aerial photography aircraft toiling with no respect. However, as managers continue to seek tools providing better, faster information for more effective decision making in an era of downsized field resources, digital aerial photography clearly demonstrates the flexibility to deliver viable cost-effective digital solutions. In many cases, as was recently demonstrated by a major power grid failure affecting the entire western U.S., increased public safety is attainable through availability of more complete information related to external plant and facilities. In this case, better information related to vegetation monitoring within a transmission corridor may have prevented the causes leading to the power outage.
Underground natural gas pipeline classification also presents a compelling need where gas companies can rely on digital aerial photography and change detection tools to save both time and money. Mandated by the U.S. Department of Transportation and defined by a rating structure dictating transmission rates, line classification is categorized by residential building density within a stated distance either side of the buried pipeline. Requirements for this type of monitoring and change detection demand flexibility, repeatability, control over timing and ability to manage image acquisition windows as weather conditions change.
Corridors, be they pipeline, transmission, waterway, railroad or others are defined areas of activity where changes are continuous and monitoring needs frequent. Digital aerial photography systems are uniquely suited to monitoring long linear features such as corridors due to the size of the sensors pixel array, changeability of camera orientation, rapid delivery, simultaneous true color and multispectral operation, and the operating capability of the aircraft. Innovations in Advanced Post-Flight Processing
Digital aerial photography systems deliver image data in industry standard file formats on 8mm tape cartridge or CD-ROM. Imagery can be delivered as individual scenes, each covering from less than .20 to over 20 square miles depending upon the sensor utilized and spatial resolution per pixel as defined by altitude.
Until recently, mosaicking and georeferencing the images for use in GIS has required use of the same photogrammetric processes developed for orthorectifying scanned film images. The resulting orthoimage mosaics, while excellent in spatial accuracy, were costly to produce in the context of their planned utilization. Significant technical differences between digital and precision film cameras make utilization of these traditional tools and methodologies much less efficient for digitally captured aerial photos.
Today, innovative software driven processes such as ADAR AutoMosaic totally automate the arduous task of individually mosaicking each digital scene. By removing human induced subjectivity and exploiting the power of todays microprocessors, stronger mosaic geometry and better scene to scene registration are possible without applying terrain or camera look angle adjustments. Deliverable within days, AutoMosaics are readily rectified to GIS vectors or existing orthoquads. They are particularly useful for areas with minimal terrain relief, and monitoring applications where low cost requirements, rapid delivery and picture content outweigh the need for precise pixel location.
As computer processing power continues its seemingly endless upward spiral, tools enabling automated change detection capabilities are adding yet another potential facet to the multiple uses expected from GIS. Trainable software applications are emerging to take advantage of inexpensive CPU cycles, comparing one image to another while condensing their differences to discrete, reportable information. Tools like AutoMosaic will play an increasing role in the automation of change detection applications in the future.
New methodologies also exist for production of digital orthomosaics via photogrammetric processes geared towards the unique advantages and characteristics of digital cameras. Produced to National Map Accuracy Standards, these orthomosaics are deliverable within a fraction of the turnaround time typically associated with film-based digital ortho processes. Applications such as E911 mapping and address updating look to benefit from these new AutoMosaic and orthomosaic post-flight processing capabilities. Future Trends
In the future, expect advances in digital aerial photography acquisition systems to include advanced flight planning, new camera control capabilities, and streamlined image management utilities designed to improve flight operations, further reduce acquisition costs, shorten delivery time and consolidate post-flight processing. Analogous to memory chips, digital sensor arrays will continue to increase in size further improving economies of flight and making new applications cost justifiable. In addition, look for smaller, easier to use, less expensive digital acquisition systems to emerge, designed for specific end-user applications. Potentially installable in hundreds or thousands of aircraft worldwide, these systems will meet the unique widespread demands driven exclusively by needs for better information to improve decision making. Summary
As the predicted boom generating imagery for the masses becomes reality, a myriad of new remote sensing and imaging technologies both currently available and predicted are coming into more rapid use on a larger scale.
Many early adopters of new technology products initially pursue the most highly technical, fully featured, maximum implementation available. This is understandable given that first generation products are more expensive than their later generation counterparts, and the initial need and instinctive desire is to get the most bang for the dollar. It is important to note however, that as applications mature and the technology addressing them becomes proven, the economies required for these applications to proliferate dictate that minimum requirements be identified and specified in order to keep costs down Ñ costs that become categorized as ongoing expenses rather than initial research investments. It is becoming increasingly important for users to evaluate and acquire application data based upon only the minimum features required for their projects rather than maximum features available. Translated to digital image data, this means the minimal number of spectral bands, coarsest acceptable spatial resolution, minimum acceptable level of horizontal accuracy, etc. required to deliver information that is acceptable within given budget constraints.
Better, faster, cheaper and uniquely suited for specific applications. New advances, particularly in the area of more automated, lower cost image mosaicking and georeferencing will continue to increase utilization of digital aerial photography in remote sensing and GIS applications. As with all viable technologies, this one will continue to improve tomorrow. But unlike many, it is commercially available today as a proven cost-effective alternative for applications that cannot, should not, or will not wait. About the Author:
Bruce Burger is a regional sales manager for Positive Systems, Inc. in Whitefish, Mont. He may be reached at 406-862-7745 or by e-mail: [email protected] The author wishes to thank the Positive Systems team, whose assistance and extra effort greatly contributed to the completion of this article. Much of this work was supported by NASA through the EOCAP program.
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