Airborne
The Value of Historic Imagery
By Terry Slonecker, Mary J. Lacerte, and Donald Garofalo

In the dynamic world of remote sensing applications, many of the primary uses of imagery (such as map-making or military reconnaissance) rely on the most up-to-date imagery to provide the best possible information for product-development or decision-support purposes. Indeed, many professionals in the remote sensing field naturally equate 'most recent 3D' with 'best 3D' when planning and acquiring imagery for one of the many growing applications of remotely sensed data. Often overlooked, however, is the often invaluable information that can be derived from historical imagery.
    Imagery archives now span several decades and, in some cases, can go back 50 years or more. These valuable records allow us to assess conditions as they were when the image was acquired, no matter how much change has taken place since that time. The implications of the use of historical imagery can be significant.

The History
The first known aerial photographs were taken in 1858 on board a tethered balloon by a Frenchman named Gaspard Felix Tournachon. Tournachon, who later became known as Nadar, successfully hotographed the landscape around Paris, France. Shortly thereafter, the aerial perspective proved to be so valuable that General George McClellan used tethered balloons to photograph and study enemy positions in the United States Civil War. From the mid-1800s to the early 1900s, balloons, kites, and even pigeons were used as the platforms to hoist cameras above the land to photograph the surface below. But these platforms were relatively stationary, limited in altitude and range, and vulnerable.
    Shortly after the advent of the airplane in 1903, the value of aircraft-based photography became readily apparent to many, including Wilbur Wright who took the first recorded photographs from an airplane in 1909. The airplane soon became the preferred platform for overhead imaging. Airplanes had much greater flexibility in terms of accessing denied areas in varying scale, altitude, and resolution requirements for specific purposes. Regular use of cameras from airplanes continued until World War I, when the formal development of reconnaissance systems and photographic interpretation science became so developed that they completely changed the tactics of battlefield warfare. Between World War I and World War II, the continuing development of both aircraft and photographic technology made the use of aerial photographs commonplace for military and intelligence applications, domestic mapping, planning, and natural resource management.
    However, the development of imaging platforms did not include aircraft platforms only. In 1906, a Saxon engineer named Alfred Maul successfully launched a camera aboard a small compressed-air rocket that rose to an altitude of 2,625 feet, where it took several pictures before returning to Earth by parachute. Although the concept of rocket-based photography never gained any significant interest, imagery acquired by Earth-orbiting satellites certainly did, and earlier than many people realize. Recently declassified images and data from the CORONA program show that satellite deployment and imaging from space was accomplished as early as 1960 by the US intelligence community.
    Beginning in 1972, civil satellite remote sensing was born with the launch of the Earth Resources Technology Satellite (ERTS-1), later renamed Landsat. To this day, the Landsat program continues to provide global multi-spectral digital imagery through the use of the two primary medium resolution sensors, the Multi-Spectral Scanner (MSS) at 80-meter pixel size, and the Thematic Mapper (TM) at 30m pixel size. Eventually, other countries began to enter the satellite remote sensing market; most notably, the French SPOT and the Indian IRS programs launched in 1986 and 1988 respectively. These programs have collected large archives of historical imagery. Since the early development of the Landsat systems, a number of other satellite imaging systems have been launched including weather satellites by the National Oceanographic and Atmospheric Administration (NOAA), radar satellites by Europe, Canada, and Japan, as well as other imaging systems. Like the US, Russia has declassified some of its imagery holdings and sells historical imagery commercially through the SPIN-2 corporation.
    It should be noted that many of the current uses of historical imagery were NOT envisioned during the time of the imagery acquisition. Indeed, imagery in general provides us with a frozen moment in time that may provide us, in the future, with a unique historical view of the world at that time. The future uses of historical imagery may yet be unforeseen, underscoring the value of preserving and protecting irreplaceable imagery archives.

Change Detection
Changes in landscape patterns and processes over local to regional areas tell us much about the quality and condition of our environment, and the relative influence of man versus natural processes as causative factors for these changes. Ecological researchers study landscape patterns and change and their significance as indicators of environmental quality. Remote sensing, in the form of satellite-acquired data, has been a major source of regional landscape information. Historical satellite data going back to the 1970s provides landscape change information over broad regional areas. Historical aerial photographs, on the other hand, having a much longer temporal history than satellite-derived data, offer the potential for more detailed landscape ecological assessments. In addition, they provide a much longer time frame for assessing the magnitude of change and for predicting change based on long-term trends.

Archaeology
Imagery, especially aerial photography, can capture fragments of what life was like long before the camera was invented. The landscape often shows subtle clues as to what people centuries ago did and what they left behind. Historical imagery can record obvious protruding cultural features, subtle indicators laying just below the surface of the ground, or identify potential cultural site locations based on general landscape relationships.
    The cultural remnants of the Pueblo people of the American southwest have been studied using aerial photographs since the 1930s. Many of these ruins are obvious structures and are easily identifiable on film. Aerial photography provides a non-destructive method of studying ruins. The science of photogrammetry provides a mechanism for precise measurements and mapping of these ruins, used independently or in concert with more intrusive methods of study. When aerial photographs are taken at regular intervals during ground studies or excavations, invaluable information can be preserved on film and utilized to recreate and accurately map the site, thereby eliminating the need for extensive ground measurements.
    Historical imagery can also aid in identifying potential sites for discovery of new ruins. Communities develop where certain essential elements can be found in close proximity. These elements, necessary for survival, include: water (reliable), food (either wild or domesticated/cultivated), shelter from enemies and weather, access to transportation routes, and a favorable climate.
    An excellent example of historical imagery aiding archaeological investigation can be found in the discovery of the ancient city of Ubar in present day Oman. Ubar is a legendary Arabian city that was buried beneath the sands by Allah when its inhabitants became wicked and greedy because of their great wealth. References to Ubar and its lost riches have appeared in literature for thousands of years, and during the 20th Century, many archaeologists attempted to find its location. In 1991, an international, multi-disciplinary team of scientists discovered the location of Ubar, largely based on information from imagery acquired by the Landsat TM, the Shuttle Imaging Radar (SIR-B), and the SPOT satellite.

Hazardous Waste Site Analysis
Imagine this scenario: You and your spouse bought a house and started to raise a family. As time passes, more and more of the neighborhood residents seek medical attention for mysterious illnesses. Soon, a pattern begins to develop in the sicknesses and the numbers are statistically more than a mere coincidence. The drinking water is tested and is found to be contaminated, but the possible source of the problem is not apparent. Unfortunately, this is not just a hypothetical situation, it occurs all too frequently in communities around the world. However, there are ways to determine the possible source of the contaminants. Aerial photography provides a record of how the landscape and its human usage has changed through the years. It can answer questions like: What was there before the houses were built? Was there a chemical plant nearby years ago? Was hazardous material dumped, or has it potentially leaked from containers and found its way into the drinking water?
    The US Environmental Protection Agency (EPA) is responsible for cleaning up hazardous waste-disposal sites, including the removal of materials that represent an immediate or potential threat to the environment and to the health and safety of surrounding populations. Under the Superfund program, EPA assesses and remediates abandoned hazardous waste disposal sites. Many of these have been abandoned for years, and often the presence of a former waste disposal site at a specific location has been completely obscured through burial activities and through natural processes of wind and water erosion and re-vegetation. A review of landscape changes over time using historical aerial photographs has been a very effective method for identifying the location of suspected abandoned sites, for determining when waste disposal activities began, their duration, and time of cessation. More importantly, the value of the historical aerial photograph for determining the precise location of specific waste disposal activities and features such as trenches, excavations, waste drums, or munitions burial, and areas of apparent spills of unknown substances onto the ground surface is extremely valuable information for guiding site-cleanup teams in safely removing hazardous materials. Determining the location of these buried features using ground-based methods would be less precise, significantly more time consuming, and substantially more expensive.

In the Courtroom
Historical imagery has proven to be a powerful form of evidence in many formal legal proceedings. Imagery has been used to demonstrate various landscape conditions, such as the cultivation of illegal drug crops. Its use for law enforcement and regulatory compliance monitoring has been approved by the US Supreme Court.

Where to Acquire Historical Imagery
Historical imagery, like many other historical documents, can be found in a variety of places and circumstances. There is no comprehensive one-stop source, and often, only good-old-fashioned research will ferret out the holdings of the imagery that may meet the specific needs of your particular project. Some government agencies and commercial vendors that have historical aerial imagery include: Aerial Photography Summary Records System (APSRS); US Geological Survey (USGS) EROS Data Center (EDC); US Department of Agriculture's Farm Service Agency, Aerial Photography Field Office (APFO); National Archives and Records Administration (NARA), Still Pictures Branch (SPB) and Cartographic and Architectural Branch (CAB); US Library of Congress (LOC); Space Imaging; Orbital Imaging Corporation (ORBIMAGE); Core Software Technology; American Society for Photogrammetry and Remote Sensing (ASPRS); and many state, local, and quasi-government organizations that routinely acquire imagery for planning, mapping, management, and monitoring purposes.

The Future
The information contained in the vast holdings of historical imagery has already made many meaningful contributions to scientific discovery and resource management. This will no doubt grow as the rapidly expanding world of remote sensing technology creates unprecedented volumes of overhead imagery from a variety of sensors and platforms. Increases in spatial and spectral resolution will spawn many new and unforeseen uses of imagery.
    Creating permanent, digital archives of existing historical imagery collections should be a high priority for the remote sensing and scientific communities. In concert with the acquisition of imagery, comprehensive and searchable metadata files must be generated and maintained. Further, great care should be taken to preserve this capability by creating reliable physical storage systems to ensure efficient access to these new imagery resources.
    Historical imagery is a unique and irreplaceable resource. We should always keep in mind that imaging scientists have a unique capability to view the past and we, as a community, should take great care to preserve this resource for future generations.

Notice
The US Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), funded and performed the research described here. It has been subjected to the Agency's peer review and approved by EPA for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

About the Author:
Terrence Slonecker, Mary J. Lacerte, and Donald Garofalo work at the US Environmental Protection Agency.

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