Searching for historical waste management records is often an exercise in futility. Chemical handling and disposal activities were rarely, if ever, recorded by industry or government facilities prior to the mid-1970s. As an environmental professional, you must rely on alternative resources to identify objects and judge their significance.
Aerial photo interpretation -- the act of examining photographic images to identify objects and judge their significance -- is a site characterization tool that can provide valuable insight into a site's past disposal practices and chronology of activities. Information derived from aerial photographs is unique because it provides a visual record or a "snapshot in time" of historical site events or conditions. Although documents and eyewitness accounts, if available, can provide similar information, they are also notoriously imprecise, inaccurate or erroneous.
Take the authors' recent experience for instance. Client documentation indicated that wastes had been historically disposed in several trenches and sinkholes within his facility. Geophysical investigations of these reported disposal sites yielded no evidence of such units. Historical aerial photography acquired soon thereafter revealed that the disposal areas were over 100 yards east of their "documented" locations. Additionally, an unreported liquid disposal pit and large open burning area were also identified from the photography in an adjacent tract. In this instance, a simple up-front review of inexpensive aerial photographs would have saved valuable time and money in developing an understanding of site conditions.
In another reference to the utility of aerial photos, the U.S. Army Corps of Engineers' Albert Reyes relates, "Historic aerial photographs are essential for evaluating changes in site conditions over time and pinpointing suspect areas where environmental investigation is necessary, particularly in the absence of a written historical record." The Corps has also been able, in several instances, to alleviate regulator concerns regarding alleged disposal operations by producing aerial photos proving the concern/assumption incorrect.
Little guidance is available to environmental professionals on how to obtain and conduct aerial photographic analyses. Although a novice may be able to identify some useful information from a photograph, an experienced photo interpreter or "PI" can frequently extract a wealth of useful data. Eugene Avery elegantly described this difference between photo interpretation and photo reading in his book, "Fundamentals of Remote Sensing and Airphoto Interpretation" (1992) as follows:
"Photo interpretation is defined as the process of identifying objects or conditions in aerial photographs and determining their meaning of significance. This process should not be confused with photo reading, which is concerned with only identifications. As such, photo interpretation is both reading the lines and reading between the lines."
Obtaining photographs
Since the late 1930s, federal agencies have extensively photographed our nation from the air. The single largest resource for tapping into this wealth of information is the U.S. Geological Survey's (USGS) Aerial Photography Summary Record System (APSRS). APSRS is the authors' standard reference for identifying potential photographs of interest. Several federal, state, and municipal agencies, as well as commercial aerial survey firms, contribute to the APSRS database. Available photographs are linked to USGS's line of 7.5-minute topographic map products. Once you locate your project site on the appropriate 7.5-minute map (or maps), records of available coverages can be reviewed. Each APSRS record describes the specific attributes of the aerial photographs, including the agency (or commercial) source of the photos, date, scale, film type (e.g., B&W, color), and percent cloud cover. Several limiting factors inherent to aerial photographs must be considered when acquiring coverage. Limiting factors including seasonal attributes (foliage, snow and ice), and atmospheric conditions can obscure site features or activities.
Acquisition costs and "turn-around" time involved in securing relevant photography varies depending on the source of photographs to be acquired. Typically, coverage from public archives (federal and state agencies) can be purchased inexpensively for about $10 to $15 each, but can take as long as six weeks to acquire. Conversely, private aerial survey holdings are usually accessible within a week, but are much more expensive to acquire (anywhere from $30 to $100+ each). Photographs may also be available from other sources including: your client, college, university, and private collections; or local historical societies/museums. These sources may take some "sleuthing" to identify and materials are generally less comprehensive. However, it may document unusual or significant events (e.g, accidents, facility "glamour/PR shots").
Other important acquisition considerations include the format and scale of the photography you wish to acquire. Paper print "contact photos" are commonly recognized by many environmental professionals. Film transparencies or "diapositives" are generally favored by professional PIs. Regarding scale, ideally all photography should be acquired at 1:24,000 scale or larger (i.e., more magnified) to show adequate detail and provide necessary spatial coverage. Regulatory agencies commonly request temporal coverage of five- to ten-year intervals.
Photo interpretation
As discussed above, even a simple review or "eyeballing" of aerial photographic prints can yield useful information. However, many investigators mistakenly believe that a cursory review is all that is needed to meet project/client needs. Although cost is always a driving force in conducting environmental investigations, a few well-placed dollars invested in a professional review of relevant aerial photographs can pay dividends by reducing unnecessary project expenditures. It is recommended that your initial site characterization activities include availing yourself of the services and expertise of an experienced PI.
Before beginning any investigation, a skilled PI should sit down with you and listen to the objectives and needs of your project. Thoroughly understanding your needs and not presuming to know your requirements in advance should be his/her primary objective. Once these needs are discussed and understood, the deliverable product can be developed.
The interpretive process generally begins with a thorough evaluation of the photographs. Interpretation of the aerial photography generally involves using diapositive images and a backlit stereoscopic instrument specifically designed for viewing aerial photographs. The three-dimensional effect created from stereoscopic viewing of the photos, when coupled with the magnifying capabilities of the optical instrument, provides the interpreter with useful spatial and dimensional information unavailable from a simple "eyeball" survey of the photography. This three-dimensional perspective is especially useful for identifying natural features such as topographic relief, drainage, and vegetative cover, as well as understanding anthropogenic features.
For example, determining whether a dark-toned feature seen in a photograph is a pile of coal, a body of liquid, or a pit is often impossible without use of a stereoscope. To provide historical site perspective, the analysis is generally conducted in a chronological fashion. It is also prudent to acquire coverage of the site in its agricultural or pre-development state. This sequencing of several years of coverage is an effective means of gaining insight into the order of events and conditions that occurred within a given facility. Additionally, evaluating properties adjacent to the target site is useful in determining whether environmental impacts beyond the property might have had a contributory effect on site conditions.
Accurately locating areas of interest on the ground once identified from the photography is an additional consideration. Changing site topography or physical improvements can frustrate any field verification ("ground-truthing") survey. Knowledge that an activity of potential environmental concern has occurred is of limited value unless the location can be accurately located and sampled as necessary. In many cases, simple geo-referencing of the photography — digitally overlaying the historic aerial photographs on USGS digital orthophoto products and matching common features (e.g., building corners, roadway intersections) — can be utilized to provide survey accuracies (five- to ten-meters) that are generally sufficient for most environmental applications. More sophisticated geo-rectification procedures can be employed if greater accuracies are required.
Interpreting and understanding conditions captured in aerial photographs are heavily dependent on the analysts' experience and careful evaluation. Both direct and indirect indicators of operational activities must be considered. Direct indicators are physical characteristics that are generally associated with anthropogenic or natural features/activities such as drums, trenches and pits, stressed vegetation, ground scars, etc. Indirect indicators are generally intangible attributes or physical properties upon which certain site assumptions and/or inferences can be made. For example, facility housekeeping practices can be cited as an indirect indicator of a site's general condition or level of activity. Topical issues generally considered in most interpretation assignments include, but are not limited to:
- Excavation/ground scarring activities
- Waste dumping and disposal units (including trenches, pits, landfills)
- Fill material (type and appearance)
- Drums, tanks, and/or other bulk containers
- Pipelines
- Surficial discolorations ("staining")
- Release "signature" patterns
- Changes in topography and surface drainage
- Industrial operations (including housekeeping considerations)
- Facility infrastructure (structures, roads, fencing, etc.)
- Impoundments
- Salvage operations
- Stressed/damaged vegetation
- Other (e.g., open burning, storage activities)
Additionally, documenting the negative (conditions that are not present) is also important to demonstrate that a range of potential concerns has been addressed.
The cost of a professionally conducted aerial analysis can vary widely. A fairly simple search and analysis by a skilled PI can run from $2,000 to $5,000, while an extremely thorough site analysis of a large, several thousand acre facility, accompanied by a detailed text presentation, may run as much as $100,000. Key analysis and cost considerations include: the number of exposures required for site coverage, size of the site, number of years to review, sources of the photography (commercial vs. government) acquired, etc. Subsequent reporting considerations include the means this information is conveyed (hard copy or electronically) and its presentation format. Reporting formats vary considerably. One common format involves extensive use of annotated photo enlargements and detailed supporting text to document site conditions. Large area (1,000+ acres) facilities often employ an inventory-style format that allows for general documentation of conditions, then later, more focused analyses on specific sites. Still other presentations can be simple mapping "X marks the spot" products accompanied by little or no supporting text. Your PI should be able to provide you with flexible and creative alternatives that can be tailored to meet your project's budget.
Regardless of the size or particular requirements of your project, early project use of historical aerial photography can pay for itself several times over by reducing initial site characterization activities, eliminating undue sampling requirements, targeting previously unknown areas of potential concern, and documenting historical site conditions for possible litigation support purposes. Availing yourself of a PI's services makes your project more cost effective and efficient and may identify liabilities not even your client knew existed. PE
