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Kinds of Aerial Photography

The term “aerial photography”seems to suggest, simply, a photo taken from the air. However, in this context, its definition is much narrower. Environmental site analysts are very interested in all forms of aerial images of industries under investigation, including Cessna tourist photos, news photos, old advertising photos, etc. However, the heavy lifting of most of the analysis work is done from systematic flights producing thousands of 9x9 inch, vertical airphotos by Government agencies and mapping/engineering companies. There are a variety of justifications for this.

In typical cases, the image quality of these photos is quite good, because they have been run through standardized quality
control processes. In addition, they almost always provide for stereo observation. As discussed elsewhere, this is a huge benefit. Also, at least for Government photography, the DATE of the flight is very firmly established and labeled on every exposure of the negatives, so that almost any product made from them is unmistakably date stamped.

Example 9x9" Aerial Photo

A typical 9x9 inch aerial print. This is the size of most original negatives. The small, red circle near the center encloses the entire site. This size is typical for most sites.

In addition, “Certifications” are available from most agencies. These are notarized documents which attest that the reproduction was made from the original negatives without alteration, except for the normal optimizing of tones or colors. One of the underlying principles here, is that the enhancement is generally done for the frame as a whole, rather than just one piece of it. The one exception to that, is that in some cases a scanning process is used to even out the tones/colors of various parts of the negative to produce a more pleasing print. However, this is not normally used to alter the appearance of a certain piece of ground but, rather, to correct broad zones of the photo which may be blemished by lighting variations or other causes. The Environmental Image Group applies digital image enhancement to virtually every photo used. However, this is always done rigorously to the zoomed portion of the 9x9 aerial, but for the whole zoomed cutout, never just a part of it to accentuate something.

“Aerial photos”, in this sense, come in three fundamental types: vertical, oblique, and panoramic. The latter category is mostly used only for specialized applications and is not widely available as historical imagery. Most of the available, historical film is vertical. This makes its use in mapping, mosaicking, and other operations, much more straightforward.

Oblique photography does allow mapping and measuring, but with much more complex procedures and equipment. Its benefit is that it is much easier for the layman to look at and interpret.  For example, it may appear similar to what you see when looking out the passenger window of an airline jet while low over a city. Thus, hand-held or drone oblique photos over a disputed site may be excellent for introducing it to juries, judges and the public. Also, in some cases, very attractive video simulations may be produced for this purpose.
Oblique Aerial Photo View
Example of an oblique airphoto taken by a small aircraft flyover during fieldwork.

Measurements from Aerial Oblique

Sample, low altitude oblique airphoto with potential ground measurements indicated. By means of the diameter and the height of a cylindrical storage tank, its capacity can be easily determined.
Example Stereo Triplet
Aerial film stereo triplet of an urban and industrial scene for analysis.

The “stereo view” is provided by two (or more) overlapping aerial photos. In the normal case this may simply be two adjoining exposures in one of the many parallel flight lines. The “overlap” in such cases is typically adjusted, at the flight planning stage, to 60%. However, when the analyst is attempting to discern small details in an industry, perhaps 75 years ago, it is often quite helpful to have additional stereoviews from other photos in the flight, which may also cover the site, or part of it.
For example, the adjoining flight line may just happen to cover part of the site if it is near the edge of one of the stereo photos originally used. The stereo might be from two of the exposures in the adjoining flight line, or between one exposure from each flight line. There are no image quality penalties for these various combinations. However, each stereopair will tend to show different aspects of the target area, because of the different viewing angle. A thorough historical analysis will attempt to use as many as possible of the various stereo possibilities from the historical aerials, depending on the level of detail requested by the client.

The first step after a large photo flight (e.g., for a US County) is the quality control of the results by specialists. Assuming that the photography is judged satisfactory, the next step is to create the Photo Index Mosaic(s) so that customers for the photography can easily identify the exposures which cover their target area. The traditional, old way of doing this, has been very simple. Paper prints of the photos are simply overlaid and stapled to a wall or copy board in such a way that the exposure numbers are readable on every photo. This entire board is then photographed, such that a 2x3 foot print allows any target to be found, and its necessary exposures identified and ordered. In the age of digital photography, computer processes can much more rapidly produce a similar graphic overview of the results of the flight. A small print of part of such a Photo Index is shown at right.

In general, the target industrial sites are small on this photography which, in the most typical case, may cover about 8 square miles. Put in another way, for the typical industrial site undergoing environmental  investigation,  the eraser of a pencil would completely cover it on the aerial photography (see example 9x9 photo print with red circle around the actual site).
       Example of Photo Index
A small portion of an aerial photography “Index Sheet”, used to select and order individual prints, film duplicates, and scans. As indicated, multiple stereo overlaps can be quite helpful in the analysis work. These provide slightly different views of the same ground objects.

As mentioned elsewhere here, it is also possible to order a Certification for an aerial photo. This attests to the fact that the reproduction is done from the original negatives without alteration, other than the normal optimizing of the tones/colors of the scene. An example is shown below.

Example of Certified Aerial Photo

A color aerial photo, with an official “Certification” grommeted to the photo. This is the document from the producing agency which establishes that the reproduction is authentic and unaltered. In some cases this may be desirable, although it is not usually a necessity.

Digital Scan of an Aerial Photo
Sample scanned aerial photo, showing the dimensions of the digital image. Notice that, for a black and white photo, the file size is around 1.5GB. For color aerial photography, the file size would be just under 5GB per photo.

In addition to traditional aerial photography shot from an airplane, there is also imagery taken by satellites, as well as another useful tool for capturing landscape features that has arisen in recent years which is the ability to take still or moving images with small remote-controlled drones.

For any current industrial contamination situation, or introductory briefings on an industrial facility, drone-video coverage can be quite useful and easily obtained. Although EIG does not offer this as a commercial service, some 3-minute illustrations may help the reader realize its potential. The first example shows a time-lapse video sequence of the construction of the first supermarket on Catalina Island, CA. The sequences were made every 2-3 weeks, over a year’s period of time, as the new Von’s Market was constructed. The second drone-video shows the last of a series of a dozen drone flights during the replacement of the Catalina Island Airport runway, as a training project, by the US Marine Corps and the US Navy. That is, it features the celebration at the official Airport Re-Opening ceremony.

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