Remote Sensing - Image Formation Techniques

Another common way of categorizing remote sensing systems (and the imagery they produce) is in terms of the "image formation technique". Image formation technique refers to how the system actually produces an image. As previously mentioned, the most common image formation device is the frame camera. It creates an image by using a lens to receive the electromagnetic energy from the ground and transmit it to the film where it is recorded. Just like a snapshot camera, the aerial frame camera takes exposures one by one on a long roll of film; each exposure typically overlaps the last one by about sixty percent. This provides for total coverage of the terrain without any gaps, and makes possible the stereo, or 3-D, observation of the aerial photography by photo interpreters. An illustration of two overlapping airphotos is provided below [See Image 5].

Image 5: Illustration of a typical frame camera aerial stereopair showing the Cape Kennedy Space Shuttle launch facilities in Florida. Flight direction is from right to left. For larger view, click on the image. (photos: U.S. Geological Survey)

The frame camera may also be intentionally pointed away from the normal, vertical orientation. This produces what is referred to as an oblique airphoto [See Images 6 and 7]. These airphotos tend to be much more natural looking and easier to interpret. They also have the effect of visually exaggerating the relief dimension of the topography which may be beneficial as illustrations for the public. Dimensioning and mapping are still possible from these photos although, the procedures become more complex because of scale variations between the foreground and background.

Image 6: Example low altitude oblique airphoto of a petroleum refinery; note the very natural appearance of this view of part of  Baytown, Texas; industrial facilities like these can be easily interpreted and dimensioned from this image. (photo: U.S. Air Force)

Image 7: Oblique airphoto of an air show display at El Toro Marine Corps Air Station, in Southern California; note the visible horizon in the background which allows the photo interpreter to know the orientation of the aircraft, and therefore measure and map more easily. (photo: U.S. Marine Corps)

The aerial camera may use one of four basic kinds of film to gather different kinds of information. The most common aerial film is standard black and white film,called panchromatic film. This is the same as the film you might use in your own camera. The four airphotos illustrated above, were taken using this kind of film. Standard color film is also used widely for aerial photography. This film can produce negatives for printing in a photo lab, or film positives like 35mm slides. However, your aerial sensing task may be somewhat more specialized: for example, you may want to penetrate the haze better, tell the difference between healthy and stressed vegetation or get a better rendition of water areas on the land. In that case, the use of an infrared film might be appropriate.

Infrared films were developed during World War II, and were originally called "camouflage detection films." This is because they could differentiate between healthy forests and cut tree limbs thrown on top of vehicles in an effort to hide them. The infrared films record brightly, the reflections from chlorophyll-rich vegetation. Thus, the bright vegetation stands out from the darker background, including things such as: bare soil and rock, water areas, pavement, roofs, and other common surfaces. Both black and white, and color infrared films exist. Color infrared (CIR), also referred to as false color film, is by far the most common [See Images 8 and 9]. Its appearance is very distinctive and recognizable: healthy vegetation appears in bright red, and water areas are typically very dark, almost black. As the name implies, none of the colors in such imagery are natural and appropriate; they are all false. However, as discussed above, this can supply great advantages to the photo interpreter. A sample of color and color infrared images are given below.

Image 8: Color aerial photo of a sewage treatment plant; note the very dark vegetation. (photo: Department of Defense)

Image 9: Color infrared (CIR) airphoto of approximately the same area as left photo; note the contrast of the bright red of the healthy vegetation with the darker areas of the image such as water and pavement. (photo: Department of Defense)

Another very common image formation technique is the use of a panoramic camera [See Image 10]. In the past, these have been much more common in the military, although their use is spreading now for civilian applications. For many mined areas around the world, historical, military panoramic imagery may be available. This kind of aerial imagery can be somewhat complex to use, but it has many advantages.

The panoramic camera uses a rotating lens (or lens prism) to obtain a narrow sweep across the flight line. That is, each exposure is a long, narrow strip perpendicular to the flight direction. In a typical panoramic camera, the sweep may begin just under the left wing, then rotate downwards, directly under the aircraft, and finally end up just under the right wing. This produces one continuous image, possibly from horizon to horizon. When this image is plotted on a reference map, it will typically result in an "hourglass" or "bow tie" shape in terms of its ground coverage. This is because the ground coverage becomes increasingly oblique, out to either side of the flight line, but the film width is constant. Therefore, the scale on such imagery is systematically smaller as you move outwards or towards the edge on the image, away from the flight line of the aircraft. Nevertheless, this kind of imagery has been used for mapping and measuring for many years by the military.

Image 10: Sample panoramic image over Lambert St. Louis Airport, Missouri; note the vertical view at the center, and the oblique views all the way to the horizon at each end. Flight direction is towards the top of the page; historical, military imagery of this kind is widely available for many former war zones. (photo: McDonnell Douglas Corp.)

Despite its added complexity, the panoramic camera has several very significant advantages over conventional cameras. Typically, it provides a much wider swath of coverage under the aircraft, which requires fewer flight lines to cover the same area. In addition, it has the capability to produce sharper imagery because, only the center of the lens is used. The wide coverage is obtained not by using a wide angle field of view, but by rotating the lens during the exposure. These cameras can use any of the same basic four films referred to above.

Former Military Imagery Available
If the humanitarian deminer would like to obtain historical aerial photography from the 1960s or early 1970s, a great deal of declassified "Corona" panoramic reconnaissance satellite imagery is now available through the U.S. Geological Survey at Sioux Falls, South Dakota. The Corona military photo reconnaissance satellite system was used to produce very high resolution imagery from 1960 until 1972. Researching and ordering this historical imagery is now possible over the Web from the U.S. Geological Survey at Sioux Falls, South Dakota. This is mostly black and white, high quality imagery on 70-millimeter film providing stereo in many cases. Similarly, the aerial photography from the CIA period of U-2 aircraft operations (1956 through 1974) has also all been declassified. It is currently being cataloged at the U.S. National Archives at College Park, Maryland. The most recent historic imagery declassification occurred in September of 2002. In this release the National Imagery and Mapping Agency (NIMA) unveiled the KH-7 Gambit (high-resolution surveillance) and the KH-9 Hexagon (low-resolution mapping) satellite imagery. The KH-7 satellite was used between 1963 and 1967, while the KH-9 was used between 1973 and 1980. The spatial resolution of the surveillance imagery is comparable with the best commercial satellites of today, between 0.6 and 1.2 meters. The public can acquire this data from the USGS EROS Data Center in Sioux Falls, South Dakota or from the National Archives and Records Administration in Adelphi, Maryland. Military radar imagery is also now available due to declassification. A great deal of radar imagery was produced during various Third World conflicts; some of this data may be available to the official humanitarian demining community. Historical aerial imagery has already been usefully employed in connection with the cleanup effort in Southeast Asia. However, because of the exotic form of these kinds of aerial film, trained personnel at a support center are required to use it effectively.



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