Aerial photos have four principal uses: 1) to show what is in each photo; 2) to function as a visible base for mapping; 3) to serve as a mate for stereo viewing; and 4) to be joined together in mosaics that retain each photo’s resolution (unless reduced in scale by being rephotographed) but cover a much larger area. This last use recasts a collection of contiguous aerial photos into a synoptic product that offers regional coverage. However, unless very careful photo processing is applied, the mosaics will likely show a patchy quality because of vignette darkening towards the edges (see Section 10). Space images, such as Landsat, SPOT, IRS, JERS, and others can also be combined into mosaics that can embrace very large areas, even to continental scale. Since the images were recorded in digital format, the data can be reprocessed to minimize and even eliminate the differences in tone, etc. from center outwards.
The notion of getting a much larger picture of a region by pasting images of individual scenes into a single composite goes back to the early days of aerial photography. The resultant product is called a mosaic. We construct mosaics a bit like jigsaw puzzles, except that we usually know the position of each air photo in advance. During an aerial mission (see page 10-1, which discusses aerial photography) we predetermine the flight lines for the airplane to follow, usually a back and forth pattern, much as a farmer plows a field.
The plane takes the photographs in sequence, such that there is always about 50% overlap (common area) between each successive picture, normally accomplished by an automated camera shutter at a timing interval controlled by altitude, air speed, and camera properties. On the next parallel line the pilot attempts to traverse the ground at a lateral distance that produces up to 40% sidelap. Both overlaps allow for obtaining a three-dimensional or stereo effect (see page 11-3 for stereoscopic procedures and examples of 3-D views). However, the resulting pictures almost always display distortions, which comes from inexact navigation and aircraft wobble (in pitch, roll, and yaw) caused by turbulence.. There is also a notable distortion in an image outward from its center (presumed looking vertically downward, but at times its canted off center) because of the increasing slant distances from center to edge. This effect decreases with higher altitudes and we can also adjust the focal length and other camera geometry to diminish it. Tonal brightness also can be discernibly lower from the interior of the photo towards its boundaries.
In practice, we construct mosaics from the central parts of the component photographs, which we trim enough to remove the more distorted, overlapping parts. Thus, they approximate rectified orthophotos. Large photo-mosaics have an added problem: the crews frequently fly the assigned flight lines on different days (or, less commonly, at different times of the same day), so that lighting and weather conditions often are not uniform. This problem is especially severe if the overflights happen weeks apart, in which case vegetation changes, along with the sun orientation due to seasonal shifts.. Photographic processing can compensate for some of these deviations, but uncontrolled (minimal adjustments) mosaics are typically rather patchy.
To retain maximum resolution, we must put together a photo-mosaic from individual scenes that we have not reduced in size. Consider making a mosaic from aerial photos having a scale of 1:62,500. Each photo, approximately 30 cm (12 in) on a side, covers almost 2.6 sq km (1 sq mi) of ground surface. To create a mosaic representing ground dimensions of 6,400 sq km (2,500 sq mi), which is 80 km (50 mi) on a side, we would need a "billboard" 15 m (50 ft) wide and high, requiring 2,500 photos, neglecting trimming requirements. This is impractical, so we almost invariably reduce mosaics in size and hence in scale (defined on page 10-1) and therefore, they have a notably lower resolution.
The scene below is a solid example of a typical uncontrolled mosaic. This is a series of high-altitude aerial photos (each about 18 km [12 mi] on a side) taken by NASA's U-2 aircraft along seven flight lines during late spring of 1972 in support of the writer's study of the geology of central Wyoming.
7-1: Evaluate the black and white aerial photo mosaic in terms of its properties and usability. ANSWER
Compare this scene, centered on the Wind River Basin, with this Landsat 1 Band 6 MSS scene that includes, and extends beyond, the mosaic. This was the prime image taken by the writer (NMS) into the field in central Wyoming just over a month after launch of ERTS-2 (to my knowledge I was the first to take any ERTS product to the area it covered so as to check on its usability):
7-2: What photo property does this ERTS image NOT have that is evident in the individual photos making up the U-2 mosaic ANSWER
To match the image with the U-2 mosaic, look for the Boysen Reservoir and Ocean Lake (round) that stand out in the MSS view but are hard to see in the mosaic. In that mosaic, the Owl Creek Mountains, with partial snow cover, lie along the top; the Sweetwater River is at the bottom; a cloud bank appears in left center. When examined full size on a light table, this black and white scene shows more ground detail, even in that tonal mode, than does the Landsat full image but the even-toned nature of the latter compensates somewhat for the information quality.
Shortly after returning from the field in 1973, the writer (NMS) arranged with a support contractor, GE Space Sciences Lab in Beltsville, MD, to reprocess black and white images of western and central Wyoming and part of adjacent Utah to optimize contrast and then to match tonal levels at joins so as to produce an ERTS mosaic. The result was both pleasing to the eye and informative. Here is that product: To the writer's knowledge, the next image is the first color mosaic of a State in the U.S. - Wyoming - ever made, again at the GE facility. This was used as a base for data plotting in the Wyoming project he conducted with members of the Geology Department of the University of Wyoming: 7-3: Why is this mosaic still somewhat patchy? ANSWER
Shortly after returning from the field in 1973, the writer (NMS) arranged with a support contractor, GE Space Sciences Lab in Beltsville, MD, to reprocess black and white images of western and central Wyoming and part of adjacent Utah to optimize contrast and then to match tonal levels at joins so as to produce an ERTS mosaic. The result was both pleasing to the eye and informative. Here is that product:
To the writer's knowledge, the next image is the first color mosaic of a State in the U.S. - Wyoming - ever made, again at the GE facility. This was used as a base for data plotting in the Wyoming project he conducted with members of the Geology Department of the University of Wyoming:
7-3: Why is this mosaic still somewhat patchy? ANSWER
Collaborators: Code 935 NASA GSFC, GST, USAF Academy