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SECTION 7

REGIONAL STUDIES: USE OF MOSAICS

(Note: Because this section is brief, and the text covers most points of discussion, there will be no questions for you to respond to.)

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 Section 10, 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 Section 11 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 it’s 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 in Section 10) 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 U-2 photos (each about 18 km [12 mi] on a side) taken by NASA along seven flight lines during late spring of 1972 in support of the writer's study of the geology of central Wyoming.

 

Example of uncontrolled mosaic image of central Wyoming, 1972.

Compare this scene, centered on the Wind River Basin, with this Landsat 1 MSS scene that includes, and extends beyond, the mosaic.

Color Landsat-1 MSS mosaic of the Wind River Basin, Wyoming.

To match the image with the 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. The mosaic here has several of the problems inherent to aerial products. The amount of sidelap was minimal, so the pictures could not be trimmed. The inherent average gray levels darken from the central area of each picture, such that the boundaries between photos are emphasized. Exact joins are difficult owing to edge distortions. This gives the mosaic an overall blocky or patchwork appearance that detracts somewhat from its presentation of the feature content. With great care and expense, photo processing can minimize this tonal vignetting, yielding a photomosaic of the quality we saw for the Los Angeles scene in Section 4.


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Primary Author: Nicholas M. Short, Sr. email: nmshort@epix.net

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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