Shadows

The issue of shadowing in an image is obviously strongly connected to the latitude, time of day, and cloud aspects discussed above. With direct illumination by the sun, all three-dimensional objects or surface features cast a shadow if they project into the paths of the light rays between sun and ground. Shadowing in aerial images is most prominent at early morning and later afternoon and increases with higher latitudes. The darkening effect of shadowing increases with longer wavelengths as these are less subject to atmospheric scattering. For the same reason, shadows on a clear and dry day are much more pronounced than with slightly overcast conditions and a dusty or humid atmosphere.

Shadowing may occur at various scale levels in an image: a steep slope may cast a shadow onto a valley floor and opposite valley side; the shadow of a tree may obscure the ground beneath; shadowing between clods of earth may reveal the texture of a freshly plowed field. Shadows might

FIGURE 4-30 Vertical images of gully erosion on an abandoned Moroccan field. (A) Shadows emphasize the various depths and degrees of ruggedness of the geomorphologic forms carved by surface and subsurface erosion processes. (B) Image taken a few minutes later under cloud cover. The indirect lighting improves the visibility of details in all parts of the gully, but takes away most depth impression from the image, making it impossible to distinguish different levels of incision. Kite aerial photographs by IM and JBR; Talaa, Taroudant, Morocco, March 2006.

FIGURE 4-30 Vertical images of gully erosion on an abandoned Moroccan field. (A) Shadows emphasize the various depths and degrees of ruggedness of the geomorphologic forms carved by surface and subsurface erosion processes. (B) Image taken a few minutes later under cloud cover. The indirect lighting improves the visibility of details in all parts of the gully, but takes away most depth impression from the image, making it impossible to distinguish different levels of incision. Kite aerial photographs by IM and JBR; Talaa, Taroudant, Morocco, March 2006.

FIGURE 4-31 Every tree appears twice in this image of a burned pine forest near Castejon de Valdejasa, Province of Zaragoza, Spain. The long shadows cast by the afternoon sun are much larger from the vertical vantage point than the same-colored, soot-blackened skeletons of the pines, obscuring great parts of the image. The forest fire, sparked by a car accident in August 2008, destroyed 2200 ha of woodland. Model airplane photography by C. Claussen, M. Niesen, JBR, and IM, February 2009.

FIGURE 4-31 Every tree appears twice in this image of a burned pine forest near Castejon de Valdejasa, Province of Zaragoza, Spain. The long shadows cast by the afternoon sun are much larger from the vertical vantage point than the same-colored, soot-blackened skeletons of the pines, obscuring great parts of the image. The forest fire, sparked by a car accident in August 2008, destroyed 2200 ha of woodland. Model airplane photography by C. Claussen, M. Niesen, JBR, and IM, February 2009.

FIGURE 4-32 Low-height, vertical view of ''Big Brutus'' at the Mining Heritage Museum near West Mineral, Kansas, United States. One of the world's two largest power shovels, it stands 50 m tall with a working weight of 5000 metric tons. Shadow creates a silhouette side view of the machine. A normal-sized power shovel is visible at lower left; note people for scale standing in upper left corner. Picture taken with a compact digital camera from an unmanned, tethered, helium blimp (Aber and Aber 2009, fig. 72).

FIGURE 4-32 Low-height, vertical view of ''Big Brutus'' at the Mining Heritage Museum near West Mineral, Kansas, United States. One of the world's two largest power shovels, it stands 50 m tall with a working weight of 5000 metric tons. Shadow creates a silhouette side view of the machine. A normal-sized power shovel is visible at lower left; note people for scale standing in upper left corner. Picture taken with a compact digital camera from an unmanned, tethered, helium blimp (Aber and Aber 2009, fig. 72).

FIGURE 4-33 Bank gully in southern Spain. (A) Indirect lighting from an overcast sky uniformly illuminates all areas in this image, which completely lacks shadows. Details of the gully bottom are clearly visible. (B) Strong sun light casts a dark shadow from the steep gully wall onto its bottom. Image classification or stereoscopic photogrammetric analysis in this area is not possible and comparison between the two monitoring dates is difficult. Hot-air blimp photographs by IM and JBR; Rambla Salada, Murcia Province, Spain, April 2002 and 2004.

FIGURE 4-33 Bank gully in southern Spain. (A) Indirect lighting from an overcast sky uniformly illuminates all areas in this image, which completely lacks shadows. Details of the gully bottom are clearly visible. (B) Strong sun light casts a dark shadow from the steep gully wall onto its bottom. Image classification or stereoscopic photogrammetric analysis in this area is not possible and comparison between the two monitoring dates is difficult. Hot-air blimp photographs by IM and JBR; Rambla Salada, Murcia Province, Spain, April 2002 and 2004.

both be desired or unwanted depending on the purpose of the image: they may emphasize as well as obscure objects and surfaces in a scene (Fig. 4-30).

For visual interpretation of aerial images, shadowing can be extremely helpful as it offers clues to the third dimension, which the two-dimensional image lacks. The shadows can help to identify form and function of buildings or types of trees in vertical images (Fig. 4-31), disclose the course of a power transmission line, or reveal archaeological and historical landscape features. Shadows may be quite dramatic in some cases (Fig. 4-32).

Geomorphologic and geologic features and landforms at various scale levels—rock crevices, erosion rills, fault lines, dells or dolines, dunes or coastal cliffs—could be rendered nearly invisible or at least unfathomable when melting into the landscape background on an image without shadowing. In contrast, late afternoon sun might model even subtle variations of surface height, if the angle of incidence is more-or-less orthogonal to the relief structures.

In digital image analysis, shadows are usually undesirable as they change the spectral response of objects with otherwise homogeneous or identical reflectivity. For spectral classification algorithms, this results in mis-classified or unclassified areas (see Chapter 11.5). To avoid this effect, masking techniques or topographic normalization may be applied (see Fig. 11-17; Colby, 1991; Zhan et al., 2005). Photogrammetric analysis both by visual stereoscopy as well as automated image matching is strongly hampered in dark shadow areas (Fig. 4-33) and may lead to considerable measurement errors (Gimenez et al., 2009). The resulting spatially varying data quality of image classifications and surface models subsequently also afflicts time-series analyses and monitoring by remote sensing.

On the other hand, shadows might actually enable automatic detection of objects or surface types by adding

FIGURE 4-34 Fallow field in northern Spain. (A) After several drought years, remains of the pattern created by plowing five years previously are still visible in the early morning sunlight. (B) Digital texture and Fourier analysis were used to create this map of different micro-topographic surface positions (ridge and furrow), corresponding to different types of soil crusts. Field of view ~24 m across; taken from Marzolff (1999, maps 1A and 4A).

FIGURE 4-34 Fallow field in northern Spain. (A) After several drought years, remains of the pattern created by plowing five years previously are still visible in the early morning sunlight. (B) Digital texture and Fourier analysis were used to create this map of different micro-topographic surface positions (ridge and furrow), corresponding to different types of soil crusts. Field of view ~24 m across; taken from Marzolff (1999, maps 1A and 4A).

FIGURE 4-35 Changing its direction by 135° in the center of this vertical image, an erosion rill on a fallow field turns from an inconspicous feature melting into the bare-soil background into a starkly prominent incision. Note sun azimuth indicated by the kite flyer's shadow in the upper right. Kite aerial photograph taken in the Bardenas Reales, Province of Navarra, Spain, by IM and JBR, February 2009.

FIGURE 4-35 Changing its direction by 135° in the center of this vertical image, an erosion rill on a fallow field turns from an inconspicous feature melting into the bare-soil background into a starkly prominent incision. Note sun azimuth indicated by the kite flyer's shadow in the upper right. Kite aerial photograph taken in the Bardenas Reales, Province of Navarra, Spain, by IM and JBR, February 2009.

typical patterns or structures to the image, which can be enhanced and extracted by filtering, image segmentation, and texture analysis (Fig. 4-34; Marzolff, 1999; Shackelford, 2004). A difficult problem both for intentionally achieving and avoiding shadowing is that the degree of shadowing is not only dependent on sun elevation and object height, but also on the relative position of the objects or surface structures to the incident rays of light. Similar to multiview-angle effects, the same structures may have different appearance throughout the image depending on their orientation (Fig. 4-35; see also Gimenez et al., 2009).

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