Visual interpretation of aerial photographs is based on recognition of objects, as seen from above, in pictorial format. This visual recognition often takes place without any conscious effort by the interpreter. Nonetheless, several basic features aid in the examination and interpretation of airphotos (based on Avery and Berlin, 1992; Teng et al., 1997; Jensen, 2007).

• Tone or color, gray tone (b/w) and color are how we recognize and distinguish objects. The tone or color of an object helps to separate it from other features in the scene, especially for features with high contrast. Color may be an important clue to the object's identity— water, soil, vegetation, rocks, etc.

• Shape, natural shapes tend to follow the lay of the land. Cultural (human) shapes, on the other hand, are often geometric in nature with straight lines, sharp angles, and regular forms. In monoscopic images only 2D shape can be appreciated fully, although an object's height or relief may be detected to some extent by shadow or relief displacement. In stereoscopic images, the full three dimensions become apparent.

• Size and height, absolute size and height are important clues that depend on the scale of the photograph. Some object sizes are common experience, for example, a car, tree, and house. But many objects seen on airphotos are not so obvious, and height may be difficult to judge in vertical airphotos. Always check photo scale for a guide to object size.

• Shadow, shadows can be useful clues for identifying objects, as seen from above. Trees, buildings, animals, bridges, and towers are examples of features that cast distinctive shadows. Shadows on the landscape also help with depth perception. Photographs without shadows often seem "blank" and difficult to interpret; however, too much shadowing may obscure features of interest.

• Pattern, the spatial arrangement of discrete objects may create a distinctive pattern. This is most apparent for cultural features, for example, city street grids, airport runways, and agricultural fields. Patterns in the natural environment also may be noticeable in some cases, for example, bedrock fractures, drainage networks, and sand dunes.

• Texture, this refers to grouped objects that are too small or too close together to create distinctive patterns. Examples include tree crowns in a forest canopy, individual plants in an agricultural field, ripples on a water surface, gravel sheets of different grain size in a river bed, etc. The difference between texture and pattern is largely determined by photo scale.

• Context, the association and site location of objects are often important for aiding interpretation. Note land cover and land use as clues to help identify related features in the scene, and refer to existing maps or census data for ancillary information. This is often a matter of common sense and experience for image interpretation.

Each of these basic features is seldom recognized on its own. Rather it is the combination of visual elements that allows interpretation of objects depicted in an aerial photograph. These elements are explored in further detail with selected small-format aerial photography (SFAP) examples later in this chapter and in the case studies. The issue of stereoscopic viewing is covered together with stereoscopic mapping in Chapter 11.

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