The disadvantage of vertical photography is that it requires a higher degree of planning and preparation than oblique. These differences can be caused by soil, environmental, vegetative, cultural, and other factors. Accurate interpretation is what determines the ultimate value of aerial photography as a crop management aid. The following paragraphs discuss briefly each interpretive element-its definition, the differences it reveals, and reasons for those differences. The referenced figures show examples of the four elements.
Tone is the color or relative brightness of an object.
It is the characteristic most noticeable for distinguishing between crops and in identifying problem areas within a crop. Soybeans, for example, reflect more green and infrared light than corn. Wilting, chlorosis, and necrosis, which are evidences of plant stress problems, produce tonal differences that color infrared can detect. A true red indicates healthy plants, whereas light red, pink, and bluish-pink indicate plants under stress. Texture is the degree of roughness or smoothness-a visual impression usually related to crop density and plant height variation.