Answer: Color infrared photography, often called false color photography because
it renders the scene in other than the normal colors seen by the human
eye, is widely used for interpretation of natural resources. Atmospheric
haze does not interfere with the acquisition of the image, therefore is
well suited to aerial photography. Because the film is high speed and
subject to degrees of degradation in handling before exposure, the
aerial photographs can vary in overall tone. This variability complicates
the interpretation of color tones between photographs. However, some
general guidelines can be given to aid the inexperienced interpreter.
The red tone of color infrared aerial photographs is almost always
associated with live vegetation. Very intense reds indicate vegetation
which is growing vigorously and is quite dense. An irrigated alfalfa
field would be an example of such vegetation. An evergreen forest, which
may be quite dense vegetatively, will not appear as a similar bright
red because its level of growth activity is less, compared to irrigated
alfalfa. Knowledge of the vigor and density of vegetation is important
to the interpretation of the red colors on color infrared aerial
photography.
As the vigor and density of vegetation decreases, the tones may change
to light reds and pinks. If plant density becomes low enough the faint
reds may be overcome by the tones of the soils on which the plants are
growing. The ground areas in this case will appear in shades of white,
blue, or green depending on the kind of soil and its moisture content.
As plant vigor decreases, the vegetation will show as lighter shades
of red and pink, various shades of greens, and possible tans. Dead
vegetation, wheat stubble as an example, will often be shades of greens
or tans.
Bare soils will appear as shades of white, blue, or green in most
agricultural regions. In general, the more moist the soil the darker
the shade of that particular soil color. Composition of the soil will
affect the color tones shown on the photographs. Dry sand will appear
white and, with more moisture, may be very light gray or possibly light
tan. Clayey soils will generally be darker in color than sands and tend
toward tans and bluegreens. Again, wetter clays will be darker shades
of the same tones. Soils high in organic matter, like silts and loams
will be even darker in color, and usually in shades of blues and greens.
Wet organic soils can be very dark blue or green in the aerial photographs.
Man-made features will show in the tones that relate to the materials they
are made of. Asphalt roads, for example, will be dark blue or black,
gravel or dirt roads will show as lighter colors, depending on the soil
materials involved in their composition, and concrete roads will appear
light
in tone, assuming clean concrete. The buildings and streets of towns can
be considered in a similar manner, their color dependent on the material
they are made of.
Water will appear as shades of blue, varying from nearly black to very pale
blue. Clear, clean water will appear nearly black. As the amount of
sediment increases, the color becomes increasingly lighter blue. Very
shallow water will often appear as the material present in the bottom of
the stream. For example, a very shallow stream with a sandy bottom will
appear white due to the high level of reflection of the sand.
Degraded film will result in photographs which have an overall blue or
green cast. When that occurs, the interpretation must consider what that
overall cast will do to a "normal" rendition of the scene. |