Geoscience Reference
In-Depth Information
Figure 3-6.
Sun glint highlights tiny islands in the
shallow Väinameri (Strait Sea) south of Vormsi, Estonia.
Kite aerial photo by J.S. Aber and S.W. Aber.
Figure 3-4.
The man-made pond in the foreground
trapped recent runoff and has a high content of
yellowish-brown suspended sediment (see Color Plate
3-4). The next pond downstream did not receive this
sediment and has a clean, dark blue appearance.
Central Kansas, United States. Kite aerial photo by J.S.
Aber and S.W. Aber.
for wetland applications (Tiner 1997), because
emergent vegetation appears in pink and red
colors (Fig. 3-8). The unique spectral signature
of active green plants has given rise to several
vegetation indices, such as the normalized dif-
ference vegetation index (NDVI), which are
based on the ratio of red to near-infrared bands
(Tucker 1979; Murtha et al. 1997).
Although color-infrared imagery is ideal for
separating emergent vegetation from other
objects in wetlands, the authors have found that
conventional color-visible imagery is often better
for recognizing i ne distinctions in vegetation.
Subtle variations in plant height, texture, pattern,
and visible color may be quite revealing (Fig.
3-9). In temperate climates, deciduous vegetation
undergoes marked phenological changes from
season to season. These changes may be used to
advantage for plant identii cation and are some-
times most striking in the autumn (Fig. 3-10).
Soil, when visible from above, is the third
main feature of interest for wetland image inter-
pretation. Like water, the most obvious aspect
of soil is its color. Yellowish-brown and reddish-
brown colors indicate ferric iron (rust) in oxi-
dized soils (Fig. 3-11). In contrast, bluish- or
greenish-gray colors are typical of reducing
conditions (Fig. 3-12), and dark brown to black
colors suggest high contents of organic matter.
In saline soils, surface evaporation may deposit
a crust of salt that appears quite bright (see Fig.
2-7). However, soil color must be treated with
some caution, as moisture content has a strong
Figure 3-5.
Sun glint from smooth water (*), and sun
glitter from ripple and wave surfaces elsewhere in
scene. Southern tip of South Padre Island, Texas,
United States. Kite airphoto by J.S. Aber and S.W. Aber.
may aid recognition of small, emergent islands
within larger water bodies (Fig. 3-6).
Many wetlands are largely, if not entirely,
covered with emergent vegetation. Photosyn-
thetically active “green” leaves absorb blue and
red light, weakly rel ect green light, and strongly
rel ect near-infrared radiation (Fig. 3-7). Green
plants are the only objects at the Earth's surface
with this spectral signature. For this reason,
color-infrared photographs are recommended
