Image Processing Reference
In-Depth Information
The spectral signatures shown in Fig.
4.2
are best explained and understood with
absorption processes that represent the specific material properties of the surfaces.
One of the non-built spectra indicates the classic spectral characteristics for green
vegetation. Characteristic features are the reflectance peak in visible green (550
nm; chlorophyll absorptions at 450 and 680 nm) and the red edge (~750 nm), and
the near infrared with absorption features at 980, 1,200, 1,400 and 1,900 nm caused
by water content, and ligno-cellulose absorptions at 2,100 and 2,300 nm in the
SWIR. The spectra of bare soil show general similarities with non-photosynthetic
vegetation (NPV) in the visible (VIS) and NIR region. NPV spectra have significant
ligno-cellulose absorption bands that clearly identify them as vegetation.
The bare soil spectra show clay mineral absorption features at 2,200 nm.
Gravel surfaces also reflect mineral absorption features similar to the soil sur-
faces. The roof spectra indicate distinct spectral signatures for red tile roofs and
wood shingle roofs. Both roof types show a significant reflectance increase in the
NIR and SWIR region. The wood shingle signature contains the ligno-cellulose
absorption feature in the SWIR that is common for all
non-photosynthetic vegetation. Liquid water and
hydroxyl absorptions, typically found in clays are lack-
ing in fired red tile bricks due to the loss of water in the
production firing process. Red tile roofs and gravel roads
show significant iron oxides absorptions in the visible
and near infrared (near 520, 670 and 870 nm). Concrete
road materials are comprised of cement, gravel, water, and various other ingredi-
ents. Significant absorptions appear in the SWIR due to calcium carbonate with
a feature at 2,300 nm for calcite and at 2,370 from dolomite. The feature near
2,170 nm can be related to content of clays in the concrete. The diagram showing
built up spectra further contains several materials with constant low reflectance
and no or only minor unique small-scale absorption features like asphalt roads,
parking lots, composite shingle roofs and dark tile roofs. Swimming pools
(AVIRIS spectra of other urban surfaces) have a large reflectance in the visible
blue near 450 nm. In this wavelength the radiation penetrates the water body and
reflects off the light blue/cyan painted bottom. Towards longer wavelengths the
water absorbs increasing amounts of radiation. The reflectance drops and basi-
cally no radiation is reflected in the NIR and SWIR region. The sport surfaces red
sport tartan and tennis court, nicely show the absorptions in the visible region that
cause their significant color. In the case of red tartan this color is caused by the
iron oxide content with absorptions near 520, 670 and 870 nm.
spectral absorp-
tion features are
important indica-
tors of material
characteristics
4.3
Urban Materials Versus Land Cover Types
From a remote sensing mapping perspective it is important to view the spectral
signatures (Fig.
4.2
) in terms of the spectral contrast. If a surface type has a unique
spectral signal it is easily distinguished in remote sensing images, i.e., the expected
