REFLECTANCE OF EARTH SURFACE FEATURE AND SPECTRAL SIGNATURE OF SATELLITE IMAGE
This article is prepared to
understand the role of reflectance and spectral signature in a satellite image.
How the reflectance vary with change in EMR and characteristics of earth surface feature.
It is very important to understand
that how the reflection which is received by sensor, converts into an image. It
is explained in earlier article that how the reflectance taking place and its behavior
of incident energy after interaction with surface. The reflectance completely
depends on incident energy and characteristics of material/earth’s surface
feature. So reflectance which is created during this process is called SIGNATURE.
Every object on the earth surface reflects and emits incident radiation is defined as signature of that object. It can be
also called re-radiation from earth surface feature. These signatures are
recorded by sensor boarded on satellites.
As it is understood that basically
only three natural land covers, it includes vegetation, soil and water. So behavior
of these three natural covers with incident energy will create reflectance. Which
is further, defined as signature of these land covers. This signature pattern
can be understood by the spectral signature curve, which is in below figure.
Typical Spectral Reflectance Curve of Soil, Water and Vegetation
Vegetation:
The reflectance received in the range from about 0.7 to 1.3 μm from vegetation and it reflects typically 40 -
50% of the energy incident upon
it. The reflectance from vegetation depends upon internal structure of
leaf and condition of plant.
There are lots of variations
in reflectance value as reflectance values goes down in these wavelengths at 1.4, 1.9 and 2.7 μm and reflectance peaks occur at about 1.6 μm
and 2.2 μm.
Soil:
The reflectance of soil is influenced by many factors such
as moisture
content,
soil texture
(proportion of sand, silt and clay), surface roughness, presence
of iron oxide and
organic matter content.
So it is understood that dry soil will have more reflectance compare to moisture
soil. Soil reflectance is higher in 1.4 to 1.8 and 2.0 to 2.1 wavelengths, whereas
in blue part of visible spectrum have lowest reflectance compare to other part
of spectrum.
Water:
Reflectance
in water is limited to visible part, where as radiation is absorbed in IR wavelengths of
spectrum. Clear water body absorbs comparatively less energy, results in high
transmittance. However turbidity in water affects the reflectance. Reflectance from a water body is
dependent on
- the type of water surface (specular reflection),
- suspended material present in the water
- depth of the water body
The ultimate reflectance, after interaction of incident energy with the
earth surface feature is received by satellite sensor. This Reflectance is
recorded and converts into an image.
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