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
The reflectance received in therangefromabout0.7to1.3μmfrom vegetation and it reflectstypically 40– 50%ofthe energy incident uponit. 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 at1.4,1.9and2.7μm and reflectance peaksoccuratabout1.6μm and2.2μm.
The reflectance of soil is influenced by many factors such as moisture content, soil texture (proportionof sand,siltand clay),surfaceroughness,presenceofironoxideand organicmattercontent. 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.
Reflectance in water is limited to visible part, where asradiation 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. Reflectancefrom a waterbody is dependent on
the type of water surface(specularreflection),
suspended material presentinthewater
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.