Energy Interactions
When electro-magnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature are possible. See Fig. 2
Fig 2: Basic interactions between electromagnetic energy and an earth surface feature
Spectral Reflectance & Colour Readability
Two points about the above given relationship (expressed in the form of equation) should be noted.
- The proportions of energy reflected, absorbed, and transmitted will vary for different earth features, depending upon their material type and conditions. These differences permit us to distinguish different features on an image.
- The wavelength dependency means that,
even within a given feature type, the proportion of reflected,
absorbed, and transmitted energy will vary at different
wavelengths.
Thus, two features may be distinguishable in one spectral range and be very different on another wavelength brand. Within the visible portion of the spectrum, these spectral variations result in the visual effect called COLOUR. For example we call blue objects 'blue' when they reflect highly in the 'green' spectral region, and so on. Thus the eye uses spectral variations in the magnitude of reflected energy to discriminate between various objects.
A graph of the spectral reflectance of an object as a function of wavelength is called a spectral reflectance curve.
The lines in this figure. 3 represent average reflectance curves compiled by measuring large sample features. It should be noted how distinctive the curves are for each feature. In general,the configuration of these curves is an indicator of the type and condition of the features to which they apply. Although the reflectance of individual features will vary considerably above and below the average, these curves demonstrate some fundamental points concerning spectral reflectance.
Fig 3: Special Reflectance Curve of common object
| Band | Wavelength (mm) | Principal applications |
| 1 | 0.45-0.52 | Sensitive to sedimentation, deciduous/ coniferous forest cover discrimination, soil vegetation differentiation |
| 2 | 0.52-0.59 | Green reflectance by healthy vegetation, vegetation vigour, rock-soil discrimination, turbidity and bathymetry in shallow waters |
| 3 | 0.62-0.68 | Sensitive to chlorophyll absorption: plant species discrimination, differentiation of soil and geological boundary |
| 4 | 0.77-0.86 | Sensitive to green biomass and moisture in vegetation, land and water contrast, landform/ geomorphic studies. |
Colour Discrimination based on Wavelengths of Spectral Reflectances.( IRS-IA/IB LISS I and LISSII*)
Platforms
The vehicles or carriers for remote sensors are called the platforms. Typical platforms are satellites and aircraft, but they can also include radio-controlled aeroplanes, balloons kits for low altitude remote sensing, as well as ladder trucks or 'cherry pickers' for ground investigations. The key factor for the selection of a platform is the altitude that determines the ground resolution and which is also dependent on the instantaneous field of view (IFOV) of the sensor on board the platform.
Sensors
- Active Sensors: Detect the reflected or emitted electromagnetic radiation from natural sources.
- Passive Sensors: Detect reflected
responses from objects that are irradiated from
artificially-generated energy sources such as radar.
Resolution
In general resolution is defined as the ability of an entire remote-sensing system to render a sharply defined image.
- Spectral Resolution: Spectral Resolution of a remote sensing instrument (sensor) is determined by the band-widths of the Electro-magnetic radiation.
- Radiometric Resolution: It is determined by the number of discrete levels into which signals may be divided.
- Spatial Resolution: It is determined in terms of the geometric properties of the imaging system.
- Temporal Resolution: Is related ot the
repetitive coverage of the ground by the remote-sensing system.