Remote sensing

Remote sensing

Science and art that permits us to obtain information about an object or phenomenon or area through the analysis of data acquired by a sensing device without its being in contact with that object or phenomenon or area.

Drawbacks of Traditional methods

• Data collection cannot be done throughout the year due to unfavourable weather conditions.

• Data collection from inaccessible areas is not possible.

• Date collection through traditional methods is time consuming and there is wide gap between data collection and its possible utilization.

Advantages of Remote sensing

• Remote sensing technique provides the synoptic view of large areas.

• The data are collected on a permanent basis.

• The data are an unbiased record of the objects.

• The data are amenable for multi-disciplinary use i.e. the same data can be used for studies in forestry, soil science, hydrology and geology.

• The process of data acquisition and analysis is faster

• Satellite data are received periodically and helps in updating the information and monitoring the changes at short intervals.

• This have unique capability of recording data in visible, as well as invisible parts of electromagnetic spectrum.

Basic processes of remote sensing

The two basic processes involved are

• Data acquisition and

• Data analysis

Data acquisition

The elements of the data acquisition process are energy sources, propagation of energy through the atmosphere, energy interaction with earth surface features, re-transmission of energy through the atmosphere, air borne and / or space borne sensors, resulting in the generation of sensor data in pictorial and / or digital form. In short we use sensors to record variations in the way earth surface features reflect and emit electromagnetic energy.

Data analysis

The data analysis process involves examining data using various viewing and interpretation devices to analyze pictorial data and / or a computer to analyze digital sensor data. Reference data about the resources being studied (such as soil maps, crop statistics and field check data) are used when and where available to acquits in the data analysis. With the aid of the reference data, the analysis extracts information about the type, extent, location and condition of the various resources over which the sensor data were collected. This information is then complied, generally in the form of hard copy maps and tables or as computer files that can be merged with other “layers” of information’s in a Geographic Information System (GIS). Finally the information is presented to the users who apply it to their decisions making process.

Energy sources and radiation principles

Visible light is only one of many forms of electromagnetic energy. Radio waves, heat, ultraviolet rays and X rays are other familiar forms. All this energy is inherently similar and radiates in accordance with basic wave theory.

This theory describes electromagnetic energy is traveling in a harmonic, sinusoidal fashion at the “Velocity of light”. The distance from one peak to the next is the wave length and the number of peaks passing a fixed point in space per unit time is the wave frequency.

Waves obey the general equation

C = v λ

Where,

C = is essentially a constant (3 x 108 m/sec.) – Velocity of light

V = frequency , λ = wave length.

In remote sensing, it is most common to categorize electromagnetic waves by their wave length location within the electromagnetic spectrum. The most prevalent unit used to measure wave length along the spectrum is the micrometer (μm). A micrometer equals to 1 x 10-6m.

Forms of electromagnetic energy            Wavelength

Television and radio waves             -  > 30 cm

Micro waves                                          -  0.1 – 30 cm

Far infrared                                        -  7.0 – 15.0 μm

Thermal infrared                                 -  3.0 – 14.0 μm

Mid infrared                                       -  1.3 – 3.0μm

Near infrared                                           -  0.7 – 1.3 μm

Visible                                                     -  0.4 – 0.7 μm

X rays

R rays                                                       - up to 0.03 μm

Cosmic rays

Most common sensing system operates in one or several of the visible, infrared or micro waves.

Micro waves: RADAR, Scateriometer, Altimeter, Micro wave radiometer.

IR range: Spectrometers, Radiometers, polarimeters, laser based active sensing system.

Visible: Mostly used for natural resource mapping.

Although many characteristics of electromagnetic radiation are most easily described by wave theory, another theory offers useful in lights into low electromagnetic energy interacts with matter. This theory (particle theory) suggests that electromagnetic radiation is composed of many discrete units called “Photons” of “quanta”.

The energy of a quantum is given by

Q = h v

Where,

Q = Energy of a quantum, Joules (J)

h = Planck’s constant, 6.626 x 10-34 J sec.

v = frequency 

 (C = v λ)  

 V = (C/ λ)

By relating wave and quantum models of electromagnetic radiation behaviour by substituting C / λ for V in the above equation.

Q = hc / λ

∴ Quantum is inversely proportional to wave length. The longer the wave length involved the lower its energy content. This has important implications in remote sensing from the stand point that naturally emitted long wave length radiation, such as microwave emission from terrain features, is more difficult to sense than radiation of shorter wave lengths, such as emitted thermal IR energy.

The sun is the most obvious source of electromagnetic radiation for remote sensing. However all matter at temperatures above absolute zero (0° K or - 273°C) continuously emits electromagnetic radiation. Thus terrestrial objects are also source of radiation though it is of considerable different magnitude and spectral composition than that of the sun. Thus energy radiated by objects is a function of the surface temperature of the object. This property expressed by Stefan – Boltzmann Law, which is

M = σ T4

M = Total radiant existence from the surface of a material watts (w) m-2.

σ = Stefan – Boltzmann constant 5.6697 x 10-8 (Wm-2 °K-4)

T4 = Absolute temperature (ok) of the emitting material.

It is important to note that the total energy emitted from an object varies as T4 and therefore increases very rapidly with increase in temperature

Energy interactions with earth surface features

When electromagnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature are possible. Various fractions of the energy incident on the element are reflected, absorbed and / or transmitted. Applying the principle of conservation of energy, the interrelationship between these three energy interactions are

EI (λ)   =  ER (λ ) + EA (λ) + ET (λ)

(Incident energy) = (Reflected) + (Absorbed) + (Transmitted)

The proportion of energy reflected, absorbed and transmitted varies with earth surface features. This depends on physical and chemical characteristics of features. Further proportion of ER, EA, ET vary at different wave lengths of electromagnetic radiation for a given feature. The above facts of electromagnetic spectrum and earth surface features responses to electromagnetic spectrum permits us to distinguish different features on an image / photography / digital data obtained.