Principles of Remote Sensing - Assignment Example

1. Introduction Remote sensing is based on the techniques used to gather information about either an object or a phenomenon without the use of physical contact. In recent years remote sensing has been used to refer to the utilisation of aerial sensor technologies in order to detect and classify various objects and phenomenon present on the surface of the Earth as well as in the atmosphere and oceans. This is done by using propagated signals such as electromagnetic radiation released from aircraft as well as satellites. (Schowengerdt, 2007) (Schott, 2007) Remote sensing can be classified into two broad categories based on the source used for the propagation which are passive and active remote sensing. (Liu & Mason, 2009) When passive remote sensing is applied, the source used is generally based on natural radiation which is being emitted and reflected by the object of interest as well as the surrounding region. Generally reflected sunlight is used as the most common source for passive remote sensing. The remote sensors employed for passive remote sensing are film photography, charge coupled devices, radiometers and infrared sensors. On the other hand when active remote sensing is applied, energy is emitted towards the object of interest as well as the surrounding regions. The amount of radiation that is reflected back or back scattered from the object of interest is then measured using sensing techniques. Typical examples of active remote sensing include RADAR and LiDAR. 2. Principles of Remote Sensing In order to discover and detect objects as well as phenomenon it is necessary that the radiant energy that is reflected or emitted by the object of interest be detected and recorded for analysis. Various materials tend to return differing amounts of energy based on the bands of the electromagnetic spectrum based on the incident energy. The properties of the material including its structural, physical and chemical properties as well as the surface roughness, intensity, wavelength and angle of incidence all help in determining the returned amount of energy. 3. Temperature and Radiated Electromagnetic Radiation As soon as the temperature of a body goes up the absolute zero, it begins to radiate electromagnetic radiation due to atomic and molecular oscillations. As the absolute temperature of a body increases, so does the amount of emitted radiation. The emitted radiation tends to peak as progressively shorter wavelengths are used. The energy from the sun comes in two forms which are radiation and illumination. These forms of energy from the sun can be captured through the use of conventional and unconventional cameras and other sensing devices. The biggest distinction that is relied upon in order to sense electromagnetic radiation is that each unique object tends to display its own specific distributions of reflected, absorbed and emitted radiations. The exploitation of these spectral properties enables to create distinctions between various objects in order to gather information relating to the shape, size as well as other chemical and physical properties of the object of interest. 4. Electromagnetic Radiation and the Electromagnetic Spectrum Electromagnetic radiation is a dynamic form of energy that tends to propagate in wave form at the speed of light i.e. 3 x 108 m/s. The wave motion in electromagnetic radiation can be characterised using the following parameters: 1. Wavelength (λ); 2. Frequency (v); 3. Velocity (c). The parameters listed above can be related mathematically as: c = vλ Figure 1 - A typical electromagnetic wave showing the two principal components which are the electric field (E) and the magnetic field (M) which are both perpendicular to the direction of propagation. The basic wave theory is used in order to deal with the radiation of electromagnetic energy. As per this theory, electromagnetic energy tends to move in a harmonic sinusoidal manner at the speed of light. Other than the wave theory, the particle theory offers greater insight into the interaction between matter and electromagnetic energy. This theory emphasises that electromagnetic radiation is constituted out of various discrete units that are known as photons or quantas. (Campbell, 1996) The energy of a photon can be expressed as: Q = hc / λ = hv where: Q is the quantum’s energy h is the Planck’s constant 5. Electromagnetic Radiation and the Atmosphere The sun acts as the major source for electromagnetic radiation and this radiation must interact with the atmosphere twice that is one on its way from Sun to Earth and secondly when it is reflected from the surface of the Earth. Remote sensing relies on electromagnetic radiation passing through the atmosphere because it reveals the state of the atmosphere and the electromagnetic radiation traversing the atmosphere tends to get changed by it. The components of the atmosphere tend to scatter and absorb the incoming radiation (both directions) and attenuate it to modify its spatial distribution such that sunlight scattered in the atmosphere and energy reflected from the ground is introduced to the field of view of radiation. (Curan, 1985) The scattering and absorption of light differ depending on the part of the spectrum in question. The incident solar energy is subject to three distinct physical processes as it traverses the atmosphere. These are scattering, absorption and refraction. 5.1. Scattering Electromagnetic radiation is redirected by interaction with large particle suspended in the air or by larger molecules of gases in the air. Scattering tends to change the ground’s spectral signature as received by the sensor. The total scattering is dependent upon the particle size, abundance, wavelength of radiation, depth of atmosphere and particle concentration. Particle concentration tends to vary with time and season and hence scattering is unevenly distributed spatially and in respect of time. The scattering process depends on the particle size and the wavelength and are summarised in the table below. Table 1 - Scattering processes Scattering Process Wavelength Particle Size Particle Type Selective Rayleigh λ-4 < 1 µm Air molecules Mie λo to λ-4 0.1 to 10 µm Haze, smoke Non-selective λo >10 µm Dust, fog, clouds 5.2. Absorption The gas molecules in the atmosphere tend to absorb the passing electromagnetic radiation within specific spectral bands. Three gases are largely responsible for absorption which are ozone, carbon dioxide and water vapour. Ozone tends to absorb the high energy, short wavelength portions of the ultraviolet spectrum with a wavelength of less than 0.24 µm. Carbon dioxide tends to absorb radiation in the mid and far infrared regions between 13 and 17.5 µm while water vapour tends to absorb in the bands between 5.5 and 7 µm as well as above the 27 µm limit. Absorption tends to reduce the amount of incident light and thus increases the dullness. 5.3. Refraction Light tends to bend at the interface of different media and this occurs in the atmosphere as light passes between layers with differing clarity, humidity and temperature. Variations in these parameters tend to change the density of the atmospheric layers and these cause light to bend. 6. Electromagnetic Radiation and the Earth’s Surface Incident solar radiation on the Earth’s surface is either reflected, transmitted or absorbed and consequently emitted by the surface. As a result the electromagnetic radiation changes in magnitude, wavelength, phase, direction and polarization. The data acquired by remote sensing contains spatial information such as size, shape and orientation as well as spectral information such as tone, colour and spectral significance. Figure 2 - Energys interaction with the Earths surface. The infrared and object visible spectrum can be divided into three categories based on wavelength. The reflective region comprises of wavelength between 0.3 and 3 µm and this band represents the energy reflected from the earth’s surface. The thermal infrared band exists between 8 and 14 µm and represents the energy due to thermal emission from the earth’s surface. Reflection and self emission both play a large play in the wavelength band between 3 and 5.5 µm. 6.1. Reflection Surface reflections are very important for remote sensing. The intensity of reflection depends on the surface’s refractive index, the angles of incidence and reflection as well as the absorption coefficient. Figure 3 - Various scattering surfaces (a) perfect specular reflector (b) near perfect specular reflector (c) Lambertian (d) quassi-Lambertian (e) complex. 6.2. Transmission If radiation traverses a medium with significant attenuation it is known as transmission. The transmittance () of a substance is given by: 7. Bibliography Campbell, J.B., 1996. Introduction to Remote Sensing. London: Taylor & Francis. Curan, P.J., 1985. Principles of Remote Sensing. London: Longman. Liu, J.G. & Mason, P.J., 2009. Essential Image Processing for GIS and Remote Sensing. Oxford: Wiley-Blackwell. Schott, J.R., 2007. Remote sensing: the image chain approach. 2nd ed. Oxford: Oxford University Press. Schowengerdt, R.A., 2007. Remote sensing: models and methods for image processing. 3rd ed. Academic Press. Read More