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Version 2 CE IIT, Kharagpur






Version 2 CE IIT, Kharagpur

Instructional objectives:

On completion of this lesson, the student shall learn about:

1. The techniques of Remote Sensing and Geographic Information System (GIS)

2. Different types of remotely sensed images

3. Application of Remote Sensing in water resources engineering

4. Application of GIS in water resources engineering 6.3.0 Introduction The term Remote Sensing is applied to the study of earth’s features from images taken from space using satellites, or from nearer the earth using aircrafts. The technique of remote sensing has picked up in the past half a decade, largely due to the availability of digital computers, improved communication systems, digital imaging techniques and space technology. Remotely sensed data can be said to have its origin in photography, where the information about a target area is interpreted from photographs. Later this technique was extended to aeroplane - borne cameras giving rise to the science of aerial photography. This technique is still used, but largely the signal cameras have been replaced by Laser operated ones where the reflectance of a Laser beam projected from the bottom of the aircraft is sensed by electronic sensors.

In this chapter we shall discuss remote sensing using satellite as India has strived ahead in this field and made good use of satellite images. The satellite launching program of our country is one of the most ambitious in the world, and is still continuing to be so in the future as well. Amongst other fields, the Water Resources Engineers have benefited greatly by using satellite imaging techniques, some applications of which have been highlighted in this chapter.

The other topic that is discussed in this lesson is the Geographic Information System (GIS) that has wide applications in planning any spatially distributed projects.

Fundamentally, a GIS is a map in an electronic form, representing any type of spatial features. Additionally, properties or attributes may be attached to the spatial features.

Apart from its spatial data analysis capabilities, it provides an interface to remotely sensed images and field surveyed data. This technique has specifically benefited the Water Resources Engineers, which has been discussed in some detail.

6.3.1 Remote sensing through satellites Remote sensing means assessing the characteristics of a place (usually meant as the surface of the earth) from a distance. Though this term was coined during the 1960’s, similar technology had been practiced earlier like fitting a camera to a balloon and allowing it to float over the earth’s surface taking pictures, which may then be developed Version 2 CE IIT, Kharagpur and interpreted for specific purpose like geology, agriculture, forestry etc.

Photogrammetry, that is, taking pictures of the land surface from a low flying aircraft and comparing subsequent pictures to obtain the terrain relief has been extensively used in the last century and many books have been written on the subject.

In satellite remote sensing, too, cameras are fitted to the orbiting satellite and are focussed towards the earth. However, the cameras are special in the sense that they are sensitive to other wavelengths of the electromagnetic spectrum as well. As may be observed from Figure1, the electromagnetic spectrum identifies the wavelength of the electromagnetic energy, of which the visible portion (or light) occupies only a small portion. Actually, electromagnetic energy refers to light, heat and radio waves. Ordinary camera or the human eye are sensitive only to the visible light. But the satellites are equipped with Electromagnetic Sensors that can sense other forms of electromagnetic radiations as well. This includes not only the Blue (0.4-0.5μm), Green (0.5-0.6μm) and Red (0.6-0.7μm) of the spectrum but also longer wavelength regions termed as the Infrared (IR) spectrum (0.7-1000μm), which can again be further subdivided into the


a) Photographic IR : 0.7-0.9μm

b) Very near IR : 0.7-1.0μm

c) Reflected/Near IR : 0.7-3.0μm

d) Thermal IR : 3.0-1000μm Still longer wavelength is the microwave portion of the spectrum, which extends from 3000μm to 3m. The common remote sensing systems operate in one or more of the visible, reflected-infrared, thermal-infrared and microwave portions of the spectrum.

Version 2 CE IIT, Kharagpur 6.3.2 Interaction of electromagnetic radiation and earth Electromagnetic energy of the sun incident on the earth’s surface reaches fully upto the top of the atmosphere. However, as illustrated in Figure 2, not all of this energy reaches the surface of the earth, since part of the energy gets either scattered, absorbed or reflected by the atmosphere or cloud cover, if any. Only a part is transmitted upto the earth’s surface. Specifically, it may be said that although the electromagnetic radiation reaching the top of the atmosphere contains all wavelengths emitted by the sun, only specific wave bands of energy can pass through the atmosphere. This is because the gaseous components of the atmosphere act as selective absorbers. Molecules of different gases present in the atmosphere absorb different wavelengths due to the specific arrangement of atoms within the molecule and their energy levels. The main gaseous component of the atmosphere is nitrogen, but it has no prominent absorption features.

Oxygen, Ozone, Carbon Dioxide and Water Vapour, the other major components absorb electromagnetic wavelengths at certain specific wavelengths. The wavelengths at which electromagnetic radiation are partially or wholly transmitted through the atmosphere to reach the surface of the earth are known as atmospheric windows, as shown in Figure 3.

Since these radiations reach the surface of the earth, they are useful for remote sensing as they would be reflected or absorbed by the features of the earth giving the typical signatures for the sensors in the satellite (or any other space borne device) to record.

This is shown graphically in Figure 4.

The remote sensing system sensors are designed in such a way that can capture information for those wavelengths of electromagnetic radiation that occur within the atmospheric windows.

Version 2 CE IIT, Kharagpur Version 2 CE IIT, Kharagpur 6.3.3 Interaction of electromagnetic radiation with a surface When electromagnetic radiation strikes a surface, it may be reflected, scattered, absorbed or transmitted. These processes as not mutually exclusive: a beam of light may be partially reflected and partially absorbed. Which processes actually occur depends on the wavelength of the radiation, the angle at which the radiation intersects the surface and the roughness of the surface. Reflected radiation is returned from a surface at the same angle as it approached, the angle of incidence thus equals the angle of reflectance.

Scattered radiation, however, leaves the surface in all directions. Whether or not incident energy’s reflected or scattered is partly a function of the roughness variations of the surface compared to the wavelength of the incident radiation. If the ratio of roughness to wavelength is low (less than one), the radiation is reflected whereas, if the ratio is greater than one, the radiation is scattered. A surface which reflects all the incident energy is known as a Specular reflector whereas one which scatters all the energy equally is a Lambertian reflector. Real surfaces are neither fully specular nor fully lambertian.

However, for remote sensing purposes, a Lambertian nature is better. A remotely sensed image of a fully specular surface gives a bright reflectance (or signature) for one position of the camera and dark image at other positions. If the surface is uniform lambertian, then the reflectance obtained for the surface will be same irrespective of the location of the camera because the radiation from the surface would be scattered equally in all directions. Most natural surfaces that are observed using remote sensing systems are approximately lambertian at visible and infrared wavelengths.

6.3.4 Interaction of electromagnetic radiation with earth surface features From the general discussion on the nature of interaction of electromagnetic energy with any surface, we turn on to the earth features as these would be useful in Water Resources Engineering.

As observed from Figure 5, it is seen that a part of the electromagnetic energy reaches the earth’s surface, a part of it gets absorbed by the body, a part gets transmitted within the body, and a part gets reflected from the surface of the body. The proportion of energy that is reflected, absorbed and transmitted varies with the particular earth feature, like whether it is vegetation, water, urban landscape, etc. Besides, the proportion of energy is also dependent on the wavelength of the electromagnetic spectrum that is interacting with the surface. Thus, for a particular feature, the proportion of energy that is reflected, absorbed or transmitted varies with the wavelength that is interacting.

This means that two different features may reflect equal proportion of energy in one wavelength range and may not be separately identified but for another wavelength range their difference reflectance may allow a sensor to distinguish between the two features.

This variation in interaction of electromagnetic energy with any surface can be explained in the way we distinguish objects by separate colours. As we know, the wavelengths in the visible range of the spectrum strike all surfaces, but we observe different colours because each surface reflect only a particular wavelength and absorb the rest.

Version 2 CE IIT, Kharagpur Most of the sensors in remote sensing systems also operate in the wavelength regions in which the reflected energy predominates and thus the reflectance property of surfaces is very important. Of course, the sensors do not capture only the reflected energy in the visible range of wavelength but different sensors are designed to capture the reflected energy in other ranges of wavelengths as well.

The reflectance characteristics of the different features of the earth surface may be quantified by measuring the portion of incident energy that is reflected by a surface. This reflected energy is measured as a function of the wavelength and is called Spectral Reflectance. Quantitatively this is defined as the ratio of the energy of the wavelength reflected from an object and the energy that is incident upon it.

Spectral reflectance of any object usually varies according to the wavelength of the electromagnetic radiation that it is reflecting. A graph showing the spectral reflectance of an object for various wavelength is known as a Spectral Reflectance Curve (Figure 6).

The pattern of a Spectral Reflectance Curve gives an insight into the spectral characteristics of the object. It also helps in selecting the wavelength bands which may be suitable for identifying the object.

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