Different types of remotely sensed images 3. Application of Remote Sensing in water resources engineering 4. Application of GIS in water resources engineering 6. 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.
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Different types of remotely sensed images 3. Application of Remote Sensing in water resources engineering 4. Application of GIS in water resources engineering 6. 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 This technique is still used, but largely the signal cameras have been replaced by Laser operated ones where the reflectance of a Laser beam projected from the bottom of the aircraft is sensed by electronic sensors.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. 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. The common remote sensing systems operate in one or more of the visible, reflected-infrared, thermal-infrared and microwave portions of the spectrum.
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. 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. 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. 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. 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.
Usually, the features that are classified through satellite remote sensing may be grouped into inanimate objects like soil, minerals, rock, water, etc. Soil is a heterogeneous mixture of minerals, containing considerable amount of organic matter and often moisture.
The proportion of these determine the spectral characteristics of the particular soil type. Rocks are assemblages of minerals and hence the reflectance spectrum of rocks is a composite of individual spectra of its constituent minerals. As for vegetation, the reflectance spectra vary according to the freshness of the leaves.
Thus the characteristics of the reflectance of various earth features for different electromagnetic wavelength bands is used to identify different earth objects and are hence also known as Spectral Signatures. A study of the spectral reflectance characteristics of natural earth surface features shows that the broad features are normally separable. In the following paragraphs, we discuss the spectral signatures of certain typical earth features, natural and artificial.
Vegetation The spectral signature or reflectance of healthy green vegetation is as given in Figure 6. In the visible range of electromagnetic wavelength spectrum, it has an absorption band in the blue and red parts because of the presence of chlorophyll. Even within the green part of the spectrum, only 10 to 15 percent of the incident light is reflected.
The reflectance peak is seen to be at 0. The reflectance property of healthy vegetation is seen to be much larger 40 percent or more in the infrared portion of the spectrum and is nearly constant from 0. In this range of electromagnetic spectrum, the reflectance variation is different for different plants and also between healthy vegetation and stressed vegetation.
Hence, a reflectance measurement in this range permits one to discriminate between different species of vegetation, though this differentiation is not very apparent in the visible range of the spectrum.
Beyond 1. Soil The spectral signature of soil is simpler in soils compared to that by vegetation since all the incoming radiation is either reflected or absorbed due to very little transmittance. A typical reflectance curve for soil shows increase in wavelength in the visible and near- infrared regions Figure 6. The reflectance property of soil varies with soil moisture content, texture that is, the relative content of sand silt and clay that makes up the soil , surface roughness, colour, content of organic matter, presence of sesquioxides, etc.
In the visible portion of the spectrum, there is a distinct decrease in reflectance as moisture content increases, since more moisture in soil makes a soil appear darker causing less reflectance. Soil texture influences the spectral reflectance by the way of difference in moisture holding capacity and due to difference in the size of the particles.
Soils with higher organic matter appears as light brown to grayish in colour. The reflectance characteristics in the visible region of the electromagnetic spectrum has been observed to be inversely proportional to the organic matter content. The presence of iron oxide in soil also significantly reduces the reflectance, at least in the visible wavelength.
Water For water resources engineer, locating areal extent of water bodies like lakes, rivers, ponds, etc. The spectral response from a water body is complex, as water in any quantity is a medium that is semi-transparent to electromagnetic radiation. Electromagnetic radiation incident on water may be absorbed, scattered and transmitted. The spectral response also varies according to the wavelength, the nature of the water surface calm or wavy , the angle of illumination and observation of reflected radiation from the surface and bottom of shallow water bodies.
Pure clear water has a relatively high reflectance in the visible wavelength bands between 0. Thus clear water appears dark on an infrared image. Therefore, location and delineation of water bodies from remotely sensed data in the higher wave bands can be done very accurately.
Man-made structures Sometimes it is required to identify artificial structures that is useful to an engineer.
For example roads, paved surfaces, canals, and even dams and barrages can be identified from remotely sensed images by their reflectance characteristics. Many of these, especially linear features, are clearly discernible in the visible waveband of electromagnetic spectrum. One of these, the Passive System, records the reflected electromagnetic energy of the earth, the source of the energy being the radiation of the Sun.
The other, called the Active System, employs its self-generated pulses and records the reflected pulse. These two systems may be compared to taking photographs in sunlight and with flashlight respectively. The active remote sensing systems mostly use radars that emit radiation in the microwave band of the electromagnetic spectrum.
This system is useful in cases where passive systems do not give sufficient information.
Module 3 - NPTel
Different types of remotely sensed images 3. Application of Remote Sensing in water resources engineering 4. Application of GIS in water resources engineering 6. 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.
Version 2 CE IIT, Kharagpur Civil Engineering (CE) Notes | EduRev
Irrigation Engineering. The total depth of water required to raise a crop over a unit area of land is usually. How much water is required for the proper growth of important crops How to estimate the water demand of crops What are the different seasons of crop growth What are the usual cropping patterns On what variables does the crop water requirement vary? Primarily, the plot or field is expected to receive water from rain falling on the land surface. But, as we know, the distribution of rain is rather uncertain both in time and space. Also some of the rain as in a light shower does not reach the ground as it may be intercepted by the leaves of the plant during a heavy downpour; much of the water might flow away as surface runoff. Hence, only a certain amount of falling rain may be effective in raising the soil moisture that is actually useful for plant growth.
Version 2 CE IIT, Kharagpur Notes | EduRev