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Rsgis Mod1-5@Az Documents
Electronic and communication (ECE)
Visvesvaraya Technological University
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MODULE 1
DEFINITION AND PROCESS OF REMOTE SENSING
INTRODUCTION
Now-a-days the field of Remote Sensing and GIS has become exciting and glamorous with rapidly expanding opportunities. Many organizations spend large amounts of money on these fields. Here the question arises why these fields are so important in recent years. Two main reasons are there behind this. 1) Now-a-days scientists, researchers, students, and even common people are showing great interest for better understanding of our environment. By environment we mean the geographic space of their study area and the events that take place there. In other words, we have come to realize that geographic space along with the data describing it, is part of our everyday world; almost every decision we take is influenced or dictated by some fact of geography. 2) Advancement in sophisticated space technology (which can provide large volume of spatial data), along with declining costs of computer hardware and software (which can handle these data) has made Remote Sensing and G.I. affordable to not only complex environmental / spatial situation but also affordable to an increasingly wider audience.
REMOTE SENSING AND ITS COMPONENTS: Remote sensing is the science of acquiring information about the Earth's surface without actually being in contact with it. This is done by sensing and recording reflected or emitted energy and processing, analyzing, and applying that information." In much of remote sensing, the process involves an interaction between incident radiation and the targets of interest. This is exemplified by the use of imaging systems where the following seven elements are involved. However that remote sensing also involves the sensing of emitted energy and the use of non-imaging sensors.-
Components of Remote Sensing
- Energy Source or Illumination (A) – the first requirement for remote sensing is to have an energy source which illuminates or provides electromagnetic energy to the target of interest.
- Radiation and the Atmosphre (B) – as the energy travels from its source to the target, it will come in contact with and interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor.
- Interaction with the Target (C) - once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation.
- Recording of Energy by the Sensor (D) - after the energy has been scattered by, or emitted from the target, we require a sensor (remote - not in contact with the target) to collect and record the electromagnetic radiation.
- Transmission, Reception, and Processing (E) - the energy recorded by the sensor has to be transmitted, often in electronic form, to a receiving and processing station where the data are processed into an image (hardcopy and/or digital).
- Interpretation and Analysis (F) - the processed image is interpreted, visually and/or digitally or electronically, to extract information about the target which was illuminated.
Frequency is normally measured in hertz (Hz), equivalent to one cycle per second, and various multiples of hertz. Wavelength and frequency are related by the following formula:
Therefore, the two are inversely related to each other. Shorter the wavelength higher the frequency. The longer the wavelength, the lower the frequency. Understanding the characteristics of electromagnetic radiation in terms of their wavelength and frequency is crucial in understanding the information to be extracted from remote sensing data. The electromagnetic spectrum ranges from the shorter wavelengths (including gamma and x- rays) to the longer wavelengths (including microwaves and broadcast radio waves). There are several regions of the electromagnetic spectrum which are useful for remote sensing.
WAVELENGTH REGIONS IMPORTANT TO REMOTE SENSING:
1 Ultraviolet or UV For the most purposes ultraviolet or UV of the spectrum shortest wavelengths are practical for remote sensing. This wavelength beyond the violet portion of the visible wavelengths hence it name. Some earth surface materials primarily rocks and materials are emit visible radiation when illuminated by UV radiation.
2 Visible Spectrums The light which our eyes - our "remote sensors" - can detect is part of the visible spectrum. It is important to recognize how small the visible portion is relative to the rest of the spectrum. There is a lot of radiation around us which is “invisible" to our eyes, but can be detected by other remote sensing instruments and used to our advantage. The visible wavelengths cover a range from approximately 0 to 0 μm. The longest visible wavelength is red and the shortest is violet. Common wavelengths of what we perceive as particular colours from the visible portion of the spectrum are listed below. It Is important to note that this is the only portion of the spectrum we can associate with the concept of colors. Violet: 0 -0 μm Blue: 0 -0 μm Green: 0 -0 μm Yellow: 0 -0 μm Orange: 0 -0 μm Red: 0 -0 μm Blue, green, and red are the primary colours or wavelengths of the visible spectrum. They are defined as such because no single primary colour can be created from the other two, but all other colours can be formed by combining blue, green, and red in various proportions. Although we see sunlight as a uniform or homogeneous colour, it is actually composed of various wavelengths of radiation in primarily the ultraviolet, visible and infrared portions of the spectrum. The visible portion of this radiation can be shown in its component colours when sunlight is passed through a prism, which bends the light in differing amounts according to wavelength.
3 Infrared (IR)
radiobroadcast.
ENERGY INTERACTIONS WITH THE ATMOSPHERE
Before radiation used for remote sensing reaches the Earth's surface it has to travel through some distance of the Earth's atmosphere. Particles and gases in the atmosphere can affect the incoming light and radiation. These effects are caused by the mechanisms of scattering and absorption.
Energy Interaction with Atmosphere SCATTERING Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path. How much scattering takes place depends on several factors including the wavelength of the radiation, the abundance of particles or gases, and the distance the radiation travels through the atmosphere. There are three (3) types of scattering which take place.
RAYLEIGH SCATTERING Rayleigh scattering occurs when particles are very small compared to the wavelength of the radiation. These could be articles such as small specks of dust or nitrogen and oxygen molecules. Rayleigh scattering causes shorter wavelengths of energy to be scattered much more than longer wavelengths. Rayleigh scattering is the dominant scattering mechanism in the upper atmosphere fact that the sky appears "blue" during the day is because of this phenomenon. As sunlight passes through the atmosphere, the shorter wavelengths (i. blue) of the visible spectrum are scattered more than the other (longer) visible wavelengths. At sunrise and sunset the light has to travel farther through the atmosphere than at midday and the scattering of the
Mie scattering occurs when the particles are just about the same size as the wavelength of the radiation. Dust, pollen, smoke and water vapour are common causes of Mie scattering which tends to affect longer wavelengths than those affected by Rayleigh scattering. Mie scattering occurs mostly in the lower portions of the atmosphere where larger particles are more abundant, and dominates when cloud conditions are overcast. The final scattering mechanism of importance is called nonselective scattering. This occurs when the particles are much larger than the wavelength of the radiation. Water droplets and large dust particles can cause this type of scattering. Nonselective scattering gets its name from the fact that all wavelengths are scattered about equally. This type of scattering causes fog and clouds to appear white to our eyes because blue, green, and red light are all scattered in approximately equal quantities (blue+green+red light = white light).
ATMOSPHERIC WINDOWS
While EMR is transmitted from the sun to the surface of the earth, it passes through the atmosphere. Here, electromagnetic radiation is scattered and absorbed by gases and dust particles. Besides the major atmospheric gaseous components like molecular nitrogen and oxygen, other constituents like water vapour, methane, hydrogen, helium and nitrogen compounds play important role in modifying electro magnetic radiation. This affects image quality. Regions of the electromagnetic spectrum in which the atmosphere is transparent are called atmospheric windows. In other words, certain spectral regions of the electromagnetic radiation pass through the atmosphere without much attenuation are called atmospheric windows. The atmosphere is practically transparent in the visible region of the electromagnetic spectrum and therefore, many of the satellite based remote sensing sensors are designed to collect data in this region. Some of the commonly used atmospheric windows are shown in the figure.
Figure. They are: 0.38-0 microns (visible), 0.72-3 microns (near infra-red and middle infra-red), and 8.00-14 microns (thermal infra-red). Transmission100%UVVisibleInfraredEnergy Blocked0 Wavelength (microns)1101001 mm
SPECTRAL SIGNATURE CONCEPTS-TYPICAL SPECTRAL REFLECTANCE CHARACTRISTICS OF WATER, VEGETATION AND SOIL: A basic assumption made in remote sensing is that a specific target has an individual and characteristic manner of interacting with incident radiation. The manner of interaction is described by the spectral response of the target. The spectral reflectance curves describe the spectral response of a target in a particular wavelength region of electromagnetic spectrum, which, in turn depends upon certain factors, namely, orientation of the sun (solar azimuth), the height of the Sun in the sky (solar elevation angle), the direction in which the sensor is pointing relative to nadir (the look angle) and nature of the target, that is, state of health of vegetation. Spectral Reflectivity •Reflectivity is the fraction of incident radiation reflected by a surface
•The reflectance characteristics of Earth’s surface features may be quantified by measuring the portion of incident energy that is reflected
•This is measured as a function of wavelength (λ) and is called spectral reflectance (rλ).
If a plant is subject to some form of stress, it may decrease chlorophyll production resulting in less chlorophyll absorption in the blue and red bands Often the red reflectance increases to the point that we see the plant turn yellow (combination of green and red)
Spectral reflectance of Vegetation
- In the range from about 0 to 1 μma plant leaf typically reflects 40 -50% of the energy incident upon it primarily due to the internal structure of plant leaves
- Because the internal structure of leaves are highly variable between plant species, reflectance measurements in this range often permit us to discriminate between species (even if they look the same in visible wavelengths)
- Many plant stresses alter the reflectance in this region, and sensors operating in this range are often used for vegetation stress detection
Spectral reflectance of Vegetation
Beyond 1 μm energy incident upon vegetation is essentially absorbed or reflected with little to no transmittance of energy
Dips in reflectance occur at 1, 1 and 2 μm because water in the leaf absorbs strongly at these wavelengths (water absorption bands)
Reflectance peaks occur at about 1μmand 2 μm, between the absorption bands
Throughout the range beyond 1 μm, leaf reflectance is approximately inversely related to the total water present in a leaf which is a function of both the moisture content and the thickness of a leaf
Spectral reflectance of Soil
•The factors that influence soil reflectance act over less specified spectral bands
•Factors affecting soil reflectance are moisture content, soil texture (proportion of sand, silt and clay), surface roughness, presence of iron oxide and organic matter content
•The presence of moisture in soil will decrease its reflectance -this effect is greatest in the water absorption bands at about 1, 1, 2 and 2 μm
•Soil moisture content is strongly related to the soil texture.
ELEMENTS OF VISUAL INTERPRETATION TECHNIQUES
A systematic study of aerial photographs and satellite imageries usually, involves several characteristics of features shown on an image and it depend upon field of application. Most of the application consider the following basic characteristics or variation in them which aid the visual interpretation process of satellites imagery Although there is a difference of opinion on the number of elements ,there is namely tone, size, shape, texture, pattern, location, association, shadow and resolution
Tone:- refers to the relative brightness or colour of objects in an image. Generally, tone is the fundamental element for distinguishing between different targets or features. Variations in tone also allows the elements of shape, texture, and pattern of objects to be distinguished.
Shape:- refers to the general form, structure, or outline of individual objects. Shape can be a very distinctive clue for interpretation. Straight edge shapes typically represent urban or agricultural (field) targets, while natural features, such as forest edges, are generally more irregular in shape, except where man has created a road or clear cuts. Farm or crop land irrigated by rotating sprinkler systems would appear as circular shapes.
Size:- of objects in an image is a function of scale. It is important to assess the size of a target relative to other objects in a scene, as well as the absolute size, to aid in the interpretation of that target. A quick approximation of target size can direct interpretation to an appropriate result more quickly. For example, if an interpreter had to distinguish zones of land use, and had identified an area with a number of buildings in it, large buildings such as factories or warehouses would suggest commercial property, whereas small buildings would indicate residential use
Pattern:- refers to the spatial arrangement of visibly discernible objects. Typically an orderly repetition of similar tones and textures will produce a distinctive and ultimately recognizable pattern. Orchards with evenly spaced trees, and urban streets with regularly spaced houses are good examples of pattern.
Texture:- refers to the arrangement and frequency of tonal variation in particular areas of an image. Rough textures would consist of a mottled tone where the grey levels change abruptly in a small area, whereas smooth textures would have very little tonal variation. Smooth textures are most often the result of uniform, even surfaces, such as fields, asphalt, or grasslands. A target with a rough surface and irregular structure, such as a forest canopy, results in a rough textured appearance. Texture is one of the most important elements for distinguishing features in radar imagery.
Shadow:- is also helpful in interpretation as it may provide an idea of the profile and relative height of a target or targets which may make identification easier. However, shadows can also reduce or eliminate interpretation in their area of influence, since targets within shadows are much less (or not at all) discernible from their surroundings. Shadow is also useful for enhancing or identifying topography and landforms, particularly in radar imagery.
Association:- takes into account the relationship between other recognizable objects or features in proximity to the target of interest. The identification of features that one would expect to associate with other features may provide information to facilitate identification. In the example given above, commercial properties may be associated with proximity to major transportation routes, whereas residential areas would be associated with schools, playgrounds, and sports fields. In our example, a lake is associated with boats, a marina, and adjacent recreational land.
Site:-refers to the vocational characteristic of object such as topography, soil, vegetation and cultural features
FALSE COLOUR COMPOSITE
THE display OF colour assignment for any bands of a spectral image can be done entirely in the orbitary manner in this case the colour of the target in the displayed image does not have any resemblance to its actual colour this resulting product is called FCC. A very common FCCs given for displaying a SPOT multispectral image is
R- near higher band
G- red bands » rivers
B- green bands
This false color composite beam allows vegetations to be detected in the image. In this type of FCC images vegetations appears in different shapes of red depending on the types and conditions
It refers to how often a given sensor obtains imagery of a particular area. Ideally, the sensor obtains data repetitively to capture unique discriminating characteristics of the phenomena of interest.
RADIOMETRIC RESOLUTION: It is the capability to differentiate the spectral reflectance/ remittance from various targets. This depends on the number of quantization levels within the spectral band. In other words, the number of bits of digital data in the spectral band will decide the Sensitivity of the sensor. It is the smallest difference in exposure that can be detected in a given film analysis. It is also the ability of a given sensing system to discriminate between density leve:s. In general, the radiometric resolution is inversely proportional to contrast, so that higher Contrast film is able to resolve smaller differences in exposure. Low contrast films have greater radiometric range while highest contrast films have smaller exposure range and lower radiometric range.
MODULE 2:
The sensors are a transducer that converts the physical property into an electrical signal. The physical property can be weight, temp, pressure, force, electric, magnetic or electromagnetic
Sensors must have following significant characters
Range: Every sensor has a range in which they work with an acceptable error. It has the maximum and minimum value over which the sensor will work if the input is not in range then the output is unpredictable
Drift: The signal level varies for the same input over a long period. This is the low frequency changed in the sensor with time. It is often associated with electronic aging of components (or) a reference standard sensor.
Sensitivity (additional): It is defined as the ratio of the incremental change in the sensor output to the incremental change in the measured input.
Selectivity: It’s a system’s ability to measure a target in the presence of others interference.
Resolution: It’s a minimal change of measure and that can produce a detectable (clear images) increment in the output.
Response and recovery time: The response time is the time taken by the sensors for its output to reach 95% of its final value.
Rsgis Mod1-5@Az Documents
Course: Electronic and communication (ECE)
University: Visvesvaraya Technological University
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