Chapter 7 Introduction To Remote Sensing | Class 11th Practical Work in Geography revision notes

NCERT Class 11 Chapter 7 Practical Geography Introduction to Remote Sensing

Unlike aerial photo which observe similar to human eyes.

Remote sensing can go much beyond & react to much wider range of radiations reflected/emitted, absorbed and transmitted by all object surfaces at a temperature above 0 Kelvin () .

The term remote sensing was first used in the early 1960s. Later, it was defined as the total processes used to acquire and measure the information of some property of objects and phenomena by a recording device (sensor) that is not in physical contact with the objects and phenomena in study

• Absorptance: The ratio of the radiant energy absorbed by a substance to the energy it receives.
• Band: The specific wavelength interval in the electromagnetic spectrum.
• Digital image: An array of digital numbers (DN) arranged in rows and columns, having the property of an intensity value and their locations.
• Digital Number: An intensity value of a pixel in a digital image.
• Digital Image Processing: The numerical manipulation of DN values for extracting information about the phenomena of the surface they represent.
• Electromagnetic Radiation (EMR) : The Energy propagated through a space or a medium at a speed of light.
• Electromagnetic Spectrum: The continuum of EMR that ranges from short wave high frequency cosmic radiations to long wavelength low frequency radio waves.
• False Colour Composite (FCC) : An artificially generated colour image in which blue, green and red colours are assigned to the wavelength regions to which they do not belong in nature. For example, in standard a False Colour Composite blue is assigned to green radiations (0.5 to ) , green is assigned to red radiations (0.6 to  and red is assigned to Near Infrared radiation (0.7 to ) .
• Gray scale: A medium to calibrate the variations in the brightness of an image that ranges from black to white with intermediate grey values.
• Image: The permanent record of a scene comprising of natural and synthetic features and activities, produced by photographic and non – photographic means.
• Scene: The ground area covered by an image or a photograph.
• Sensor: Any imaging or non – imaging device that receives EMR and converts it into a signal that can be recorded and displayed as photographic or digital image.
• Reflectance: The ratio of the radiant energy reflected by a substance to the energy it receives.
• Spectral Band: The range of the wavelengths in the continuous spectrum such as the green band ranges from 0.5 to  and the range of NIR band 0.7 to .

Stages in Remote Sensing

• Source of Energy (sun/self-emission) – artificially by flashguns and energy beams in radar (radio detection and ranging)
• Transmission of energy from the source to the surface of the earth – propagates at speed of light called Electromagnetic Radiation (EMR) – vary in size and frequency
• Interaction of energy with the earth՚s surface
• Propagation of reflected/emitted energy through atmosphere – atmosphere comprises of gases, water molecules, and dust particles. The energy reflected from the objects comes in contact with the atmospheric constituents and the properties of the original energy are modified. , H, and  molecules absorb energy in the middle infrared region; the dust particles scatter the blue energy. Hence, the energy that is either absorbed or scattered by the atmospheric constituents never reaches to sensor placed onboard a satellite and the properties of the objects carried by such energy waves are left unrecorded
• Detection of the reflected/emitted energy by the sensor
• Conversion of energy received into photographic/digital form of data – Digital image comprises digital numbers that are arranged in rows and columns. These numbers may also be converted into an analogue (picture) form of data product. The sensor onboard an earth-orbiting satellite electronically transmits the collected image data to an Earth Receiving Station located in different parts of the world. In India, one such station is located at Shadnagar near Hyderabad
• Extraction of the information contents from the data products – errors are eliminated in data collected, image is corrected – digital image processing is done
• Conversion of information into Map/Tabular forms – converted into different layers of thematic maps (use quantitative measures as well)

Based on the size of the waves and frequency, the energy waves are grouped into Gamma, X – rays, Ultraviolet rays, visible rays, Infrared rays, Microwaves, and Radio waves. Each one of these broad regions of spectrum is used in different applications. However, the visible, infrared and microwave regions of energy are used in remote sensing

Interaction of Energy with the Earth՚s Surface: The propagating energy finally interacts with the objects of the surface of the earth. This leads to absorption, transmission, reflection, or emission of energy from the objects. We all know that all objects vary in their composition, appearance forms, and other properties. Hence, the objects՚ responses to the energy they receive are also not uniform.

For example, a fresh water body absorbs more energy in the red and infrared regions of the spectrum and appears dark/black in a satellite image whereas turbid water body reflects more in blue and green regions of spectrum and appears in light tone

Spectral Signatures Are Important

• Detection of Reflected/Emitted Energy by the Sensor: The sensors recording the energy that they receive are placed in a near – polar sun synchronous orbit at an altitude of 700 – 900 km. These satellites are known as remote sensing satellites (e. g. Indian Remote Sensing Series) . As against these satellites, the weather monitoring and telecommunication satellites are placed in a Geostationary position (the satellite is always positioned over its orbit that synchronizes with the direction of the rotation of the earth) and revolves around the earth (coinciding with the direction of the movement of the earth over its axis) at an altitude of nearly 36,000 km (e. g. INSAT series of satellites)
• Sun Synchronous – 700 – 900 km, cover  N to , earth resource applications
• Geostationary – 36000 km, cover  ^rdof the globe, orbital period is 24 hours, used in Telecommunication and weather monitoring
• The images so acquired by space-borne sensors are in digital format as against the photographic format obtained through a camera-based system

Images of Himalayas and Northern Indian Plain by IRS Satellite taken in May (Left) and November (Right) show differences in the types of vegetation. The red patches in May image refer to Coniferous vegetation. In November image the additional red patches refer to deciduous plants and the light red colour is related to the crops

Satellite Multispectral Scanners

• A sensor is a device that gathers electromagnetic radiations, converts it into a signal, and presents it in a form suitable for obtaining information about the objects under investigation. Based upon the form of the data output, the sensors are classified into photographic (analogue) and non – photographic (digital) sensors
• A photographic sensor (camera) records the images of the objects at an instance of exposure. On the other hand, a non – photographic sensor obtains the images of the objects in bit-by-bit form. These sensors are known as scanners.
• MultiSpectral Scanners (MSS) are used as sensors. These sensors are designed to obtain images of the objects while sweeping across the field of view. A scanner is usually made up of a reception system consisting of a mirror and detectors. A scanning sensor constructs the scene by recording a series of scan lines. While doing so, the motor device oscillates the scanning mirror through the angular field of view of the sensor, which determines the length of scan lines and is called swath. It is because of such reasons that the mode of collection of images by scanners is referred bit – by – bit. Each scene is composed of cells that determine the spatial resolution of an image. The oscillation of the scanning mirror across the scene directs the received energy to the detectors, where it is converted into electrical signals. These signals are further converted into numerical values called Digital Number (DN Values) for recording on a magnetic tape.
• Whiskbroom (Across track) Scanners: The whiskbroom scanners are made up of a rotating mirror and a single detector. The mirror is so oriented that when it completes a rotation, the detector sweeps across the field of view between  and  to obtain images in a large number of narrow spectral bands ranging from visible to middle infrared regions of the spectrum. The total extent of the oscillating sensor is known as the Total Field of View (TFOV) of the scanner. While scanning the entire field, the sensor՚s optical head is always placed at a particular dimension called the Instantaneous Field of View (IFOV) .
• Push broom (Along track) Scanners: The push broom scanners consist of a number of detectors, which are equivalent to the number obtained by dividing the swath of the sensor by the size of the spatial resolution. For example, the swath of High Resolution Visible Radiometer – 1 (HRVR – 1) of the French remote sensing satellite SPOT is 60 km and the spatial resolution is 20 metres. If we divide  metres, we get a number of 3000 detectors that are deployed in SPOT HRV – 1 sensor. In push broom scanner, all detectors are linearly arrayed and each detector collects the energy reflected by the ground cell (pixel) dimensions of 20 metres at a nadir՚s view.

Satellite Sensors Types of Resolution

Types of Sensor Resolution

• Temporal – how many image in a given time
• Spectral – shades can be recorded – frequency and sensitivity – quantization Data can be 0 to 16 or can be 0 to 255 – Color – The principles in obtaining such images is the extension of the dispersion of light in nature resulting in the appearance of the ‘rainbow’ and the use of prism in the lab. Strong absorption properties of fresh water in band 4 (Infrared) and mixed strong reflectance in band 2 (green) by dry surfaces
• Radiometric resolution corresponds to the sensitivity of a sensor, i.e.. its ability to measure and to enable distinction within the same spectral band of differences (intensity or energy)
• Spatial – distance – With an increasing resolution the identification of even smaller object surfaces become possible

Band 2: 0.52 to 0.59 

Band 3: 0.62 to 0.68 

Band 4: 0.77 to 0.86 

Dispersion of Light (The principle that is utilized in obtaining Multispectral Images)

The overall mechanism of obtaining images in a number of bands derives strength from the principle of the dispersion of light. You must have seen the rainbow. It is formed through a natural process of dispersion of light rays through water molecules present in the atmosphere. The same phenomena may be experimented by putting a beam of light at one side of a prism. At the other side of the prism, you may notice the dispersion of energy into seven colours that form white light

Elements of Image Interpretation

• Tone or color: smooth and dry object surfaces reflect more energy in comparison to the rough and moist surfaces. Healthy vegetation reflects strongly in the infrared region because of the multiple-layered leaf structure and appears in a light tone or bright red colour in standard false colour composite and the scrubs appear in greyish red colour. Similarly, a fresh water body absorbs much of the radiations received by it and appears in dark tone or black colour, whereas the turbid water body appears in light tone or light bluish colour in FCC due to mixed response shown by the water molecules as well as suspended sand particles
• Texture: dense residential areas in a large city form fine texture due to the concentration of the houses in a smaller area and the low-density residential areas produce a coarse texture. Similarly, in high-resolution images the sugarcane or millet plants produce coarse texture in comparison to the fine texture of rice or wheat plants. One can also notice the coarse texture in the images of scrubbed lands if compared with the fine texture of lush green evergreen forests.
• Size: It helps in distinctively identifying the industrial and industrial complexes with residential dwellings, stadium in the heart of the city with brick kilns at an urban fringe, size and hierarchy of the settlements
• Shape: railway line can be readily distinguished from a road due to its long continuous linearity in shape with gradual change in its course.
• Shadow: The shape of some of the objects is so typical that they could not be identified without finding out the length of the shadow they cast. For example, the Qutub Minar located in Delhi, minarets of mosques, overhead water tanks, electric or telephone lines, and similar features can only be identified using their shadow
• Pattern: planned residential areas with the same size and layout plan of the dwelling units in an urban area can easily be identified if their pattern is followed. Similarly, orchards and plantations produce arrangements of the same type of plants with uniform inter plant distances.
• Association: an educational institution always finds its association with its location in or near a residential area as well as the location of a playground within the same premises. Similarly, stadium, race course and golf course holds good for a large city, industrial sites along highway at the periphery of a growing city, and slums along drains and railway lines