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After reading this article you will learn about:- 1. Definition of Remote Sensing 2. Basics 3. Applications.
Definition of Remote Sensing:
Remote Sensing can be defined as the science and art of acquiring information about an object made from a distance without physical contact with the object.
In practice, remote sensing is the utilization at a distance (as from aircraft, spacecraft, satellite, or ship) of any device for gathering information about the environment.
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The information may be acquired from aircraft or artificial satellite. The principle of remote sensing is same with that of human eye vision, where the reflected sun rays are sensed by the eye sensor that sends the information to brain for identification of the feature.
Basics of Remote Sensing:
Electro-magnetic radiation which is reflected or emitted from an object is the usual source of remote sensing data. However, any media such as gravity or magnetic fields can be utilized in remote sensing.
Electromagnetic radiation as waves is energy transported through space in the form of periodic disturbances of electric and magnetic fields. The wide frequency range of electromagnetic waves constitutes the Electromagnetic Spectrum.
The Electromagnetic Spectrum:
A device to detect the electro-magnetic radiation reflected or emitted from an object is called a “remote sensor” or “sensor”. Cameras or scanners are examples of remote sensors.
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A vehicle to carry the sensor is called a “platform”. Aircrafts or satellites are used as platforms.
Types of Platform:
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The vehicle or carrier for remote sensors is called the platform. Typical platforms are satellite and aircraft, but they also include radio controlled airplanes, balloons, kites for low altitude remote sensing, as well as ladder trucks or “cherry pickers” for ground investigations.
Types of Remote Sensing:
(1) Ground remote sensing:
Where sensor is placed in a handheld Radiometer to measure the amount of reflectance.
(2) Aerial remote sensing:
Where sensor/camera is placed on aircraft.
(3) Satellite remote sensing:
Where sensor is placed on artificial satellite.
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Interpreting Optical Remote Sensing Images:
The reflectance from an earth’s feature is recorded by the sensor and sent back to satellite data receiving station on earth. The initial data is received at the stations in digital form which is processed and corrected geometrically and radiometrically to produce final product like False colour composite paper product or computer compatible digital product.
These satellite data products are then used for interpretation and final map preparation. The characteristics of remote sensing data are referred in terms of nos of sensors, spectral resolution, spatial resolution, radiometric resolution and temporal resolution.
Optical remote sensing systems are classified into the following types, depending on the number of spectral bands used in the imaging process.
Panchromatic Imaging System:
The sensor is a single channel detector sensitive to radiation within a broad wavelength range, giving rise to black and white images.
Multispectral Imaging System:
The sensor is a multichannel detector with a few spectral bands. Each channel is sensitive to radiation within a narrow wavelength band. The resulting image is a multilayer image.
Superspectral Imaging Systems:
A superspectral imaging sensor has many more spectral channels (typically >10) than a multispectral sensor. The bands have narrower bandwidths, enabling the finer spectral characteristics of the targets to be captured by the sensor.
Hyperspectral Imaging Systems:
A hyperspectral imaging system is also known as an “imaging spectrometer”. It acquires images in about a hundred or more contiguous spectral bands. The precise spectral information contained in a hyperspectral image enables better characterisation and identification of targets.
False Colour Composite:
The display colour assignment for any band of a multispectral image can be done in an entirely arbitrary manner. In this case, the colour of a target in the displayed image does not have any resemblance to its actual colour.
The resulting product is known as a false colour composite image. There are many possible schemes of producing false colour composite images. However, some scheme may be more suitable for detecting certain objects in the image.
A very common false colour composite scheme for displaying a SPOT multispectral image is shown below:
R = NIR band
G = red band
B = green band
Digital image processing:
Digital image processing involves the manipulation and interpretation of digital images with the aid of a computer. Several computer assisted operations are applied in the digital image processing system.
Important among them are:
1. Image rectification and restoration.
2. Image enhancement.
3. Image classification.
4. Data merging and GIS integration.
Image interpretation:
Image interpretation is defined as the extraction of qualitative and quantitative information in the form of a map, about the shape, function, quality, condition, relationship of and between objects, etc. by using human knowledge of experience.
Types of information extraction by remote sensing:
Interpretation elements:
The following eight elements are mostly used in image interpretation:
1. Size:
A proper photo-scale should be selected depending on the purpose of the interpretation. Approximate size of an object can be measured by multiplying the length on the image by the inverse of the photo-scale.
2. Shape:
The specific shape of an object as it is viewed from above will be imaged on a vertical photograph. Therefore, the shape looking from a vertical view should be known. For example, the crown of a conifer tree locks like a circle, while that of a deciduous tree has an irregular shape. Airports, harbors, factories and so on, can also be identified by their shape.
3. Shadow:
Shadow is usually a visual obstacle for image interpretation. However, shadow can also give height information about towers, tall buildings etc., as well as shape information from the non-vertical perspective such as the shape of a bridge.
4. Tone:
The continuous gray scale varying from white to black is called tone. In panchromatic photographs, any object will reflect its unique tone according to the reflectance.
5. Colour:
Colour is more convenient for the identification of object details.
6. Texture:
Texture is a group of repeated small patterns.
7. Pattern:
Pattern is a regular, usually repeated shape with respect to an object.
8. Associated relationships or context:
A specific combination of elements, geographic characteristics, configuration of the surroundings or the context of an object can provide the user with specific information for the image.
Application of Remote Sensing:
1. Land cover/land use mapping
2. Wetland studies
3. Watershed management.
4. Geology and Geomorphology
5. Soil survey
6. Agriculture
7. Forestry.