Types of sensors

A sensor is a device that gathers energy (EMR) and converts it to a signal and presents it in a form suitable for obtaining information about the object under investigation.

CLASSIFICATION OF SENSORS

Remote sensors can broadly be classified as passive sensors and active sensors.

Passive sensors measure the natural light emitted from the sun.

Active sensors have their own source of light and the sensors measure the reflected energy.

The Earth’s surface interacts with the incoming Electro-Magnetic Radiation (EMR) from the Sun.  This is known as incident energy (E_i). The three fundamental interactions with incident energy are:

1. Reflected energy (E_r)
2. Absorbed energy (E_a) and
3. Transmitted energy (E_t)

 Incident energy formula:

E_i = E_r + E_a + E_t

Passive sensors measure this natural energy at specific frequencies or wavelengths. Wavelength is conventionally measured in ‘m’ or multiples thereof ‘nm’ etc. The frequencies radiations typically sensed are listed below:

Visible radiation – 390 to 700 nm

Infra-red radiation – 750 to 1 mm

Ultra-violet radiation – 100 to 400 nm

These wavelength ranges are known as “bands”

Sensors can have multiple bands (3 to 10 bands) and this is known as MSS (Multi-Spectral Sensing).

Hundereds of finer bands are known as Hyper-spectral imaging.

RELATIONSHIP BETWEEN REFLECTED LIGHT AND SPECTRAL REFLECTANCE

The solar radiation that is incident on Earth, is reflected back to the passive sensors and this reflected energy is detected by the passive sensors.

Reflected energy formula

E_r = E_i - E_a - E_t

_{Different objects on Earth, reflect, transmit and absorb different amounts of energy and this implies that each feature on Earth a unique property called spectral reflectance (p).}

_{Spectral reflectance formula:}

_{p =} E_r / Ei

Examples of active sensors are:

1. Radar
2. Camera with flashlight

Examples of passive sensors are:

1. Camera without flashlight
2. ALL remote sensing sensors

Non-scanning or framing sensors: These sensors measure the radiation coming from the entire scene at once.
Examples of non scanning sensors are:

1. Our eyes
2. Photographic cameras

Imaging sensors: These sensors form image by collected radiation. They may be scanning sensors or non-imaging sensors. In scanning sensors, the image is sensed point-by-point. These scanners may be along track scanners in which the image is acquired line by line or across track scanners in which the image is acquired pixel by pixel.

Non imaging sensors: These type of sensors do not form the image. They are used to recors spectral quantity as afunction of time.

Examples are: Sensors for temperature measurement, study of the atmosphere, etc.

Image plane scanning: In this type of sensor, the lens is used after the scan mirror to focus the light on the detector

Object plane scanning: In this type of sensor, the lens is placed before the scan mirror to focus the light on the detector.

Most of the active sensors operate in the microwave portion of the electromagnetic spectrum. A few of the active sensors are listed below:

1. Laser Altimeter
2. Radar
3. Lidar
4. Ranging Instrument
5. Scatterometer and
6. Sounder

Passive sensors include different types of radiometers and spectrometers. Passive remote sensors in remote sensing operate in the visible, infrared, thermal infrared and microwave regions of the electromagnetic spectrum. Passive remote sensors used are listed below:

1. Accelerometer
2. Hyperspectral radiometer
3. Imaging radiometer
4. Radiometer
5. Sounder
6. Spectrometer and
7. Spectroradiometer

Interaction of EMR with Earth's surface

The interaction of Electro-magnetic radiation (EMR) with the atmosphere and the earth's surface play a very important role in Remote sensing. Each molecule has a set of absorption bands in the electromangetic spectrum. Hence, only the wavelength regions outside the absorption bands of atmospheric gases can be used for remote sensing. These regions are known as Atmospheric Transmission Windows.
These windows are found in the visible, near infrared, certain bands of thermal infrared and microwave region.
The figure below shows the fate of atmospheric radiation

The gases in the atmosphere interact with solar irradiation and with the radiation reflected from the Earth's surface. The electromagnetic radiation (EMR ) will experience varying degrees of transmission, absorption, emittance and/or scattering.

Software Scenario Functions: Visibility Analysis

The portion of the terrain that can be seen from any particular elevation is known as viewshed. The process of visibility and intervisibility at a particular point on a topographic surface is called viewshed analysis or visibility analysis.

Some of the uses of this technique are:

1. Siting television, radio and cellular phone transmitters and receiving stations
2. Locating towers for observing forest fires
3. Routing highways that are not visible to nearby residents

Visibility analysis is useful in planning that requires features to be either visible or concealed .

The simplest method is to connect an observer location to every possible target location.

In the next step, ray tracing is carried out. This is done by following the line from the target point back to the observer point. Higher points obstruct the observer's view.

Among the many possible ways to determine intervisibility, the ray tracing technique is simple and useful, although it is less accurate.

Visibility analysis requires the use of a Triangulated Irregular Network (TIN) data model in which the surface is defined by triangular vertices.

Example: Consider a builder constructing houses at the foothills of a mountain range and desires to present a beautiful view of the landscape from the location of each house. After shortlisting the potential locations, the TIN model for each location is used by the GIS software to look in all directions at the vertices for a view from the vertices of the model. The software retrieves the elevation values and compares these values with the elevation of potential building sites. All the areas higher in elevation are classified as invisible. The resulting polygon map shows visible areas for each coverage tested.

Raster methods of visibility analysis are similar except that they are less elegant and more computationally expensive.

Electromagnetic radiation and its characteristics

Electromagnetic radiation: Electromagnetic radiation (EMR) is a a form of energy propogated through free space (vaccum) or a medium in the form of electromagnetic waves.EMR is termed as such because it is composed of an electric field and a magnetic field that oscillate simultaneously in planes mutually perpendicular to each other as well as to the direction of propogation of the radiation.

The two defining characteristics of electromagnetic radiation are its:

1. Frequency and
2. Wavelength

Frequency is the number of waves that pass a point in a specified time. It is measured in Hertz (Hz) or cycles per second.

Wavelength is the distance between two successive peaks of a wave. It is measured in meters (m) or its multiples (nm, mm, cm etc)

The range of electromagnetic waves is called electromagnetic spectrum.

ALL ELECTROMAGNETIC WAVES TRAVEL WITH THE SAME VELOCITY

(Velocity of light (C)~ 3*10^8 m/s)

Velocity, wavelength and frequency are related by the equation: C = frequency * wavelength

It is evident from this equation that frequency and wavelegth are inversely proportional

This follows that:

• a wave with a longer wavelength has lower frequency and thus lower energy
• a wave with a shorter wave wavelength has higher frequency and thus higher energy.

The electromagnetic spectrum is divided into seven regions. They are:

1. Radio waves
2. Microwaves
3. InfraRed (IR) waves
4. Visible light
5. UltraViolet (UV) rays
6. X rays and
7. Gamma rays

Usually. low energy radiation (Radio waves) is expressed as wavelengths while microwaves, infrared (IR), visible and ultraviolet (UV)  radiations are expressed as frequencies.

Radio waves
Radio waves are at the lowest range of the EM spectrum, with wavelengths greater than about 10 mm. Radio is used primarily for communications including voice, data and entertainment media.

Microwaves
Microwaves have wavelengths of about 10 mm to 100 micrometers (μm). Microwaves are used for high-bandwidth communications, radar and as a heat source for microwave ovens and industrial applications.

Infrared
Infrared is in the range of wavelengths of about 100 μm to 740 nanometers (nm). IR light is invisible to human eyes, but we can feel it as heat if the intensity is sufficient.

Visible light
Visible light is found in the middle of the EM spectrum, between IR and UV. It has wavelengths of about 740 nm to 380 nm. Visible light is defined as the wavelengths that are visible to most human eyes.

Ultraviolet
Ultraviolet light is in the range of the EM spectrum between visible light and X-rays. It has wavelengths of about 380 nm to about 10 nm. UV light is a component of sunlight; however, it is invisible to the human eye. It has numerous medical and industrial applications, but it can damage living tissue.

X-rays
X-rays are roughly classified into two types: soft X-rays and hard X-rays. Soft X-rays comprise the range of the EM spectrum between UV and gamma rays. Soft X-rays have wavelengths of about 10 nm to about 100 picometers (pm). Hard X-rays occupy the same region of the EM spectrum as gamma rays. The only difference between them is their source: X-rays are produced by accelerating electrons, while gamma rays are produced by atomic nuclei.

Gamma-rays
Gamma-rays are in the range of the spectrum above soft X-rays. Gamma-rays have wavelengths of less than 100 pm (4 × 10−9 inches). Gamma radiation causes damage to living tissue, which makes it useful for killing cancer cells when applied in carefully measured doses to small regions. Uncontrolled exposure, though, is extremely dangerous to humans.

Maps - Basic components, Types of maps & Map analysis

Basic components of a map:
The basic components of any map are listed below:

1. The Title of the map indicates what the map is trying to show
2. The Key explains the symbols shown in the map
3. The Scale gives the relationship between distance on the map to the actual distance on the Earth.
4. Tha map shows the Latitudes (parallels N or S of the equator) and Longitudes (meridians E or W of the prime meridian)
5. Compass rose showing the directions on a map.

The types of maps are:

1. Physical map
2. Political map
3. Thematic map
4. Cartogram and
5. Flow-line map

Map Analysis involves answering questions based on:

1. Title of the map
2. Type of map
3. Location of an object, area or phenomena
4. Meanings of the symbols and inferences from patterns
5. Relationship between locations and events over a period of time
6. The main idea or theme being represented by the map.

GIS-Unit 5-Syllabus-OU

UNIT 5

Introduction to remote sensing: Electromagnetic radiation, Characteristics, Interaction with Earth's surface, Sensor types, Satellite characteristics, IRS series, Data products, Interpretation of data.

Software Scenario Functions: Watershed modelling, Environmental modelling and Visibility analysis.

Map transformations

MAP TRANSFORMATIONS

Map transformation involves transformation of points from one map to points on another map while minimizing the differences between the two sets of points.

• The 'Helmert transformation' is the best choice for the vast majority of applications.
• The 'Helmert transformation' translates the points of one map horizontally and vertically and also rotates and scales the points (it uses 4 parameters).
• The 'affine transformation' is useful in cases where the paper has pronounced directional shrinking due to orientation of the fibers.
• The 'affine transformation' is also useful to compensate for some shearing in the map or for computing the shearing angle.
• The 'affine transformation' with five parameters translates points in the x direction and y direction, a rotation and two scale factors (one in the x direction and one in the y direction)
• The 'affine transformation' with six parameters consists of:
• translation in x direction
• translation in y direction
• two rotation and two scale factors (both axes are rotated and scaled separately)

Other map transformations used are:

1. Robust Helmert
2. Huber estimator
3. Vestimator and
4. Hampel estimator