Remote Sensing

The technology of observing

and gathering information

about a target by a device

separated from it by a

distance.

(no physical contact)

Advantages of Remote Sensing

1. Large area coverage at reasonable cost.

2. Provides important physical quantities.

3. Extraction of information within short

time.

4. High accuracy if well-calibrated.

- Energy Source or Illumination (A)

- Radiation and the Atmosphere (B)

- Interaction with the Target (C)

- Recording of Energy by the Sensor (D)

- Transmission, Reception, and Processing (E)

- Interpretation and Analysis (F)

- Application (G)

Remote Sensing Process Components

4

Sensors

• Passive

– Sun’s energy which is reflected

(visible) or Absorbed and re-emitted

as thermal infrared wavelengths

– ASTER, Landsat, AVHRR

• Active

– Emit radiation.

– Radiation reflected is detected and

measured.

– RADAR, .

Types of Waves

• There are two main types of waves

– Mechanical waves Sound Waves

• Some physical medium is being disturbed.

• The wave is the propagation of a disturbance

through a medium.

– Electromagnetic waves R.S

• No medium required.

• Examples are light, radio waves, x-rays.

The Spectrum of EM Waves

Notes on the EM Spectrum

• Radio Waves

– Wavelengths of more than 104 m

to about 0.1 m .

– Used in radio and television

communication systems.

• Microwaves

– Wavelengths from about 0.3 m to

10-4 m.

– Well suited for radar systems.

– Microwave ovens are an

application.

Notes on the EM Spectrum, 2

• Infrared waves

– Wavelengths of about 10-3 m to 7 x 10-7 m

– Incorrectly called “heat waves”

– Produced by hot objects and molecules

– Readily absorbed by most materials

• Visible light

– Part of the spectrum detected by the human eye

– Most sensitive at about 5.5 x 10-7 m (yellow-

green)

More About Visible Light

• Different wavelengths

correspond to different

colors.

• The range is from red

(λ ~ 7 x 10-7 m) to violet

(λ ~4 x 10-7 m) .

Visible Light, cont

Notes on the EM Spectrum, 3

• Ultraviolet light

– Covers about 4 x 10-7 m to 6 x10-10 m.

– Sun is an important source of uvlight.

– Most UV light from the sun isabsorbed in the stratosphere byozone.

• X-rays

– Wavelengths of about 10-8 m to10-12 m

– Most common source isacceleration of high-energyelectrons striking a metal target.

– Used as a diagnostic tool inmedicine.

Notes on the EM Spectrum, final

• Gamma rays

– Wavelengths of about 10-10 m to 10-14 m.

– Emitted by radioactive nuclei.

– Highly penetrating and cause serious damage

when absorbed by living tissue.

The main regions of EMR spectrum used in RS

Name Wavelength ()

in m

Abbreviation

Visible 0.4 - 0.7 --

Near Infrared 0.7 -1.1 NIR

Short wave infrared 1.5 – 1.8 SWIR

Mid infrared 2 - 5 MIR

Thermal infrared 8 - 14 TIR

Microwave 1 cm - 20 cm --

Electromagnetic Radiation Models

• the wave model, and

• the particle model.

Wave Model

Wave Model , Cont.

From basic physics, waves obey the generalequation:

c = v

Since c is essentially a constant (3 x 108 m/sec),frequency v and wavelength for any given waveare related inversely, and either term can be usedto characterise a wave into a particular form.Frequency is the number of wavelengths that pass a point per unit time.

Particle Model

Particle (Quantum) Model suggests that EM radiation is

composed of many discrete units called photons or

quanta. The energy of a quantum is given as:

Q = hvwhere:

Q = energy of a quantum or photon (Joules - J)

h = Planks constant, (6.626 x 10-34 J/sec)

v = frequency

Particle Model, Cont.

We can combine the Wave and Particle models for EM

radiation by substituting v = c/ in the above equation.

This gives us:

Q = hc

From this we can see that the energy of a quantum is

inversely proportional to its wavelength. Thus, the longer

the wavelength of EM radiation, the lower its energy

content.

Radiation terminology

Name, units Symbol Definition

Radiant energy (J) Q Capacity of radiation to do work.

Radiant flux (w or

J/s)

F, Time rate of flow energy on to, off of, or

through a surface.

Irradiance (W.m-2) E Radiant flux incident upon a surface per

unit area of that surface.

Exitance (W.m-2) M Radiant flux leaving a surface per unit

area of that surface.

Radiance or

Brightness (Wm-

2sr-1)

B, L Radiant flux leaving a projected source

area per unit solid angle in a

specified direction.

Radiation laws

1.Plank’s equationIt describes the maximum amount of energy that can be emitted by

a black body (object), which is a perfect radiator.

)1( /5

1

2

TCe

CM

M is the spectral radiant exitance of the object measured in

Wm-2 m-1.

C1, C2 are constants and equal to 3.7413 x 10 8 Wm-2 m4

and 1.4388 x 10 4 mK.

Blackbody Radiation Curves

2- Stefan-Boltzmann equationThe integration of plank’s law over all wavelengths to give the

total emitted energy from a unit surface area of an object in Wm-2.

M = σ T4

where σ is the Stefan-Boltzmann constant, 5.6693 x 10 -8 W

m-2 K -4.

Thus, the amount of energy emitted by an object such as the

Sun or the Earth is a function of its temperature.

3- Wien’s displacement law

In addition to computing the total amount of energy exiting a theoretical

blackbody such as the Sun, we can determine its dominant wavelength

(max) based on Wein's displacement law:

where k is a constant equaling 2898 m K, and T is the absolute

temperature in kelvin.

Therefore, as the Sun approximates a 6000 K blackbody, its dominant

wavelength (max ) is 0.48 m:

T

kmax

K

Kmm

6000

2898483.0

Reflectance• The reflectance of an object is a physical property of the object

independent of the illumination conditions. It is a unitless quantity

ρ has values between 0 and 1.

Reflectance is a useful property because it describes a

characteristic of the object alone. Therefore, it is desirable to

find a relationship between the reflectance p of the object

and the radiance L as measured by the satellite.

Types of Reflection

Specular Reflection

smooth surface

Diffuse Reflection

rough surface

Spectral Reflectance

Snow & Clouds

Image structure, Display, Resolution and

data type

Image Structure •An image may be represented and displayed in a digital

format by subdividing the image into small equal-sized

and shaped areas, called pixels.

•Most remote sensors work by seeing only very narrow

ranges of wavelengths or colours. .

•The information from a narrow wavelength range is

gathered and stored in a channel, also sometimes

referred to as a band.

• The computer displays each digital value as different

brightness levels.

Resolution of Remotely Sensed Data

1. Spatial: related to the size of pixel.

Image scale: in remote sensing, resolution is more

important than scale, however, it is useful to visualize 1 to

2 pixels per 1mm.

Possible

scale

Resolution (m)

1: 1.5 M1000

1:200,00080

1:50,00030

1:25,00010-20

1:50001-4

2.Spectral: related to the band width and the number of bands.

Spectral Resolution

A B C

Resolution, Cont.

Resolution, Cont.

3.Temporal Resolution: frequency or revisit period.

• The dynamic characteristics of the target under study

have to be considered.

• 1- Geological mapping: no need for high temporal

resolution.

• 2- Crop species determination: more than one image

in the growing season.

• 3- Disaster monitoring in real time requires daily and

hourly images are needed.

Resolution, Cont.

• Radiometric Resolution: related to sensor sensitivity and

data quality.

• It is associated with the level of quantization of an image

which is in turn related to how to use the minimum

amount of data storage to represent the maximum

amount of information.

• Usually the data of remote sensing is binary and might

be:

– 6-bit (64 grey levels): this is known type of MSS data..

– 8-bit: Landsat and SPOT: 256 grey levels .

– 10-bit : Maximum = 1023, grey levels = 1024 .

Radiometric Resolution

8-bit

(0 - 255)

9-bit

(0 - 511)

10-bit

(0 - 1023)

0

0

0

7-bit

(0 - 127)0

Concept of binary numbers: Hand out

Image Processing

• ERDAS IMAGINE 2011• ERDAS IMAGINE is a remote sensing image processing

application.

• It allows the user to produce useful information from the

data.

• Image Enhancement and Correction (geometric and

Radiometric ).

Image Enhancement

• Enhancements are used to make it easier for

visual interpretation and understanding of

imagery.

Digital Classifications

• Classification is the process of assigning

each pixel of an image to a particular

group or class.

• In this case the classes are land cover or

crop types, so that the aim is eventually to

map the land cover types of the whole

image .

• you will depend on your knowledge with

the land cover classes

Digital Classifications ,cont.

Digital Classifications ,cont.

Digital Classifications ,final

Image filtering

• It is important to improve the appearance

of the image .

• More informative map .

Geometric Correction

• A digital image processing technique used to

compensate for image distortion and projecting

the image to real world referencing system.

Radiometric Correction

Radiation-Matter Interactions

Radiation-Matter Interactions

• The amount on interaction depends upon;

– the composition and physical properties of the

medium.

– the wavelength or frequency of the incident

radiation.

– the angle at which the incident radiation

strikes a surface.

Atmospheric Scattering

• Important scattering agents include;

– gas molecules.

– suspended particulates.

– Clouds.

• There are three types of atmospheric scattering

important to remote sensing;

– Rayleigh or molecular scattering.

– Mie or non-molecular scattering.

– Non-selective scattering.

Radiometer components

• Upward and downward facing sensor to

measure both incoming and reflected

radiation, nearly simultaneously;

Radiometer components

system couples the latest micro-

computer technology and

analog/digital electronics with

customized software.