lecture 3 - app. rs

8/7/2019 Lecture 3 - App. RS

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25/1/0525/1/05 Applied Remote SensingApplied Remote Sensing 11

EMR Characteristics:

EMR wave is characterized by its:

Intensity Amount or degree of strength of

electricity, light, heat, or sound/unit area;

Polarization Ray of light exhibiting

different properties in different direction;

Freque

ncy

How often a periodic event

occurs i.e. a signal going through a complete

cycle;

Lecture 3

Nature of Reflectance


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Energy striking anobject can be;

Absorbed Transfer of energy to an

object, usually in the form of heat; Emitted re-emitted as a function of

temperature and structure at a different

wavelength;

Scattered the travel direction of energy

is changed randomly;

- degree of scatter is related to the

wavelength and size of the objects it strike;

Reflected energy rebounds unchanged

from the object with the angle of reflection

equal to the angle of incidence;


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Transmitted energy passing through the

object but its velocity is changed (refracted).

-Change of velocity as a result of refractioninduces changes in EMR wavelength according

to the equation c = f

c = speed of light, f = frequency, and =

wavelength on EMR;Reradiated Energy first absorbed then re-

emitted, generally as thermal (heat) radiation;


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Interactionof EMR:

With The Atmosphere:

Absorption;Scattering;

Re-emission

Refraction

Reflection

With Water:Transmission;

Absorption;

Reflection;

With Opaque Objects:Absorption;

Reflection;


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EnergyInteraction with the Atmosphere:

All radiation propagates through the atmosphere;

Quality of images and data generated by sensors

are affected by atmospheric condition;

The nature of interaction between EMR with the

atmosphere is therefore important; Solar energy passing through the atmosphere can

be modified by the following physical processes;

1)Scattering;

2) Absorption;3) Refraction;


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Scattering:

The redirection of electromagnetic energy bysuspended particulate matter in the atmosphere orby large gaseous molecules in the atmosphere;

The amount of scattering depends on thefollowing:

- size and amount of particles;- wavelength of radiation;

- depth of atmosphere energy it is passing

through;


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Rayleigh Scattering (Clear Atmosphere

Scattering):

Occurs in the absence of impurities in theatmosphere i.e. atmospheric gasses causing the

scattering of light;

Interaction with atmospheric particles and tiny

particles much smaller in diameter than thewavelength of the interacting radiation;

Inversely related to wavelength;

Primary cause of haze in imagery visually

diminishing crispness and contrast of animage;

Wave length dependent;


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Mie Scattering:

Occurs when atmospheric particles are thesame diameter as the wavelength of the energy

interacting with the atmosphere; Caused mostly by water vapour and dust

particles, pollen, smoke ect.;

Affects longer wavelength energy;

Non-selective Scattering: Occurs when atmospheric particles are larger in

diameter than the wavelength of the energyinteracting with the atmosphere;

Caused mostly by water droplets;

Scattering not wavelength dependent;

Also causes haze;


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GENERAL EFFECTSOFSCATTERING:

Radiation in the blue and ultraviolet portion ofthe spectrum usually not considered in RS;

- Wavelength dependency of Rayleigh

scattering;

- recording brightness of the atmosphere notthe image;

Decreasing spatial detail recorded by sensors;

Can make dark objects appear brighter and

vice versa; Degrade the quality of images;


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Refraction:

The bending of light rays at the contact surfacebetween two media that transmit light;

Also occur as light passes through atmospheric

layers of varying clarity, humidity, and

temperature

i.e. variation in density of theatmospheric layers;

Index of refraction (n): the ratio between the

velocity of light in a vacuum (c) to its velocity in

the medium (cn):

n = c/cn


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Absorption:

Unlike scattering, absorption results in the lossof energy to the atmosphere;

Mostly caused by three atmospheric gasses;

OZONE - absorbs UV, portions of the UVspectrum ( < 0.24 m) prevents transmission to

the lower atmosphere; CARBON DIOXIDE Effects strongest at lower

atmosphere, absorbs energy in the 13 - 17.5micrometer region (mid and far infrared);

WATER VAPOR -Lower atmosphere. Mostlyimportant in humid areas, very effective atabsorbing in portions of the spectrum between5.5 and 7.0 m and above 27.0 m;


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Raises the temperature of the particles;

Causes a "loss" of available energy to theparticles;

Particles will re-radiate the absorbed energy, butat a different wavelength;

Atmospheric Window:

Areas where wavelengths are easily transmittedthrough the atmosphere;

Position, extents, and effectiveness determinedby the absorption spectra of atmosphericgasses;

Defines wavelengths that can be used forforming images;


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Wavelength not within the window is severely

attenuated by the atmosphere;


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Interactions with Surfaces:

Reflection:

Redirection of light rays as it strikes a

nontransparent surface;

Surface irregularities (i.e. roughness or

smoothness) affects the direction of reflection;

Specular reflection: smooth surface redirects

most incident radiation in a single direction;

- Angle of incidence = angle of reflection;

Diffuse or isotropic reflector: relatively roughsurfaces that scatter energy in more or less all

direction common for many natural surfaces;


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LambertainSurface: A perfectly diffused

reflector having equal brightness from any

angle;

SpectralProperties ofobjects:

Learning about objects and features by studying

the radiation reflected and /or emitted;

Every phenomena has its own distribution of

reflected, emitted, and absorbed radiation;

Information can be obtained about shape, size,

and other physical and chemical properties;


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SpectralResponses of Water, Soils, and

Vegetation: Water:

Clear water appears blue because it reflects

EMR energy primarily in the short, blue

wavelength (0.4-0.5 m); Appears black in the reflected IR region (0.8-3.0

m) because of absorption;

Shallow water or the presence of suspended

solids will change the reflectance curve; Turbid waters reflect more EMR at all visible

wavelengths;


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Studies of water quality, depth, and turbidityare best studied using reflectance in the blue

and green region of the visible spectrum (0.4-0.6 m);

IR region is best to detect the interfacebetween land and water;

Soils and Rocks:

Dry, tan, silty soils show a progressiveincrease in reflectance at longer wavelengths;

A brighter response in the visible red (0.6-0.7m) and reflected IR (0.7-1.1 m);

As moisture content of soil increases spectralreflectance changes towards a characteristicwater response;


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Vegetation:

Lush vegetation appear green becausechlorophyll - reflects the green light;

Radiation reflectance of vegetation reaches a

maximum in the near-IR between 0.8 and 1.1

m;

Maturity and health of natural vegetation can be

assessed from the changes in the reflectance;

- Pigment, and water content vary according to

species, growth stage (young and bright or

dying and yellow), and diseased or under stressfrom drought;


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Source: Wilke & Finn, 1996


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Types ofInformation RemotelySensed:

Provides basic measurements of a range ofbiological and physical characteristics of alandscape e.g. position, shape, elevation,temperature, and moisture content;

Combination of measurements produces a

hybrid or synthetic information describing thelandscape e.g. land use

- Color, shape, location size etc.

Quantitative biophysical data and informationrequire an understanding of:

i) How EMR is absorbed, reflected, and emitted

from objects in the landscape; and


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ii) How radiation reflected or emitted from

landscape objects is altered and attenuated

by the intervening atmosphere and by the

spatial and spectral resolution of the

sensor;

The amount of radiation from the terrain

recorded by a sensor is a function of:

i) Spectral, radiometric, spatial, and temporal

resolution;

ii) Size, shape, color, orientation, chemical

composition, and water content of terrain

objects;


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iii) Density, distribution, and juxtaposition of

terrain features within the landscape;iv) Noise introduced by intervening

atmosphere;


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lecture 3 - app. rs

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