Indian Institute of Remote Sensing

India

Shefali Agrawal

MODULE 2Principles of Remote Sensing

What is Remote Sensing?

•Remote sensing is the art and science of acquiring informationabout the Earth's surface without actually being in contact withit.•“Science of measuring the GEOMETRIC and THEMATICproperties of objects in the environment without touchingthem and using various devices in the air or space•This is done by sensing and recording reflected or emittedenergy and processing, analyzing, and applying thatinformation

It enables us to observe and study nature in ways that would otherwise be beyond humancapability, across great distances and at wavelengths of light invisible to human eyes

The Remote Sensing Process

Target

Energy Source

SatCom

RecievingStation

Sensor

Application•Land use•Atmosphere Sc•Geology•Hydrology•Agriculture•Forestry•Disaster Management

Information Extraction

What is recorded Interactions of EMR with land cover features

• Remote Sensing is essentially studying interaction of ElectromagneticRadiation with Different Objects – Land, Water, Atmosphere, Etc.

3/4/2013

• Parameter –what is being measured and/or derived;

• Spatial Extent – global to local areas.

• Spatial resolution – “size of the individual pixel”;

• Revisit frequency – defines the repetition of observations;

•

• Time Series – the time period for which consistent observations are available,;

• Timeliness – the speed that a product is made available for a user.

– Realtime

Variations in remotely sensed data

When you listen to the radio, watch TV, or cook dinnerin a microwave oven, you are using electromagneticwaves.

Electromagnetic Spectrum

Ultraviolet Infrared (IR)

X-RaysCosmicrays

Gammarays

U-V Infrared Micro-waves TV Radio Electric power

Visible spectrum

0.3m 0.4 0.5m 0.6 0.7m 10.0 15.0

W a v e l e n g t h

Blue Green Red

300nm 500nm 700nm

‘Optical range’

Visible Range• The light which our eyes - our "remote sensors" - can detect is

part of the visible spectrum.

• The visible wavelengths cover a range from approximately 0.4 to0.7 m.

• This is the only portion of the spectrum we can associate withthe concept of colours.

Violet: 0.4 - 0.446 mBlue: 0.446 - 0.500 mGreen: 0.500 - 0.578 mYellow: 0.578 - 0.592 mOrange: 0.592 - 0.620 mRed: 0.620 - 0.7 m

•Blue, green, and red are the primarycolours or wavelengths of the visiblespectrum.

•Although we see sunlight as a uniform orhomogeneous colour, it is actuallycomposed of various wavelengths ofradiation in primarily the ultraviolet,visible and infrared portions of thespectrum.

Infra-red Region

• Infrared region which covers the wavelength range fromapproximately 0.7 m to 100 m

• The infrared region can be divided into two categoriesbased on their radiation properties –

• the reflected IR : 0.7 m to 3.0 m. , and

• the emitted or thermal IR : 3.0 m to 100 m.

• The thermal IR region is quite different than the visibleand reflected IR portions, as this energy is essentially theradiation that is emitted from the Earth's surface in theform of heat.

Microwave Region

• The portion of the spectrum of more recent interest toremote sensing is the microwave region from about 1mm to 1 m.

• The shorter wavelengths have properties similar to thethermal infrared region while the longer wavelengthsapproach the wavelengths used for radio broadcasts.

• Longer wavelength microwave radiation can penetratethough cloud, fog, haze etc as the longer wavelengthsare not susceptible to atmospheric scattering whichaffects shorter optical wavelengths.

VioletViolet :: 0.4 0.4 -- 0.446 0.446 m m BlueBlue :: 0.446 0.446 -- 0.500 0.500 m m GreenGreen :: 0.500 0.500 -- 0.578 0.578 m m YellowYellow :: 0.578 0.578 -- 0.592 0.592 m m Orange:Orange: 0.592 0.592 -- 0.620 0.620 m m RedRed :: 0.620 0.620 -- 0.7 0.7 mm

Near Infrared: 0.7 – 1.5 mShort Wave Infrared: 1.5 – 3 mMid Wave Infrared: 3 – 8 mThermal Infrared: 8 – 15 mFar Infrared : >15 m

Microwaves

P band 0.3 – 1 GHz (30 – 100 cm)

L band 1 – 2 GHz (15 – 30 cm)

S band 2 –4 GHz (7.5 – 15 cm)

C band 4 – 8 GHz (3.8 – 7.5 cm)

X band 8 – 12.5 GHz (2.4 – 3.8 cm)

Ku band 12.5 – 18 GHz (1.7 – 2.4 cm)

K band 18 – 26.5 GHz (1.1 – 1.7 cm)

Ka band 26.5 – 40 GHz (0.75 – 1.1 cm)

(Note: 1 GHz = 109 Hz)

Optical Infrared

Types of Remote Sensing

• Passive: source of energy is either the Sun, Earth, or atmosphere – Sun

- wavelengths: 0.4-5 µm

– Earth or its atmosphere

- wavelengths: 3 µm -30 cm

• Active: source of energy is part of the remote sensor system – Radar

- wavelengths: mm-m

– Lidar

- wavelengths: UV, Visible, and near infrared

Types of Remote Sensing Data

1) Optical ( Visible/Infrared)

– passive

• solar energy reflected by the surface

• determine surface (spectral) reflectance

– active

• LIDAR - active laser pulse

• time delay (height)

• induce florescence (chlorophyll)

2) Thermal infrared

– energy measured - temperature of surface and emissivity

3) Microwave

– active

• microwave pulse transmitted

• measure amount scattered back

• infer scattering

– passive

• emitted energy at shorter end of microwave spectrum

Advantages over optical: active system,not affected by atmosphere, penetrates thecanopy (wavelength)

RADAR Remote Sensing

Optical versus SAR data ( Rice crop)

Physics of Remote Sensing

Hina Pande

• For Passive reflective and active microwave remote sensing energymust pass through the atmosphere twice.

• For Passive thermal and passive radar remote sensing energy mustpass through the atmosphere once.

The atmosphere affects the spectral composition and the intensity ofradiation sensed by all remote sensor devices.

Interactions with the AtmosphereBefore radiation used for remote sensing reaches the Earth'ssurface it has to travel through some distance of the Earth'satmosphere

Particles and gases in the atmosphere can affect the incoming lightand radiation

Interactions with the Atmosphere

The two major atmospheric effects are scattering and absorption

A) ScatteringA) Scattering B) AbsorptionB) Absorption

Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path

Absorbption causes molecules in the atmosphere to absorb energy at various wavelengths. Ozone, carbon dioxide, and water vapour are the three main atmospheric constituents which absorb radiation

SCATTERINGScattering happens when the radiation passing through A mediumencounters A `particle’ with different index of refraction compared to themedium

In scattering the energy is not lost but the direction is altered, hence lost from the original direction.

Incoming radiant energy is depleted by A change in direction due tointeractions with the ‘particles’(gas molecules and aerosols). This is calledscattering

i0

s

p

Scattering: Scattering depends on the relative size of the particle with respect to

• There are three (3) types of scattering which take place.

• Rayleigh Scattering occurs when particles are very small compared tothe wavelength of the radiation.

• These could be particles such as small specks of dust or nitrogen andoxygen molecules.

• Rayleigh scattering causes shorter wavelengths of energy to bescattered much more than longer wavelengths.

• Rayleigh scattering is the dominant scattering mechanism in the upperatmosphere.

• The fact that the sky appears "blue" during the day is because of thisphenomenon.

Physics of Remote Sensing

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As sunlight passes through the atmosphere, the shorterwavelengths (i.e. blue) of the visible spectrum are scatteredmore than the other (longer) visible wavelengths.

At sunrise and sunset the light has to travel farther through theatmosphere than at midday and the scattering of the shorterwavelengths is more complete; this leaves a greater proportionof the longer wavelengths to penetrate the atmosphere.

• Occurs when the particles are just about the samesize as the wavelength of the radiation.

• Dust, pollen, smoke and water vapour arecommon causes of Mie scattering which tends toaffect longer wavelengths than those affected byRayleigh scattering.

• Mie scattering occurs mostly in the lowerportions of the atmosphere where larger particlesare more abundant, and dominates when cloudconditions are overcast.

2. Mie scattering

Physics of Remote Sensing

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•Occurs when the particles are much larger than the wavelengthof the radiation.

•Water droplets and large dust particles can cause this type ofscattering.

•Nonselective scattering gets its name from the fact that allwavelengths are scattered about equally.

•This type of scattering causes fog and clouds to appear white toour eyes because blue, green, and red light are all scattered inapproximately equal quantities (blue+green+red light = whitelight).

3.Non selective scattering

0.4 0.7 1 2 3 5 10 10,000

100

50

Atmosphere transmission (%)

µm(...)0

Atmospheric windows suitable for Earth observations from space,

Physics of Remote Sensing

Hina Pande

Molecules OIR (m) TIR (m) Microwave (GHz)

H2O 0.9, 1.1, 1.4, 1.9, 2.7

~ 6

- 22.235183.3

O2 0.8 ~60, 118.75

CO2 2.7, 4.3 14 -

N2O 4.6, 7.7 - -

O3 - 9.5 -

MAJOR MOLECULAR ABSORPTION LINES IN THE ATMOSPHERE

• Incoming Radiation is either– Reflected from– Transmitted through– Absorbed by

Surface Interactions

Every kind of surface reflects light differently, absorbing and reflecting it more or less in different wavelengths.

The proportions of each interaction will depend on the wavelength of the energy and the material and condition of the feature.

Smooth surface

i r

i=r

Scattered Waves

Rough Surface

Reflected wave

Lambertian Surface

Specular reflection : all (oralmost all) of the energy isdirected away from thesurface in a single direction

Diffuse reflection : energy is reflected almost uniformly in all directions

Surface Reflection depends on the surface roughness of the feature

Interactions with the surface

R.S. Instrument

Su

n

Clouds

transmittedradiation

Scattered radiation*

Atmospheric absorption

Earth Reflection processes Emission processes

Thermal emission

Atmospheric emission

Reflectedradiation

scatteredradiation**

Interaction of Vegetation With EMR Interaction of Vegetation With EMR

IR = InfraredR = Red lightG = Green lightB = Blue light

Infrared, red, green,and blue light fromthe sun hit the leaves.

Chlorophyll strongly absorbs radiation in the red and bluewavelengths and reflects green wavelengths. This is whyhealthy vegetation appears green.Healthy leaves are excellent reflectors of near-infraredwavelengths.Near-infrared reflectance can be used to determine thehealth (or unhealthy) of vegetation.

Spectral reflectance of vegetation

Spectral Reflectance

Characteristics of Leaves

Needle-leaf trees canopies reflectsignificantly less near-infrared radiationcompared to broad-leaf vegetation.

Coniferous forest Deciduous forest

Immature leaves contain less chlorophyll andfewer air voids than older leaves, they reflectmore visible light and less infrared radiation.

Immature plant

Mature plant

SoilsThe five characteristics of a soil that determine its

reflectance properties are, in order of importance: • Moisture contentSoil moisture decreases

reflectance• Organic content A soil with 5% or more organic

matter usually appears black in colour• Structure Coarse soil (dry) has relatively high

reflectance• Iron oxide content Reflectance in green region

decreases, but increases in the red region• Texture The soil reflectance increases as particle

size decreases

Water

• Transmission at visible bands and a strong absorption at NIR bands

• Water surface, suspended material, and bottom of water body can affect the spectral response

Reflection of Light - dependsWater Depths – Shallow , DeepSuspended materialChlorophyll ContentSurface Roughness

• Very high reflectance in visible band

• Reflectance drops off quickly in near IR

• Reflectance varies as snow ages

• Changes in snow crystal size affects scattering

• Changes in amount of liquid water in snowpack

• Mid-IR band useful for separation of snow and clouds

• Confusion with clouds

Spectral signature of snow

Spectral Signature of Major land cover Features

Spectral Signature of Land Cover Features

REFLECTANCE (%): The ratio of energy reflected by a surface at agiven wavelength

Rocks and Soils : reflectance affected by : minerals, surfacealteration, texture, structure, water content...

Vegetation : Factors affecting : photosynthetic activity-phenology, morphology; leaf shape, thickness, and moisturecontent...

Water : low reflectance: most of the radiation is absorbed ortransmitted. Reflectance affected by: suspended materials(loams, algae) and depth

Spectral signature of an object is a function ofThe wavelengthThe wavelength dependency means that, even within agiven feature type, the proportion of reflected, absorbed,and transmitted energy will vary at differentwavelengths.

Material of the objectThe proportions of energy reflected, absorbed, andtransmitted will vary for different earth features,depending upon their material type and conditions.

These differences permit us to distinguish differentfeatures on an image.

Factors Influencing the Spectral Signature

Height of the Sun (date, time) Atmospheric Condition Relief-Shadow

Terrain Slope Vegetation Phenology Land cover Features

Landsat ETM (IR R G)

High Resolution Data

Forested Landscape Settlement (Urban)

Agricultural Land

Forest Urban Area

IRS LISS-3 image over part of Himalayas. (a) is in band-2 (Green) and (b) in band-5(SWIR). Both cloud and snow have higher reflectance in visible and hence cannot bediscriminated (except from shadow). In SWIR, low reflectance of snow candiscriminate snow from cloud.

(a)(b)

Forest area

Agricultural areas

Ikonos (1 m)

(IRS LISS III (23.5 m)

Canal

Reservoir

Water Features

(IRS LISS III (23.5 m)

Urban Features

Ikonos (1 m)

Snow features

Spot VGT ( 1 Km)

Microwave Images

• This SAR image shows an area of the seanear a busy port.

• Many ships can be seen as bright spots inthis image due to corner reflection.

• The sea is calm, and hence the ships canbe easily detected against the darkbackground

www.iirs.gov.in