Spectral Characteristics In Remote Sensing. Everything emits radiant energy. Technically speaking,...

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Spectral Characteristics In Remote Sensing

Transcript of Spectral Characteristics In Remote Sensing. Everything emits radiant energy. Technically speaking,...

Spectral CharacteristicsIn

Remote Sensing

Everything emits radiant energy.

Technically speaking, energy is emitted by all objects above absolute zero

We can measure light in wavelengths.

Wavelength• Wavelength (λ) is defined as the

distance between the maximums or minimums of a roughly periodic pattern.

• Normally measured in micrometers (µ) or nanometers (nm).

The light we can see is a small part of the light on Earth (and in the universe!)

Remote Sensing uses our knowledge of this whole great Electromagnetic

Spectrum

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

Every kind of surface has its own “spectral signature.”

Spectral Signatures of Some Common Foods

People have learned to relate different kinds of surfaces and their spectral signatures to what they look like in a satellite scene.

Guy with hand-held spectrometer

Seeing leaves with remote sensing

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

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

Green and Infrared light are reflected from the leaves, so that’s what we see.

Put another way…

Chlorophyll strongly absorbs radiation in the red and blue wavelengths and reflects green wavelengths. This is why healthy vegetation appears green.

Healthy leaves are excellent reflectors of near-infrared wavelengths.

So we can use near-infrared reflectance to determine how healthy (or unhealthy) vegetation may be.

Remote Sensing Pathway of Light

Sensors sense some segment of the Electromagnetic Spectrum

Reads the “spectral signature” of the surface that is reflecting/emitting light

Sensors Record Intensity ofReflected Energy Numerically

The amount of the reflected energy (intensity) is recorded for each pixel, in each band, on a scale of 0-255.

Assigning Colors to Bands

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3,2,13,2,1

Visible Infrared

Green Data is shown as GreenBlue Data is shown as BlueRed Data is shown as Red

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4,3,24,3,2

Visible Infrared

NIR Data is shown as RedRed Data is shown as GreenGreen Data is shown as Blue

Visible wavelengths Infrared wavelengths

So we see more than we could otherwise.

Same scene, different wavelengths

True-Color Composite (3,2,1) True-color composite images approximate the range of vision for the human eye, and hence these images appear to be close to what we would expect to see in a normal photograph. True-color images tend to be low in contrast and somewhat hazy in appearance. This is because blue light is more susceptible than other bandwidths to scattering by the atmosphere. Broad-based analysis of underwater features and landcover are representative applications for true-color composites.

Near Infrared Composite (4,3,2) Adding a near infrared (NIR) band and dropping the visible blue band creates a near infrared composite image. Vegetation in the NIR band is highly reflective due to chlorophyll, and an NIR composite vividly shows vegetation in various shades of red. Water appears dark, almost black, due to the absorption of energy in the visible red and NIR bands.

Shortwave Infrared Composite (7,4,3 or 7,4,2) A shortwave infrared composite image is one that contains at least one shortwave infrared (SWIR) band. Reflectance in the SWIR region is due primarily to moisture content. SWIR bands are especially suited for camouflage detection, change detection, disturbed soils, soil type, and vegetation stress.

Some different band combinations…

Landsat 7, Path 35 Row 34, 09.12.00

3,2,1

4,3,2

7,5,2

7,4,2

Energy SourcesEnergy Sources

►Need an energy source that provides Need an energy source that provides illuminationillumination

►Or a source that emits electromagnetic Or a source that emits electromagnetic energyenergy

Electromagnetic Energy and the Electromagnetic Energy and the atmosphereatmosphere

►Radiation reacts to the atmosphereRadiation reacts to the atmosphere

both to the target and the sensorboth to the target and the sensor

Atmospheric ScatteringAtmospheric Scattering

►Rayleigh Scattering-shorter Rayleigh Scattering-shorter wavelength to scatterwavelength to scatter

►Mie Scattering-longer wavelengths to Mie Scattering-longer wavelengths to scatterscatter

►Non-Selective Scattering-Larger than Non-Selective Scattering-Larger than the wave lengthsthe wave lengths

Atmospheric AbsorptionAtmospheric Absorption

►AbsorptionAbsorption

►Carbon DioxideCarbon Dioxide

►OzoneOzone

Atmospheric WindowsAtmospheric Windows

►Comparing the sun and the earthComparing the sun and the earth

►What is visible and our eyes are most What is visible and our eyes are most sensitive to corresponds with the peak sensitive to corresponds with the peak energy level of the sunenergy level of the sun

Electromagnetic Energy and Electromagnetic Energy and earth objectsearth objects

►EMR interacts the same with features EMR interacts the same with features on the earth as it does with the on the earth as it does with the atmosphere.atmosphere.

►Specular-smoothSpecular-smooth

►Diffuse- RoughDiffuse- Rough

Emission of Electromagnetic Emission of Electromagnetic RadiationRadiation

►TemperatureTemperature

►Warmer-more energy shorter wavelengthsWarmer-more energy shorter wavelengths

►Cooler- less energy longer wavelengthsCooler- less energy longer wavelengths

►Emissivity- Varies with composition (Water)Emissivity- Varies with composition (Water)

Spectral Properties of ObjectsSpectral Properties of Objects

► The spectral properties of an object allow us The spectral properties of an object allow us to conduct analysis.to conduct analysis.

► Objects absorb and reflect electromagnetic Objects absorb and reflect electromagnetic energy.energy.

► Spectral Signature-a pattern of spectral Spectral Signature-a pattern of spectral response.response.

► Spectral Response Curve-Percentage of Spectral Response Curve-Percentage of radiation of different wavelengthsradiation of different wavelengths

Spectral Bands Commonly used Spectral Bands Commonly used in Remote Sensingin Remote Sensing

►Visible Green/RedVisible Green/Red►Near infraredNear infrared►Mid-infrared bandsMid-infrared bands►Blue and thermal infraredBlue and thermal infrared

Remote Sensing in the Remote Sensing in the Ultraviolet SpectrumUltraviolet Spectrum

►UV Radiation is absorbed and UV Radiation is absorbed and scattered by the earth’s atmosphere.scattered by the earth’s atmosphere.

►Some materials when illuminated by Some materials when illuminated by ultraviolet radiation absorb UV ultraviolet radiation absorb UV radiation and reemit it as visible light, radiation and reemit it as visible light, this is known asthis is known as

fluorescence.fluorescence.

Remote Sensing in the Visible Remote Sensing in the Visible SpectrumSpectrum

►What the human eye can seeWhat the human eye can see►Photographic FilmPhotographic Film►CamerasCameras►Digital CamerasDigital Cameras►ScannersScanners

Applications of Visible BandsApplications of Visible Bands

►Visible Blue Band (.45-.52 microns)Visible Blue Band (.45-.52 microns)►Visible Green Band (.52-.60 microns)Visible Green Band (.52-.60 microns)►Visible Red Band (.63-.69 microns)Visible Red Band (.63-.69 microns)►Panchromatic Bands (.50-.90 microns)Panchromatic Bands (.50-.90 microns)

Visible Blue Band (.45-.52 Visible Blue Band (.45-.52 microns)microns)

►Greatest water penetrationGreatest water penetration►Greatest atmospheric scatteringGreatest atmospheric scattering►Greatest absorptionGreatest absorption►Used for : water depthUsed for : water depth water characteristicswater characteristics detection of subsurface featuresdetection of subsurface features soil and vegetation discriminationsoil and vegetation discrimination forest, geology, cultural featuresforest, geology, cultural features

Visible Green Band (.52-.60 Visible Green Band (.52-.60 micrometers)micrometers)

►Vegetation discriminationVegetation discrimination►Vigor AssessmentVigor Assessment►Cultural FeaturesCultural Features►Urban InfrastructureUrban Infrastructure►Less affected by atmospheric Less affected by atmospheric

scatteringscattering►Sediment and Chlorophyll Sediment and Chlorophyll

ConcentrationConcentration

Visible Red Band (.63-.69 micrometers)Visible Red Band (.63-.69 micrometers)

► Chlorophyll absorption band of healthy Chlorophyll absorption band of healthy green vegetation.green vegetation.

► Vegetation typeVegetation type► Plant conditionPlant condition► Soil and Geologic BoundariesSoil and Geologic Boundaries► Cultural FeaturesCultural Features► Least affected by atmospheric scatteringLeast affected by atmospheric scattering► Less water penetration but good near Less water penetration but good near

surface information ie. Water quality, surface information ie. Water quality, sediment, and chlorophyll.sediment, and chlorophyll.

Panchromatic Bands (.50-.90 Panchromatic Bands (.50-.90 micrometers)micrometers)

►Wide range of sensitivityWide range of sensitivity►Visible to Near IRVisible to Near IR►Higher spatial resolutionHigher spatial resolution►Can be combined with other multi-Can be combined with other multi-

spectralspectral

bands.bands.

Remote Sensing in the Infrared Remote Sensing in the Infrared SpectrumSpectrum

►Near infrared band (.7-1.0 micrometer)Near infrared band (.7-1.0 micrometer)►Short-wave infrared band (1.0-3.0 Short-wave infrared band (1.0-3.0

micrometer)micrometer)►Medium Wave (3.0-8 micrometer)Medium Wave (3.0-8 micrometer)►Long Wave (8-14 micrometer)Long Wave (8-14 micrometer)

Remote Sensing in the Remote Sensing in the Microwave RegionMicrowave Region

►No significant solar radiation in this No significant solar radiation in this portion of the spectrum.portion of the spectrum.

►Use antenna and sophisticated Use antenna and sophisticated electronicselectronics

to focus on microwave energy.to focus on microwave energy.

Valuable for sea surface temperatureValuable for sea surface temperature