3 Question: Earth Surface Equilibrium temperature (ET) The energy that is absorbed by the Earth will be one that reaches the Earth from the Sun then subtracted from that which is reflected from the top of the atmosphere. The fraction of light reflected from the top of the atmosphere called the albedo (A). The albedo of Earth is 0.3 (approximately 30%). What would be the ET?
4 Answer the total power absorbed by the Earth is given by equation Pabsorbed= S.(1-A).RT2The power emitted by Earth is given byPemitted = 4πRT2.σeT4To be in radiative equilibrium the conditionPabsorbed = PemittedQuestion?But the actual average temperatureof Earth is 288 kelvin (15ºC) andnot 255 kelvin. How is this possible?This can be rearranged to produceAnswer: Due to greenhouse effect.T=255 K
8 Radiative TransferEasier to consider the specific problem of the radiance ata sensor at the top of the atmosphere viewing the surface
9 There will be three components of greatest interest in the Radiation componentsThere will be three components of greatest interest in thesolar reflective part of the spectrumUnscattered, surfacereflected radiation LλsuDown scattered,surface reflected LλsdskylightUp scattered path LλspradianceRadiance at the sensoris the sum of these three
10 Much of the previous discussion centered around the Spectral signatureMuch of the previous discussion centered around theselection of the specific spectral bands for a given themeThe key will be thatdifferent materials havedifferent spectral reflectancesAs an example, considerthe spectral reflectancecurves of three differentmaterials shown in the graph
11 Spectral SignatureFor any given material, the amount of solar radiation that it reflects, absorbs, transmits,or emits varies with wavelengtha general example of areflectance plot forsome (unspecified) vegetation type(bio-organic material)
12 Spectral SignatureFor example, at some wavelengths, sand reflects more energy than green vegetation but at other wavelengths it absorbs more (reflects less) than does the vegetation. In principle, we can recognize various kinds of surface materials and distinguish them from each other by these differences in reflectance.
13 Spectral Signature - geologic Minerals and rocks can have distinctive spectral shapes basedon their chemical makeup and water contentFor example, chemically bound water can cause a similar feature to show up in several diverse sample typesHowever, the specific spectral location of the features and their shape depends on the actual sample 1
14 Spectral signature - Vegetation Samples shown here are for a variety of vegetation typesAll samples are of the leaves onlyThat is, no effects due to the branches and stems is included
15 Vegetation spectral reflectance Note that many of the themes for Landsat TM were based onthe spectral reflectance of vegetationShow a typical vegetation spectra - KNOW THIS CURVEAlso show the spectral bands of TM in the VNIR and SWIR as well as some of the basic physical process in each part of the spectrum
16 Recall the graph presented earlier showing the transmittance Spectral signature - AtmosphereRecall the graph presented earlier showing the transmittanceof the atmosphereCan see that there are absorption features in the atmosphere that could beused for atmospheric remote sensingAlso clues us in to portions of the spectrum to avoid so that the ground isvisible
17 Spectral SignatureSpectral signature is the idea that a given material has aspectral reflectance/emissivity which distinguishes it fromother materialsSpectral reflectance is the efficiency by which a material reflects energy as a function of wavelengthChallenges:Unfortunately, the problem is not as simple as it may appear since other factors beside the sensor play a role, such asSolar angleView angleSurface wetnessBackground and surrounding materialAlso have to deal with the fact that often the energy measured by the sensor will be from a mixture of many different materialsThis discussion will focus on the solar reflective for the time being
18 Have to keep in mind that a spectral signature is not always enough A signature is not enoughHave to keep in mind that a spectral signature is not always enoughSignature of a water absorption feature in vegetation may not indicate thedesired parameterVegetation stress and healthVegetation amountSignatures are typically derived in the laboratoryField measurements can verify the laboratory dataLaboratory measurements may not simulate what the satellite sensorwould seeGood example is the difficult nature of measuring the relationship betweenwater content and plant healthOnce the plant material is removed from the plant to allow measurementit begins to dry outUsing field-based measurements only is limited by the quality of thesensorsThe next question then becomes how many samples are needed todetermine what signatures allow for a thematic measurement
19 Signature and resolution The next thing to be concerned about is the fact that we will notfully sample the entire spectrum but rather use fewer bandsIn this case, all fourbands will allow us todifferentiate clay andgrassUsing bands 1, 3, and4 would also besufficient to do thisEven using just bands 3 and 4 would allow us to separate clay and grass
20 Band selection and resolution for spectral signatures should Signature and resolutionBand selection and resolution for spectral signatures shouldbe chosen first based on the shapes of the spectraThat is, it is not recommended to rely on the absolute difference betweentwo reflectance spectra for discriminationNumerous factors can alter the brightness of the sample while notimpacting the spectral shapeShadow effects and illumination conditionsAbsolute calibrationSample purityBands showngiveGypsum- Low, high, lowerMontmorillonite- High, high, lowQuartz- high, high,not so high
21 Spectral Signature is important property of matter makes it possible to identify different substances or classes and to separate them by their individual spectral signatures, as shown in the figure below.
22 Vegetation: NDVI NDVI - Normalized Difference Vegetation Index Video Negative values of NDVI (values approaching -1) correspond to water. Values close to zero (-0.1 to 0.1) generally correspond to barren areas of rock, sand, or snow. Lastly, low, positive values represent shrub and grassland (approximately 0.2 to 0.4), while high values indicate temperate and tropical rainforests (values approaching 1).
23 Vegetation Spectral Signature where RED and NIR stand for the spectral reflectance measurements acquired in the red and near-infrared regions, respectively.These spectral reflectances are themselves ratios of the reflected over the incoming radiation in each spectral band individually,hence they take on values between 0.0 and 1.0. By design, the NDVI itself thus varies between -1.0 and +1.0.The pigment in plant leaves, chlorophyll, strongly absorbs visible light(from 0.4 to 0.7 µm) for use in photosynthesis.The cell structure of the leaves, on the other hand, strongly reflectsnear-infrared light (from 0.7 to 1.1 µm).The more leaves a plant has, the more these wavelengths of light are affected, respectively.
24 Class ParticipationCalculate NDVI in these two trees.
25 Spectral signatures, image display, data systems Remote Sensing ModelsLecture 3Spectral signatures, image display, data systems
26 Terrestrial Radiation Energy radiated by the earth peaks in the TIREffective temperature of the earth-atmosphere system is 255 KPlanck curves below relate to typical terrestrial temperatures
27 Solar-Terrestrial Comparison When taking into account the earth-sun distance it can be shown that solar energydominates in VNIR/SWIR and emitted terrestrial dominates in the TIRSun emits moreenergy than the earthat ALL wavelengthsIt is a geometry effectthat allows us to treatthe wavelengthregions separately
28 Solar-Terrestrial Comparison Plots here show the energy from the sun at the sun and at the top of the earth’s atmosphereAlso show the emitted energy from the earth
29 Vertical Profile of the Atmosphere Understanding the verticalstructure of the atmosphereallows one to understand betterthe effects of the atmosphereAtmosphere is divided into layersbased on the change intemperature with height in thatlayerTroposphere is nearest thesurface with temperaturedecreasing with heightStratosphere is next layer andtemperature increases with heightMesosphere has decreasingtemperatures
30 Atmospheric composition Atmosphere is composed of dust and molecules which vary spatiallyand in concentrationDust also referred to as aerosolsAlso applies to liquid water, particulate matter, airplanes, etc.Primary source of aerosols is the earth's surfaceSize of most aerosols is between 0.2 and 5.0 micrometersLarger aerosols fall out due to gravitySmaller aerosols coagulate with other aerosols to make larger particlesBoth aerosols and molecules scatter light more efficiently at short wavelengthsMolecules scatter very strongly with wavelength (blue sky)Molecular scattering is proportional to 1/(wavelength)4Aerosols typically scatter with 1/(wavelength)Both aerosols and molecules absorbMolecular (or gaseous absorption is more wavelength dependentDepends on concentration of material
31 default ozone 60-degree zenith angle and no scattering AbsorptionMODTRAN3 output for US Standard Atmosphere, 2.54 cm column water vapor,default ozone 60-degree zenith angle and no scattering
32 Same curve as previous page but includes molecular scatter AbsorptionSame curve as previous page but includes molecular scatter
33 More material, lower transmittance Longer path, lower transmittance Angular effectChanging the angle of the path through the atmosphere effectively changes the concentrationMore material, lower transmittanceLonger path, lower transmittance
34 having complete absorption At longer wavelengths, absorption plays a stronger role with some spectral regionshaving complete absorptionThree absorption bands, at µm, µm, and µm
36 The MWIR is dominated by water vapor and carbon dioxide absorption
37 In the TIR there is the “atmospheric window” from 8-12 μm AbsorptionIn the TIR there is the “atmospheric window” from 8-12 μmwith a strong ozone band to consider
38 InfraredInfrared: 0.7 to 300 µm wavelength. This region is further divided into the following bands:Near Infrared (NIR): 0.7 to 1.5 µm.Short Wavelength Infrared (SWIR): 1.5 to 3 µm.Mid Wavelength Infrared (MWIR): 3 to 8 µm.Long Wanelength Infrared (LWIR): 8 to 15 µm.Far Infrared (FIR): longer than 15 µm.
39 Visible LightVisible Light: This narrow band of electromagnetic radiation extends from about 400 nm (violet) to about 700 nm (red). The various colour components of the visible spectrum fall roughly within the following wavelength regions:Red: nmOrange: nmYellow: nmGreen: nmBlue: nmIndigo: nmViolet: nm