UCL DEPARTMENT OF GEOGRAPHY GEOGG 141 GEOG 3051

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UCL DEPARTMENT OF GEOGRAPHY GEOGG 141/ GEOG 3051 Principles & Practice of Remote Sensing

UCL DEPARTMENT OF GEOGRAPHY GEOGG 141/ GEOG 3051 Principles & Practice of Remote Sensing EM Radiation (ii) Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7679 0592 Email: mdisney@ucl. geog. ac. uk http: //www 2. geog. ucl. ac. uk/~mdisney/teaching/GEOGG 141. html http: //www 2. geog. ucl. ac. uk/~mdisney/teaching/3051/GEOG 3051. html

UCL DEPARTMENT OF GEOGRAPHY EMR arriving at Earth • We now know how EMR

UCL DEPARTMENT OF GEOGRAPHY EMR arriving at Earth • We now know how EMR spectrum is distributed • Radiant energy arriving at Earth’s surface • NOT blackbody, but close • “Solar constant” • solar energy irradiating surface perpendicular to solar beam • ~1373 Wm-2 at top of atmosphere (TOA) • Mean distance of sun ~1. 5 x 108 km so total solar energy emitted = 4 r 2 x 1373 = 3. 88 x 1026 W • Incidentally we can now calculate Tsun (radius=6. 69 x 108 m) from SB Law • T 4 sun = 3. 88 x 1026/4 r 2 so T = ~5800 K 2

UCL DEPARTMENT OF GEOGRAPHY Departure from blackbody assumption • Interaction with gases in the

UCL DEPARTMENT OF GEOGRAPHY Departure from blackbody assumption • Interaction with gases in the atmosphere – attenuation of solar radiation 3

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: spatial relations • Now cover what happens when

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: spatial relations • Now cover what happens when radiation interacts with Earth System • Atmosphere • On the way down AND way up • Surface • Multiple interactions between surface and atmosphere • Absorption/scattering of radiation in the atmosphere 4

UCL DEPARTMENT OF GEOGRAPHY Radiation passing through media • Various interactions, with different results

UCL DEPARTMENT OF GEOGRAPHY Radiation passing through media • Various interactions, with different results From http: //rst. gsfc. nasa. gov/Intro/Part 2_3 html 5

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: spatial relations • Definitions of radiometric quantities •

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: spatial relations • Definitions of radiometric quantities • Radiant energy emitted, transmitted of received per unit time is radiant flux (usually Watts, or Js-1) • Radiant flux density is flux per unit area (Wm-2) • Irradiance is radiant flux density incident on a surface (Wm-2) e. g. Solar radiation arriving at surface • Emittance (radiance or radiant exitance) (Wm-2) is radiant flux density emitted by a surface • For parallel beam, flux density defined in terms of plane perpendicular to beam. What about from a point? 6

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: point source Point source d. F d d.

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: point source Point source d. F d d. A r • Consider flux d. F emitted from point source into solid angle d , where d. F and d very small • Intensity I defined as flux per unit solid angle i. e. I = d. F/d (Wsr-1) • Solid angle d = d. A/r 2 (steradians, sr) 7

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: plane source Plane source d. S d. F

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: plane source Plane source d. S d. F d. S cos • What about when we have a plane source rather than a point? • Element of surface with area d. S emits flux d. F in direction at angle to normal • Radiant emittance, M = d. F / d. S (Wm-2) • Radiance L is intensity in a particular direction (d. I = d. F/ ) divided by the apparent area of source in that direction i. e. flux per unit area per solid angle (Wm-2 sr-1) • Projected area of d. S is direction is d. S cos , so…. . • Radiance L = (d. F/ ) / d. S cos = d. I/d. S cos (Wm-2 sr-1) 8

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: radiance • So, radiance equivalent to: • intensity

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: radiance • So, radiance equivalent to: • intensity of radiant flux observed in a particular direction divided by apparent area of source in same direction • Note on solid angle (steradians): • 3 D analog of ordinary angle (radians) • 1 steradian = angle subtended at the centre of a sphere by an area of surface equal to the square of the radius. The surface of a sphere subtends an angle of 4 steradians at its centre. 9

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: solid angle • Cone of solid angle =

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: solid angle • Cone of solid angle = 1 sr from sphere • Radiant intensity • = area of surface A / radius 2 From http: //www. intl-light. com/handbook/ch 07. html 10

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: terms and units 11

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: terms and units 11

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: cosine law • Emission and absorption • Radiance

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: cosine law • Emission and absorption • Radiance linked to law describing spatial distn of radiation emitted by Bbody with uniform surface temp. T (total emitted flux = T 4) • Surface of Bbody then has same T from whatever angle viewed • So intensity of radiation from point on surface, and areal element of surface MUST be independent of , angle to surface normal • OTOH flux per unit solid angle divided by true area of surface must be proportional to cos 12

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: cosine law X Radiometer d. A Y X

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: cosine law X Radiometer d. A Y X Radiometer Y • Case 1: radiometer ‘sees’ d. A, flux proportional to d. A/cos • Case 2: radiometer ‘sees’ d. A/cos (larger) BUT T same, so emittance of surface same and hence radiometer measures same • So flux emitted per unit area at angle to cos so that product of emittance ( cos ) and area emitting ( 1/ cos ) is same for all • This is basis of Lambert’s Cosine Law Adapted from Monteith and Unsworth, Principles of Environmental Physics 13

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: Lambert’s Cosine Law • When radiation emitted from

UCL DEPARTMENT OF GEOGRAPHY Radiation Geometry: Lambert’s Cosine Law • When radiation emitted from Bbody at angle to normal, then flux per unit solid angle emitted by surface is cos • Corollary of this: • if Bbody exposed to beam of radiant energy at an angle to normal, the flux density of absorbed radiation is cos • In remote sensing we generally need to consider directions of both incident AND reflected radiation, then reflectivity is described as bi-directional Adapted from Monteith and Unsworth, Principles of Environmental Physics 14

UCL DEPARTMENT OF GEOGRAPHY Recap: radiance • Radiance, L • power emitted (d. F)

UCL DEPARTMENT OF GEOGRAPHY Recap: radiance • Radiance, L • power emitted (d. F) per unit of solid angle (d ) and per unit of the projected surface (d. S cos ) of an extended widespread source in a given direction, ( = zenith angle, = azimuth angle) d Projected surface d. S cos • L = d 2 F / (d d. S cos ) (in Wm-2 sr-1) • If radiance is not dependent on i. e. if same in all directions, the source is said to be Lambertian. Ordinary surfaces rarely found to be Lambertian. Ad. From http: //ceos. cnes. fr: 8100/cdrom-97/ceos 1/science/baphygb/chap 2. htm 15

UCL DEPARTMENT OF GEOGRAPHY Recap: emittance • Emittance, M (exitance) • emittance (M) is

UCL DEPARTMENT OF GEOGRAPHY Recap: emittance • Emittance, M (exitance) • emittance (M) is the power emitted (d. W) per surface unit of an extended widespread source, throughout an hemisphere. Radiance is therefore integrated over an hemisphere. If radiance independent of i. e. if same in all directions, the source is said to be Lambertian. • For Lambertian surface • Remember L = d 2 F / (d d. S cos ) = constant, so M = d. F/d. S = • M = L 16 Ad. From http: //ceos. cnes. fr: 8100/cdrom-97/ceos 1/science/baphygb/chap 2. htm

UCL DEPARTMENT OF GEOGRAPHY Recap: irradiance • Radiance, L, defined as directional (function of

UCL DEPARTMENT OF GEOGRAPHY Recap: irradiance • Radiance, L, defined as directional (function of angle) • from source d. S along viewing angle of sensor ( in this 2 D case, but more generally ( , ) in 3 D case) Direct • Emittance, M, hemispheric • Why? ? • Solar radiation scattered by atmosphere Diffuse • So we have direct AND diffuse components Ad. From http: //ceos. cnes. fr: 8100/cdrom-97/ceos 1/science/baphygb/chap 2. htm 17

UCL DEPARTMENT OF GEOGRAPHY Reflectance • Spectral reflectance, ( ), defined as ratio of

UCL DEPARTMENT OF GEOGRAPHY Reflectance • Spectral reflectance, ( ), defined as ratio of incident flux to reflected flux at same wavelength • ( ) = L( )/I( ) • Extreme cases: • Perfectly specular: radiation incident at angle reflected away from surface at angle - • Perfectly diffuse (Lambertian): radiation incident at angle reflected equally in all angles 18

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere From http: //rst. gsfc. nasa. gov/Intro/Part

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere From http: //rst. gsfc. nasa. gov/Intro/Part 2_4. html 19

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere R 4 R 1 R 2

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere R 4 R 1 R 2 target R 3 target • Notice that target reflectance is a function of • Atmospheric irradiance • reflectance outside target scattered into path • diffuse atmospheric irradiance • multiple-scattered surface-atmosphere interactions From: http: //www. geog. ucl. ac. uk/~mdisney/phd. bak/final_version/final_pdf/chapter 2 a. pdf 20

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere: refraction • Caused by atmosphere at

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere: refraction • Caused by atmosphere at different T having different density, hence refraction • path of radiation alters moving from medium of one density to another (different velocity) • index of refraction (n) is ratio of speed of light in a vacuum (c) to speed cn in another medium (e. g. Air) i. e. n = c/cn • note that n always >= 1 i. e. cn <= c • Examples • nair = 1. 0002926 • nwater = 1. 33 21

UCL DEPARTMENT OF GEOGRAPHY Refraction: Snell’s Law • Refraction described by Snell’s Law •

UCL DEPARTMENT OF GEOGRAPHY Refraction: Snell’s Law • Refraction described by Snell’s Law • For given freq. f, n 1 sin 1 = n 2 sin 2 • where 1 and 2 are the angles from the normal of the incident and refracted waves respectively • (non-turbulent) atmosphere can be considered as layers of gases, each with a different density (hence n) • Displacement of path - BUT knowing Snell’s Law can be removed Incident radiation Optically less dense n 2 1 n 1 Optically more 2 dense Optically less dense n 3 3 Path unaffected by atmosphere Path affected by atmosphere 22 After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective.

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere: scattering • Caused by presence of

UCL DEPARTMENT OF GEOGRAPHY Interactions with the atmosphere: scattering • Caused by presence of particles (soot, salt, etc. ) and/or large gas molecules present in the atmosphere • Interact with EMR anc cause to be redirected from original path. • Scattering amount depends on: • of radiation • abundance of particles or gases • distance the radiation travels through the atmosphere (path length) After: http: //www. ccrs. nrcan. gc. ca/ccrs/learn/tutorials/fundam/chapter 1_4_e. html 23

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 1: Rayleigh • Particle size << of radiation

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 1: Rayleigh • Particle size << of radiation • e. g. very fine soot and dust or N 2, O 2 molecules • Rayleigh scattering dominates shorter and in upper atmos. • i. e. Longer scattered less (visible red scattered less than blue ) • Hence during day, visible blue tend to dominate (shorter path length) • Longer path length at sunrise/sunset so proportionally more visible blue scattered out of path so sky tends to look more red • Even more so if dust in upper atmosphere • http: //www. spc. noaa. gov/publications/corfidi/sunset/ • http: //www. nws. noaa. gov/om/educ/activit/bluesky. htm 24 After: http: //www. ccrs. nrcan. gc. ca/ccrs/learn/tutorials/fundam/chapter 1_4_e. html

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 1: Rayleigh • So, scattering -4 so scattering

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 1: Rayleigh • So, scattering -4 so scattering of blue light (400 nm) > scattering of red light (700 nm) by (700/400)4 or ~ 9. 4 25 From http: //hyperphysics. phy-astr. gsu. edu/hbase/atmos/blusky. html

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 2: Mie • Particle size of radiation •

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 2: Mie • Particle size of radiation • e. g. dust, pollen, smoke and water vapour • Affects longer than Rayleigh, BUT weak dependence on • Mostly in the lower portions of the atmosphere • larger particles are more abundant • dominates when cloud conditions are overcast • i. e. large amount of water vapour (mist, cloud, fog) results in almost totally diffuse illumination After: http: //www. ccrs. nrcan. gc. ca/ccrs/learn/tutorials/fundam/chapter 1_4_e. html 26

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 3: Non-selective • Particle size >> of radiation

UCL DEPARTMENT OF GEOGRAPHY Atmospheric scattering 3: Non-selective • Particle size >> of radiation • e. g. Water droplets and larger dust particles, • All affected about equally (hence name!) • Hence results in fog, mist, clouds etc. appearing white • white = equal scattering of red, green and blue s After: http: //www. ccrs. nrcan. gc. ca/ccrs/learn/tutorials/fundam/chapter 1_4_e. html 27

UCL DEPARTMENT OF GEOGRAPHY Atmospheric absorption • Other major interaction with signal • Gaseous

UCL DEPARTMENT OF GEOGRAPHY Atmospheric absorption • Other major interaction with signal • Gaseous molecules in atmosphere can absorb photons at various • depends on vibrational modes of molecules • Very dependent on • Main components are: • CO 2, water vapour and ozone (O 3) • Also CH 4. . • O 3 absorbs shorter i. e. protects us from UV radiation 28

UCL DEPARTMENT OF GEOGRAPHY Atmospheric absorption • CO 2 as a “greenhouse” gas •

UCL DEPARTMENT OF GEOGRAPHY Atmospheric absorption • CO 2 as a “greenhouse” gas • strong absorber in longer (thermal) part of EM spectrum • i. e. 10 -12 m where Earth radiates • Remember peak of Planck function for T = 300 K • So shortwave solar energy (UV, vis, SW and NIR) is absorbed at surface and re-radiates in thermal • CO 2 absorbs re-radiated energy and keeps warm • $64 M question! • Does increasing CO 2 increasing T? ? • Anthropogenic global warming? ? • Aside. . 29

UCL DEPARTMENT OF GEOGRAPHY Atmospheric CO 2 trends • Antarctic ice core records •

UCL DEPARTMENT OF GEOGRAPHY Atmospheric CO 2 trends • Antarctic ice core records • Keeling et al. • Annual variation + trend • Smoking gun for anthropogenic change, or natural variation? ? 30

UCL DEPARTMENT OF GEOGRAPHY Atmospheric “windows” Atmospheric windows • As a result of strong

UCL DEPARTMENT OF GEOGRAPHY Atmospheric “windows” Atmospheric windows • As a result of strong dependence of absorption • Some totally unsuitable for remote sensing as most radiation absorbed 31

UCL DEPARTMENT OF GEOGRAPHY Atmospheric “windows” • If you want to look at surface

UCL DEPARTMENT OF GEOGRAPHY Atmospheric “windows” • If you want to look at surface – Look in atmospheric windows where transmissions high • If you want to look at atmosphere however. . pick gaps • Very important when selecting instrument channels – Note atmosphere nearly transparent in wave i. e. can see through clouds! – V. Important consideration. . 32

UCL DEPARTMENT OF GEOGRAPHY Atmospheric “windows” • Vivisble + NIR part of the spectrum

UCL DEPARTMENT OF GEOGRAPHY Atmospheric “windows” • Vivisble + NIR part of the spectrum – windows, roughly: 400 -750, 800 -1000, 1150 -1300, 1500 -1600, 2100 -2250 nm 33

UCL DEPARTMENT OF GEOGRAPHY Summary • Measured signal is a function of target reflectance

UCL DEPARTMENT OF GEOGRAPHY Summary • Measured signal is a function of target reflectance – plus atmospheric component (scattering, absorption) – Need to choose appropriate regions (atmospheric windows) • μ-wave region largely transparent i. e. can see through clouds in this region • one of THE major advantages of μ-wave remote sensing • Top-of-atmosphere (TOA) signal is NOT target signal • To isolate target signal need to. . . – Remove/correct for effects of atmosphere – A major part component of RS pre-processing chain • Atmospheric models, ground observations, multiple views of surface through different path lengths and/or combinations of above 34

UCL DEPARTMENT OF GEOGRAPHY Summary • Generally, solar radiation reaching the surface composed of

UCL DEPARTMENT OF GEOGRAPHY Summary • Generally, solar radiation reaching the surface composed of – <= 75% direct and >=25 % diffuse • attentuation even in clearest possible conditions – minimum loss of 25% due to molecular scattering and absorption about equally – Normally, aerosols responsible for significant increase in attenuation over 25% – HENCE ratio of diffuse to total also changes – AND spectral composition changes 35

UCL DEPARTMENT OF GEOGRAPHY Reflectance • When EMR hits target (surface) • Range of

UCL DEPARTMENT OF GEOGRAPHY Reflectance • When EMR hits target (surface) • Range of surface reflectance behaviour • perfect specular (mirror-like) - incidence angle = exitance angle • perfectly diffuse (Lambertian) - same reflectance in all directions independent of illumination angle) Natural surfaces somewhere in between 36 From http: //www. ccrs. nrcan. gc. ca/ccrs/learn/tutorials/fundam/chapter 1_5_e. html

UCL DEPARTMENT OF GEOGRAPHY Surface energy budget • Total amount of radiant flux per

UCL DEPARTMENT OF GEOGRAPHY Surface energy budget • Total amount of radiant flux per wavelength incident on surface, ( ) W m-1 is summation of: • reflected r , transmitted t , and absorbed, a • i. e. ( ) = r + t + a • So need to know about surface reflectance, transmittance and absorptance • Measured RS signal is combination of all 3 components After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective. 37

UCL DEPARTMENT OF GEOGRAPHY Reflectance: angular distribution • Real surfaces usually display some degree

UCL DEPARTMENT OF GEOGRAPHY Reflectance: angular distribution • Real surfaces usually display some degree of reflectance ANISOTROPY • Lambertian surface is isotropic by definition (a) (b) (c) (d) • Most surfaces have some level of anisotropy Figure 2. 1 Four examples of surface reflectance: (a) Lambertian reflectance (b) non-Lambertian (directional) reflectance (c) specular (mirror-like) reflectance (d) retro-reflection peak (hotspot). 38 From: http: //www. geog. ucl. ac. uk/~mdisney/phd. bak/final_version/final_pdf/chapter 2 a. pdf

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Reflectance of most real surfaces is

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Reflectance of most real surfaces is a function of not only λ, but viewing and illumination angles • Described by the Bi-Directional Reflectance Distribution Function (BRDF) • BRDF of area A defined as: ratio of incremental radiance, d. Le, leaving surface through an infinitesimal solid angle in direction ( v, v), to incremental irradiance, d. Ei, from illumination direction ’( i, i) i. e. • is viewing vector ( v, v) are view zenith and azimuth angles; ’ is illum. vector ( i, i) are illum. zenith and azimuth angles • So in sun-sensor example, is position of sensor and ’ is position of sun After: Jensen, J. (2000) Remote sensing of the environment: an Earth Resources Perspective. 39

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Note that BRDF defined over infinitesimally

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Note that BRDF defined over infinitesimally small solid angles , ’ and interval, so cannot measure directly • In practice measure over some finite angle and assume valid viewer exitant solid angle incident diffuse radiation direct irradiance ( Ei) vector v i 2 - v surface tangent vector i surface area A Configuration of viewing and illumination vectors in the viewing hemisphere, with respect to an element of surface area, A. 40 From: http: //www. geog. ucl. ac. uk/~mdisney/phd. bak/final_version/final_pdf/chapter 2 a. pdf

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Spectral behaviour depends on illuminated/viewed amounts

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Spectral behaviour depends on illuminated/viewed amounts of material • Change view/illum. angles, change these proportions so change reflectance • Information contained in angular signal related to size, shape and distribution of objects on surface (structure of surface) • Typically CANNOT assume surfaces are Lambertian (isotropic) Modelled barley reflectance, v from – 50 o to 0 o (left to right, top to bottom). 41 From: http: //www. geog. ucl. ac. uk/~mdisney/phd. bak/final_version/final_pdf/chapter 2 a. pdf

UCL DEPARTMENT OF GEOGRAPHY Directional Information 42

UCL DEPARTMENT OF GEOGRAPHY Directional Information 42

UCL DEPARTMENT OF GEOGRAPHY Directional Information 43

UCL DEPARTMENT OF GEOGRAPHY Directional Information 43

UCL DEPARTMENT OF GEOGRAPHY Features of BRDF • Bowl shape – increased scattering due

UCL DEPARTMENT OF GEOGRAPHY Features of BRDF • Bowl shape – increased scattering due to increased path length through canopy 44

UCL DEPARTMENT OF GEOGRAPHY Features of BRDF • Bowl shape – increased scattering due

UCL DEPARTMENT OF GEOGRAPHY Features of BRDF • Bowl shape – increased scattering due to increased path length through canopy 45

UCL DEPARTMENT OF GEOGRAPHY 46

UCL DEPARTMENT OF GEOGRAPHY 46

UCL DEPARTMENT OF GEOGRAPHY Features of BRDF • Hot Spot – mainly shadowing minimum

UCL DEPARTMENT OF GEOGRAPHY Features of BRDF • Hot Spot – mainly shadowing minimum – so reflectance higher 47

UCL DEPARTMENT OF GEOGRAPHY The “hotspot” See http: //www. ncaveo. ac. uk/test_sites/harwood_forest/ 48

UCL DEPARTMENT OF GEOGRAPHY The “hotspot” See http: //www. ncaveo. ac. uk/test_sites/harwood_forest/ 48

UCL DEPARTMENT OF GEOGRAPHY 49

UCL DEPARTMENT OF GEOGRAPHY 49

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Good explanation of BRDF: • http:

UCL DEPARTMENT OF GEOGRAPHY Directional reflectance: BRDF • Good explanation of BRDF: • http: //geography. bu. edu/brdfexpl. html 50

UCL DEPARTMENT OF GEOGRAPHY • Hotspot effect from MODIS image over Brazil 51

UCL DEPARTMENT OF GEOGRAPHY • Hotspot effect from MODIS image over Brazil 51

UCL DEPARTMENT OF GEOGRAPHY Measuring BRDF via RS • Need multi-angle observations. Can do

UCL DEPARTMENT OF GEOGRAPHY Measuring BRDF via RS • Need multi-angle observations. Can do three ways: • multiple cameras on same platform (e. g. MISR, POLDER 2). BUT quite complex technically. • Broad swath with large overlap so multiple orbits build up multiple view angles e. g. MODIS, SPOT-VGT, AVHRR. BUT surface can change from day to day. • Pointing capability e. g. CHRIS-PROBA, SPOT-HRV. BUT again technically difficult 52

UCL DEPARTMENT OF GEOGRAPHY Albedo • Total irradiant energy (both direct and diffuse) reflected

UCL DEPARTMENT OF GEOGRAPHY Albedo • Total irradiant energy (both direct and diffuse) reflected in all directions from the surface i. e. ratio of total outgoing to total incoming • Defines lower boundary condition of surface energy budget hence v. imp. for climate studies - determines how much incident solar radiation is absorbed • Albedo is BRDF integrated over whole viewing/illumination hemisphere • Define directional hemispherical refl (DHR) - reflectance integrated over whole viewing hemisphere resulting from directional illumination • and bi-hemispherical reflectance (BHR) - integral of DHR with respect to hemispherical (diffuse) illumination DHR = BHR = 53

UCL DEPARTMENT OF GEOGRAPHY Albedo • Actual albedo lies somewhere between DHR and BHR

UCL DEPARTMENT OF GEOGRAPHY Albedo • Actual albedo lies somewhere between DHR and BHR • Broadband albedo, , can be approximated as • where p( ) is proportion of solar irradiance at ; and ( ) is spectral albedo • so p( ) is function of direct and diffuse components of solar radiation and so is dependent on atmospheric state • Hence albedo NOT intrinsic surface property (although BRDF is) 54

UCL DEPARTMENT OF GEOGRAPHY Typical albedo values 55

UCL DEPARTMENT OF GEOGRAPHY Typical albedo values 55

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information • Causes of spectral variation in reflectance?

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information • Causes of spectral variation in reflectance? • (bio)chemical & structural properties • e. g. In vegetation, phytoplankton: chlorophyll concentration • soil - minerals/ water/ organic matter • Can consider spectral properties as continuous • e. g. mapping leaf area index or canopy cover • or discrete variable • e. g. spectrum representative of cover type (classification) 56

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: vegetation 57 vegetation

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: vegetation 57 vegetation

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: vegetation 58 vegetation

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: vegetation 58 vegetation

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: soil 59 soil

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: soil 59 soil

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: canopy 60

UCL DEPARTMENT OF GEOGRAPHY Surface spectral information: canopy 60

UCL DEPARTMENT OF GEOGRAPHY Summary • Last week • Introduction to EM radiation, the

UCL DEPARTMENT OF GEOGRAPHY Summary • Last week • Introduction to EM radiation, the EM spectrum, properties of wave / particle model of EMR • Blackbody radiation, Stefan-Boltmann Law, Wien’s Law and Planck function • This week • radiation geometry • interaction of EMR with atmosphere • atmospheric windows • interaction of EMR with surface (BRDF, albedo) • angular and spectral reflectance properties 61