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•By carefully measuring the wavelengths and intensity of visible and near-infrared light reflected by the land surface back up into space a "Vegetation Index" may be formulated to quantify the concentrations of green leaf vegetation around the globe.
Normalized Difference Vegetation Index (NDVI)
•Distinct colors (wavelengths) of visible and near-infrared sunlight reflected by the plants determine the density of green on a patch of land and ocean.•The pigment in plant leaves, chlorophyll, strongly absorbs visible light (from 0.4-0.5 and from to 0.6-0.7 μm) for use in photosynthesis. The cell structure of the leaves, on the other hand, strongly reflects near-infrared light (from 0.7 to 1.1 μm). •The more leaves a plant has or the more phytoplankton there is in the column, the more these wavelengths of light are affected, respectively.
Measuring Vegetation
Daytime Visibility
White Sunlight
Top of Atmosphere
Color and Intensity
Distance to the Dark Object
consider scattering by aerosols
Daytime Visibility
White Sunlight
Top of Atmosphere
Increased contribution ofwhite light
Object appears lighterwith distance
Longer Distance to the Dark Object
What the satellite sees
White Sunlight
Top of Atmosphere
molecular and aerosol scattering 400→ 500 nm
ocean water 450-480 nmplants 500-600 nm
near IRtransparent
Atmospheric Aerosol Correction Procedure
Blue Green Red Near-IR
UpwellingRadianceat Satellitedue to molecular and aerosol scattering
Cloudy
Cloudless-Polluted
Atmospheric Aerosol Correction Procedure
Blue Green Red Near-IR
Cloudy
More Polluted
UpwellingRadianceat Satellitedue to molecular and aerosol scattering
Angstrom Exponent
1 1
2 2
( )
( )
(765 ) 765
(865 ) 865
(865 )ln
(765 )
765865
A
A
A
A
A
A
nm nm
nm nm
nmnm
nmnm
0 clouds
increases with increases in aerosol load
500 nm
RV Ron Brown
Central Pacific
AOT=0.08
Sea of Japan
AOT=0.98
AMF
Niamey, Niger
AOT=2.5-3
Sky Imaging
Atmospheric Correction Methods
• Develop Theoretical Atmosphere including: • Rayleigh Scattering - (Strongest in Blue region) • Ozone • Aerosols - (Absorption and Scattering Characteristics)
• Use Data from Infrared (IR) band and assume that all of this signal comes from the atmosphere to get knowledge of aerosols.
• Solve Radiative Transfer Equation • Geometry • Location (types of aerosols possible)
NDVI
• NDVI is calculated from the visible and near-infrared light reflected by vegetation.
• Healthy vegetation (left) absorbs most of the visible light that hits it, and reflects a large portion of the near-infrared light.
• Unhealthy or sparse vegetation reflects more visible light and less near-infrared light.
• Real vegetation is highly variable.
NDVINDVI = (NIR — VIS)/(NIR + VIS)
Calculations of NDVI for a given pixel always result in a number that ranges from minus one (-1) to plus one (+1)
--no green leaves gives a value close to zero.
--zero means no vegetation
--close to +1 (0.8 - 0.9) indicates the highest possible density of green leaves.
NASA Earth Observatory (Illustration by Robert Simmon)
NOAA 11NOAA 11AVHRRAVHRR
1980 200019901985 201020051995
NOAA 7NOAA 7AVHRRAVHRR
NOAA 9NOAA 9AVHRRAVHRR
NOAA 14NOAA 14AVHRRAVHRR
SeaWiFSSeaWiFS
SPOTSPOT
MODISesMODISesNOAA-16NOAA-16
NPPNPP
NOAA 9NOAA 9 NOAA-17NOAA-17
Satellite Satellite NDVI NDVI data data
sourcessources
NOAA-18NOAA-18
C. Tucker
• In December 1999, NASA launched the Terra spacecraft, the flagship in the agency’s Earth Observing System (EOS) program. Aboard Terra flies a sensor called the Moderate-resolution Imaging Spectroradiometer, or MODIS, that greatly improves scientists’ ability to measure plant growth on a global scale. Briefly, MODIS provides much higher spatial resolution (up to 250-meter resolution), while also matching AVHRR’s almost-daily global cover and exceeding its spectral resolution.
Average NDVI 1981-2006Average NDVI 1981-2006
NDVI = NDVI = (ir- red)(ir- red)(ir+red)(ir+red)
~40,000 orbits of ~40,000 orbits of satellite datasatellite data
C. Tucker
Marked contrasts between the dry and Marked contrasts between the dry and wet seasonswet seasons
(~300 mm/yr @ Senegal)(~300 mm/yr @ Senegal)C. Tucker
Spectral Indices: NDVI
redNIR
redNIR
RR
RRNDVI
Creosote
Ag
NDVI = 0.922
NDVI = 0.356
Meg Andrew, UC Davis
Ocean Color• Locates and enables monitoring of regions of
high and low bio-activity. – Food (phytoplankton associated with chlorophyll) – Climate (phytoplankton possible CO2 sink)
• Reveals ocean current structure and behavior. – Seasonal influences – River and Estuary influences – Boundary currents
• Reveals Anthropogenic influences (pollution) • Remote sensing reveals large and small scale
structures that are very difficult to observe from the surface.
a) The light path of the water-leaving radiance. b) Shows the attenuation of the water-leaving radiance. c) Scattering of the water-leaving radiance out of the sensor's FOV. d) Sun glint (reflection from the water surface). e) Sky glint (scattered light reflecting from the surface). f) Scattering of reflected light out of the sensor's FOV. g) Reflected light is also attenuated towards the sensor. h) Scattered light from the sun which is directed toward the sensor. i) Light which has already been scattered by the atmosphere which is then scattered toward the sensor. j) Water-leaving radiance originating out of the sensor FOV, but scattered toward the sensor. k) Surface reflection out of the sensor FOV which is then scattered toward the sensor. Lw Total water-leaving radiance. Lr Radiance above the sea surface due to all surface reflection effects within the IFOV. Lp Atmospheric path radiance. (Gordan and Wang)
A break in the clouds over the Barents Sea on August 1, 2007 revealed a large, dense phytoplankton bloom to the orbiting MODIS aboard the Terra satellite. The bright aquamarine hues suggest that this is likely a coccolithophore bloom. The visible portion of this bloom covers about 150,000 square kilometers (57,000 square miles) or roughly the area of Wisconsin.
ENVI-1200 Atmospheric Physics
Aerosol Hygroscopic Growth
• Deliquescence– Dry crystal to solution
droplet
• Hygroscopic– Water-attracting
• Efflorescence– Solution droplet to
crystal (requires ‘nucleation’)
• Hysteresis– Particle size and
phase depends on humidity history
Atmospheric Correction Methods
• Develop Theoretical Atmosphere. Include: • Rayleigh Scattering - (Strongest in Blue region) • Ozone • Aerosols - (Absorption and Scattering Characteristics)
• Use Data from Infrared (IR) band and assume that all of this signal comes from the atmosphere to get knowledge of aerosols.
• Solve Radiative Transfer Equation • Geometry • Location (types of aerosols possible)
• Other considerations: – Sun Glint. Avoid - Use wind speed to estimate surface roughness. – White Caps. Measure - Use wind speed to estimate coverage.
Atmospheric Aerosol Correction Procedure
Blue Green Red Near-IR
UpwellingRadianceAt Satellite
Cloudy
Cloudless-PollutedClear H2O
Biological
History of the NDVIHistory of the NDVI& Vegetation Indices& Vegetation Indices
Compton TuckerCompton TuckerNASA/UMD/CCSPONASA/UMD/CCSPO
Index Formula Details Citation
Simple Ratio Green vegetation cover.Various wavelengths,depending on sensor. (e.g.NIR = 845nm, R=665nm)
Pearson, 1972
NormalizedDifference
Vegetation Index
Green vegetation cover.Various wavelengths,depending on sensor. (e.g.NIR = 845nm, R=665nm)
Tucker 1979
EnhancedVegetation Index
C1 =6; C2=7; L=1; G=2,5Huete 1997
PerpendicularVegetation Index
Perpendicular distance fromthe pixels to the soil line.
Richardsonand Wiegand
1977
Soil AdjustedVegetation Index
L = soil adjusted factor Huete 1988
Modified SoilAdjusted
Vegetation Index
L = (1-2a x(NIR-aR) x NDVI)Self adjusting L:f on tooptimize for soil effects.Higher dynamic range.
Qi et al 1994
Transformed SoilAdjusted
Vegetation Index
a=slope of soil lineb=intercept of soil line
Baret andGuyot 1991
Soil andAtmospherically
ResistantVegetation Index
More independent of surfacebrightness
Huete et al1997
BRNIR
RNIR
.7615.2
) 1 ( 08 . 0 ) ( 2 a b NIR a R
b aR NIR a
R
NIR R
R
RNIR
RNIR
RR
RR
22 )( NIRvNIRsRvRs
LLRNIR
RNIR
1
Vegetation Indices from Susan UstinVegetation Indices from Susan Ustin
C. Tucker
This figure shows four typically observed wavelength bands and the water leaving radiance in high (dotted) and low (solid) chlorophyll waters without the atmospheric signal (lower curves) and with the atmospheric signal (upper curves). The satellite observes the water leaving radiance with the signal due to the atmosphere (upper curves). [Gordon and Wang]