Final Flashcards
Electromagnetic Energy - Properties
1) Wavelength
2) Amplitude
3) Phase
4) Frequency
Radiant Energy
Q = h * v Q = Radiant Energy h = Planck's constant v = Frequency
Radiant Flux
Rate of transfer of the energy
Radiant Flux Density
The amount of energy entering or leaving an object
Blackbody
A hypothetical entity that absorbs all radiation
Kirchhoff’s Law
For blackbodies the emissivity = absorption = 1
Perfect reflection emissivity = 0
e= emissivity = M/ Mb
where:
• M = emitted radiation of the real object
• Mb= emitted radiation of the blackbody @the same temperature of the real object
Stefan-Boltzmann Law – emitted radiation
Mb = sT^4
Mb = Total emitted radiation in Watts/m2 (a radiant flux density) s= Stefan-Boltzmann constant (5.67 10-8 W/m2/K4) T = Absolute body temperature [K]
Wien’s Displacement Law - Peak of Exitance
lmax= 2898/T
lmax= wavelength T = Absolute body temperature [K]
Kirchhoff + Stefan-Boltzmann
For real material we use the concept of emissivity (e) to the blackbody law in order
to relate the actual radiance of a real body (greybody) at temperature T
M = esT4 M = Total emitted radiation in Watts/m2 (a radiant flux density) s= Stefan-Boltzmann constant (5.67 10-8 W/m2/K4) T = Absolute body temperature [K] e= emissivity
Brightness Temperature
The temperature of the equivalent blackbody that would give the same radiance at the wavelength under consideration
Typically used in thermal and passive microwave RS
For sufficient long wavelengths Tb~eT
Infrared
Near Infrared (NIR) 0.7-1.3 μm Mid Infrared (MIR) 1.3-7.0 μm Far Infrared (TIR) 7-1000 μm
NIR
Sensitive to plant’s health
Similar to VIS
Essentially solar radiation reflected from the earth’s surface
Mid Infrared/SWIR
Soil moisture applications and veg water content
Detecting plant stress and burnt area
Can detect active fires
Far Infrared/LWIR
Thermal Infrared
surface temperature, evapotranspiration, heat fluxes, etc.
Interaction of EM Radiation
Absorbed
Transmitted
Reflected
“Conservation of Energy”
Sum to 1
Divide by the incident to = the proportion of 1
Scattering
Depends on Wavelength, Size Particles, # of Particles, Depth of the Atmosphere
Function of: radiation wavelength, particle size
Rayleigh Scattering
-When the molecule diameter is smaller
than the wavelength
- Primarily caused by 02 and N2
- Scattering happens through absorption and re-emission
- Responsible for blue skies and red sunsets
Mie Scattering
- Caused by larger particles with diameters approximately equal to the radiation wavelength EX. water vapor, dust, smoke
- Causes pretty sunsets
Absorption
When the atmosphere absorbs radiation
Caused by: Ozone and oxygen <300nm
Carbon dioxide 13-17.5 um (MIR & TIR)
Water vapor 5.5-7um and 27um
Reflection
Product of: surface roughness, # of leaves, geometry of incident, viewing angle
Specular Reflection
When an object is smooth all (or almost all) of the radiation is reflected in one direction
Diffuse Reflection
Also called Lambertian
When a surface is rough energy is reflected in every direction
Bidirectional Reflectance Distribution Function (BRDF)
Ratio of reflected radiance to the incident irradiance of a flat surface
Describes the optical behavior of a surface with respect to angles of illumination and observation -> Hence BIdirectional (and wavelength)
Volume Scattering
Scattering occurring within the medium as the EMR transmits from one medium to another
EX inside of snow, between the snow particles
Albedo
The directional integration of reflectance over ALL sun and view angles and sun spectrum (VIS and NIR)
Atmospheric Correction
Converts measurements from description of earth-atmosphere system to of the earth’s surface
Dark Pixel Method
Minimum Digital Number value is assumed to be atmospheric distortion and subtracted from all pixels
Atmospheric Profile - Other method of Atmospheric Correction
Spectral Reflectance: Vegetation
- Low reflectance in VIS due to chlorophyll absorption
- depends on plant health, greater jump healthier plant
- High reflectance in NIR due to leaf scattering
- Lower reflectance in NIR w/ dips due to water absorption
- Changes because of health, physiology, and structure
Vegetation Indices
- The Red Edge is foundational
- Measure “greenness”
Simple Ratio Index
NIR/Red
Larger value = healthier veg (no upper bound)
Smaller value = Not veg
NDVI
(NIR-Red)/(NIR+Red)
-1 to 1
1 = Healthy veg
-1 = water
<0.1 = Not veg
NBR
dNBR
(NIR-SWIR)/(NIR+SWIR)
PrefireNBR-PostfireNBR
-Sensitive to water, high severity pixels may be water, mask water before
Enhanced Vegetation Index (EVI)
Stabilizes aerosol influence on NDVI
Spectral Reflectance: Water
Low Reflectance in VIS, drops off in NIR and SWIR
Distribution of reflectance can be used to infer water contents
Normalized Difference Water Content
2 Types: (NIR-SWIR)/(NIR+SWIR) - for veg in drought conditions
-1 to 0 = bright surface w/ no water
1 = water!
(green-nir)/(green+nir) - for flood water/level changes
<0.3 = No water
Spectral Reflectance: Snow
- Tough to distinguish from clouds -> different signatures in MIR -> cloud reflectance(x) of cloud type
- High in VIS, decreases in NIR, dark on SWIR
- Varies with grain size -> volumetric scattering
Normalized Differenced Snow Index
(R550-R1640)/(R550+R1640)
Spectroscopy
The study of how radiated energy and matter interact
Spectrometry
Deals with the measurement of a specific spectrum
All materials reflect, absorb or emit photons in ways characteristic of their
molecular makeup
Multispectral Imagers
- Record incoming reflected or emitted EMR at a few wide and separated wavelengths
- Typical band width is 100s of nm
- Discrete bands
Hyperspectral Imagers
- Records EMR at a series of very narrow and contiguous bands
- band width is typically 10s of nm
- Continuous spectrum
Data Cube
-How hyperspectral data is presented
Compiled from hyperspectral data, when a spectrometer collected bands as narrow as 1nm to create an image with as many bands as acquired channels
Hyperspectral data
- Typically very noisy
- Bands are averaged to limit noise
- Wider wavelength ranges increase SNR
Digital Image
2-D array of pixels
-Each pixel has an intensity value and location address from its row and column reference
Digital Number
Represents a physical quantity such as solar reflectance in a pixel
- Pixel values are coded as Digital Numbers of Brightness Values
- Stored with binary digits that must be converted to real numbers
Natural Color Model
Pros: It’s what our eyes would see
Cons: Blue region subject to atmospheric scattering, Instruments collect radiation across the spectrum
False Color Images
Cons: They look fucked up and weird
Pros: Can see things invisible to our eyes normally
Color Infrared Image
Shows living vegetation and water bodies very clearly
Landsat
- 30m Spatial Res
- 16 Day Temporal Res
- Operational Land Imager, ETM+, Thermal IR Sensor
- 8 has 9 bands
MODIS
- 250m-1 k spatial
- aqua and terra orbit every day
- 36 bands
- Moderate Resolution Imaging Spectroradiometer
AVHRR
- 1km spatial
- 4 satellites, 16 images a day
- 6 bands
- cloud and surface, land water boundaries, snow and ice, night cloud mapping
- Advanced Very High Resolution Radiometer
ASTER
- Dead satellite (02-09)
- 15 to 90m spatial
- surface temperature, emissivity, reflectance, and elevation
VIIRS
- Visible Infrared Imaging Radiometer Suite
- 1km spatial resolution
- daily or bidaily temp res
- Many bands
Hyperspectral Sensors
Mostly airborne
Very few satellites
Hyperspectral Sensors
Mostly airborne
Very few satellites
Hyperspectral Sensors
Mostly airborne
Very few satellites
Hyperion
Hyperspectral spaceborne sensor
Dead Sensor (00-17)
30m spatial, 242 bands
Spectral Mixing
- Assumes linear mixing of various fractional land covers to form the actual measured reflectance
- Finds the best fitting surface fractions of endmembers
LiDAR Distance Formula
Distance = (t*c)/2
t = elapsed time c = speed of light
Characteristics of emitted and returned LiDAR pulses
timing of pulses, wavelength, and angles
Key Elements of Airborne LiDAR
1) GPS Unit
2) Inertial Measurement Units (position of aircraft)
3) Infrared or Green laser
4) Ground Control Points
LiDAR Observations
Range Distance and Intensity (fraction of photon returned)
LiDAR Return Types
Primary Returns - from the first object a pulse contacts
Secondary Returns - from the portion of a pulse that passes through a porous surface (tree canopy), 5 max
LiDAR: Waveform Return
- records the power of the pulse received as a function of time
- Can show more info than discrete returns
- Typically have larger footprints
Processing LiDAR Data
Gathered as a zigzag
Must be resampled, gridded, and single shots are interpolated
LiDAR Applications
Mapping aboveground biomass
Forest heights
Power line mapping
Thermal Region Wavelengths
3.5-20um, typically 8-13um
Emitted, not reflected
3-5um lots of absorption in CO2
8-13um very little absorption in Ozone
Thermal RS Theory
- All objects above 0K emit radiation
- Intensity and spectral composition depends on the material and its temperature
- Observed radiance is a function of actual temperature and emissivity
- Little atmospheric scattering
- Some restrictions due to atmospheric absorption
Emissivity
Dependent on:
Physical properties of surface: water content, density, color, roughness, compaction
-wavelength, surface temperature, and angle of observation
-Polarization dependent
Brightness Temperature
The temperature of the equivalent blackbody that would give the same radiance at the wavelength under consideration
- Always lower than absolute temperature
- If emissivity of an object is know, then absolute temp can be derived from the radiation an object emits
- If emissivity is not known, then only brightness temp can be known
Thermal Inertia
Determined by thermal conductivity, thermal diffusivity, and thermal capacity
-Objects with higher inertia will have a smaller diurnal range in temperature
Thermal RS: Example Sensors
GOES
Landsat 8
AVHRR
Ecostress
Thermal RS: Applications
Soil moisture, land surface temp, mineral mapping, ET, forest fires, Sea Surface Temperature, cloud structure
Microwave EM Spectrum Region
1mm-1m W band (3mm) to P band (75cm)
Weather Radar Microwave
Use K band or S band
GPS Microwave
L-band
Soil Moisture
L band
Penetration Depth Of Microwaves
X band, C band, L band (Deepest penetration)
Polarization
Horizontal and vertical plane of EMR
Passive Microwave RS
Focuses on detecting and monitoring emissivity and temperature changes
Radiometer
Energy is very low so footprint must be very large
Active Microwave RS
Detects and monitors changes in reflectivity and backscatter
Radar
Dielectric Constant
water content, has an inverse relationship with emissivity
3 Dominant Source of Microwave Radiation
1) Earth’s Surface
2) Atmosphere
3) Extraterrestrial
Rayleigh Jean’s Expression
- Substitutes for Plank’s function at higher wavelengths
- Integration over microwave region shows a linear relationship with temperature
- At polarization, brightness temp/MWR is a product of physical temp * emissivity
Radiative Transfer Model
Relative emissivity to soil moisture
AMSR-E 2
Advanced Microwave Scanning Radiometer
Earth Observing
Currently orbiting
SMOS
Soil Moisture Ocean Salinity
L band
SMAP
Soil Moisture Active Passive
Really only passive
GPM
Global Precipitation Measurement
6-36km res
Microwave Polarization Difference Index
(Tbv-Tbh)/(Tbv+Tbh)
Radiative Transfer Models
Similar to MPDI, but needs more info. on temperature, microwave roughness, vegetation opacity, and scattering albedo
Snow Water Equivalent
- Snow depth from PMW is inaccurate
- Dry snow scatters
- Wet snow absorbs and emits
Active Microwave RS
Can have finer spatial resolution
More random noise
Distortion and shading issues
Backscatter
Indicates how much energy is scattered and reflected back to a sensor from a target object
Depends on: Composition (roughness), Polarization, wavelength, Viewing angle
-Increases with soil moisture and surface roughness
Validation
Accuracy
Precision
Stability
Sources of Error
1) Sensor Limitations
2) Method of Analysis
3) Landscape Complexity
4) Verification Process
Validation Steps
1) Design your sampling strategy
2) Collect reference data
3) Extract Information
4) Compare
5) Analyze the causes
GRACE
Gravimetry satellite detects elevation changes and areas of high gravity generally
