Remote Sensing
Thermal Remote Sensing
1. Review of Radiation Laws
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All objects at temperature above absolute 0oK emit
(-273.59oC, -459.67oF)
Stefan-Boltzmann law: W = sT4
W-total emitted radiation
s-a constant,
T-temperature in oK
► The total emitted radiation from a blackbody is
proportional to the fourth power of its absolute
temperature
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Radiation Laws
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Wien's displacement law: l = 2,897.8/T
l-peak wavelength, T-temperature in oK
As temperature of objects increases, the
wavelength of peak emittance becomes shorter
Radiation Laws
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Emissivity: e = M/Mb
e-emissivity
M-emittance of a given object
Mb-emittance of blackbody
e = 1 (blackbody)
e = 0 (whitebody, perfect reflector)
The ratio between the emittance of a given object
and that of blackbody at the same temperature
Emissivity of Common Materials
Clear water 0.98-0.99
Wet snow 0.98-0.99
Human skin 0.97-0.99
Rough ice 0.97-0.98
Vegetation 0.96-0.99
Wet soil
0.95-0.98
Asphalt concrete 0.94-0.97
Brick
0.93-0.94
Wood
0.93-0.94
Basalt rock 0.92-0.96
Dry mineral soil 0.92-0.94
paint
0.90-0.96
Dry vegetation 0.88-0.94
Dry snow
0.85-0.90
Granite rock
0.83-0.87
Glass
0.77-0.81
Sheet iron (rusted) 0.63-0.70
Polished metals 0.16-0.21
Aluminum foil
0.03-0.07
Highly polished gold 0.02-0.03
2. Heat
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Kinetic temperature
Radiant temperature
Heat capacity
Specific heat
Thermal conductivity
Heat ..
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Kinetic temperature (oF oC oK)
- thermal energy of molecules within a substance
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Radiant temperature
- the emitted energy
Heat ..
► Heat
capacity C (cal·g-1·oC-1)
- the ratio of the change in heat energy per unit
mass to the corresponding change in temperature at
constant pressure
► Water
has a higher heat capacity than many other
materials
Heat ..
► It
takes more energy for water to warm up to the
same temperature than many other materials do
► Rank
of heat capacity
Water > forest > grass > land
► In
other words, for a given amount of energy, water
warms up slower than many other materials
Heat ..
► Specific
heat
- the ratio of the heat capacity of a substance to
that of a reference substance, i.e. pure water
► Specific
heat of water = 1
Specific heat for many other materials < 1
Heat ..
► Thermal
conductivity K (cal·cm-1 ·sec-1·oC-1)
- the rate at that a substance transfers heat
► Water
has a lower K value than many other
materials
► It
takes a longer time for water to transfer heat over
a given distance than many other materials
3. Geometry of Thermal Images
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Tangential scale distortion
- caused by varied viewing distance
Aircraft instability
- roll: side by side motion
- crab (yaw): by compensating drift
- pitch: head/tail motion
Geometry of Thermal Images
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Relief displacement
- differs from that of aerial photography
- vertical features on thermal images displaced
from the nadir for each scan
- vertical features on air photo displaced radially
from the principal point
4. Thermal Image Interpretation
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Limitations
- thermal images contain noise and errors
- differences in emitted energy is not directly
related to differences in temperature, must
know emissivity of each material
- sensors only record the radiance at the surface
Color-coded temperature maps derived from NIMBUS
http://rst.gsfc.nasa.gov/Sect14/Sect14_4.html
Landsat band 6 full scene acquired at 9:32 P.M.
on August 22, 1982
Thermal Image Interpretation
Landscape factors
► Timing
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Thermal Image Interpretation
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Landscape factors
- surface material: land vs. water
- topography: sunlit vs. shadowed
- vegetation cover: land, grass, forest, water
- moisture
Thermal Image Interpretation
► Timing
Early afternoon:
- high thermal contrast but with thermal shadow
and slope orientation effects
- water is cool; bare soil, meadow and forest are
warmer
Thermal Image Interpretation
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Timing
Before dawn:
- lower thermal contrast but little slope
orientation effects or thermal shadow
- water is warm; open meadow and bare soil are
cool, forests are warm
Thermal Image Interpretation ..
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Radiant temperature fluctuates less for water
than for many other materials due to its high heat
capacity
► Radiant
temperature peaks later for water than for
many other materials due to its high heat capacity
Readings
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Chapter 5
Spectral Reflectance Curve