Landsat-based thermal change of Nisyros Island (volcanic)
November 2014 lava flow on Kilauea (USGS Volcano Observatory)
(http://hvo.wr.usgs.gov)
Thermal Remote Sensing
Distinguishing materials on the
ground using differences in
emissivity and temperature
Learning Objectives
• What is emissivity and why is it relevant to
thermal RS?
• What is the difference between kinetic
temperature and radiant temperature?
• How do you interpret thermal images?
• What is thermal lag and how can it be
used to ID materials?
• How can you use thermal RS to estimate
actual evapotranspiration?
Thermal = Emitted Infrared
• IR = 0.720 μm to 1000 μm (wide range)
– Reflective IR = 0.72 μm – 3.00 μm
– Thermal IR for remote sensing = 7– 18 μm
• Sometimes called far IR (vs. near and mid
IR)
• Experiences almost no atmospheric
scattering
• But…lots of absorption by atmospheric
gases (e.g., CO2)
– Must use atmospheric windows for rem. sens.
The Infrared portion of
the electromagnetic
spectrum
Emitted Thermal
Atmospheric transmission by λ
Thermal Properties of Objects
• All objects with temperature > 0o K emit
thermal radiation
– Amount depends on temperature (StefanBoltzman Law)
• M = εσT4
– Peak wavelength emitted also depends on
temperature (Wien’s Displacement Law)
• Peak λ(µm) = 3000/T(oK)
Wien’s
Displacement
Law
Emissivity
• Emissivity is the ratio of the energy
emitted by an object to that of a Black
Body at the same temperature
– A black body has є = 1
– A white body has є = 0
– Water has ε close to 1
– Most vegetation has є close to 1
– Many minerals have є << 1
• Depends on wavelength
• Can find tables of emissivities in reference
books and textbooks
Kinetic Temperature vs. Radiant
Temperature
• Kinetic temperature is caused by the vibration of
molecules
– sometimes called “true temperature”
– measured using conventional temperature scales
(e.g. oF, oC, oK)
• Radiant temperature is the emitted energy of an
object
– sometimes called “apparent temperature”
– what we measure with thermal remote sensing
– depends on kinetic temperature and emissivity
Thermal Remote Sensing
• Incoming radiation from the sun is
absorbed (converted to kinetic energy)
and object emits EMR
• Objects vary in the amount of sun they
“see” (different slopes, etc.) and in their
emissivity
• Thermal remote sensing is sensitive to
differences in emissivity.
Interpreting Thermal Images
• Thermal images are often single-band and
so displayed as monochrome images.
– Bright areas = relatively warmer places
– Dark areas = relatively cooler places
– Can be the opposite for thermal weather
images!
• Must know if the image is a negative or a
positive!
• Should know the time of day the image
was acquired – day vs. night alters the
interpretation
Atlanta -- Daytime
Atlanta -- Nighttime
Daily change in radiant temperature of common objects
North
Thermal Infrared Multispectral Scanner (TIMS) image of Death Valley
Daytime Positive – Bright = warm, Dark = cool
Multi-band thermal
• Thermal imagery can also be multi-band
(different parts of the thermal IR spectrum)
• When displayed in color, colors primarily
represent differences in emissivity.
North
TIMS image of Death Valley made by combining thermal
bands from different wavelengths after “decorrelation
stretching”
Interpretation (cont.)
• It is difficult to accurately calculate the
kinetic temperature of objects from their
radiant temperature
– Must know the emissivity of the target(s)
– Often have to estimate or assume emissivity
values
Complicating Factors
• Topography (effects amount of incoming
radiation from sun)
• Fine scale differences in emissivities of materials
in scene
• Cloud cover history
• Precipitation history – differences in soil
moisture
• Vegetation canopy geometry
• Geothermal areas
• Many others
Thermal Sensors
• Thermal Infrared Multispectral Scanner (TIMS)
(Airborne – 18 m spatial res.)
• Landsat 3 MSS (237 m spatial resolution)
• Landsat TM (Band 6) (120 m spatial)
• Landsat 8 (Bands 10 & 11) (100 m spatial)
• Landsat ETM+ (Band 6) (60 m spatial)
• ASTER (5 thermal bands at 90 m spatial)
• MODIS (many thermal bands at 1 km spatial
resolution)
• Many others…
Applications
• Agricultural water stress (energy balance)
• Heat loss from urban areas
• Identifying and mapping materials based on their
emissivities (e.g. minerals)
• Earthquake and volcanic activity prediction
• Mapping moisture amounts
• Ocean current mapping
• Plumes of warm water from power plants, etc.
• Atmospheric studies, weather forecasting, etc.
Evapotranspiration (ET)
estimation using thermal RS
• If you know how much energy is being
used to evaporate water, you can estimate
how much water is evaporating!
E=H+L+r+G
Where E = irradiance, H = sensible heat, L =
latent heat, r = reflected energy, and G =
ground storage of energy.
R
-R
Thermal
Image of Lava
Flows
ASTER
Airborne
thermal
image of
warm creek
flowing into
ocean near
Anchorage,
AK
ASTER images of San
Francisco.
Bottom right is thermal
image used for water
temperature
Summary – Thermal Remote
Sensing
• Typically used to map surface materials that
differ in thermal properties (like emissivity)
• Usually NOT used to map absolute kinetic
temperature
• Many applications but not especially good for
distinguishing among vegetation types because
all veg has about the same emissivity
• Gives us another tool to help distinguish
materials that may be spectrally similar in the
reflected wavelengths!