Fundamentals of Remote Sensing

1
Fundamentals of Remote Sensing

• Dr. Walter Goedecke
• Fall 2007

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Topics

• Overview of Remote Sensing
• Electromagnetic Energy, Photons, and the Spectrum
• Visible Wavelengths
• Infrared Sensing
• Thermal Radiation
• Radiation from Real Materials
• Microwave Remote Sensing
• Atmospheric Effects
• Remote Sensing Images

3
OVERVIEW OF REMOTE SENSING

• We perceive our surrounding world through our
  five senses
• Sight and hearing do not require close contact
  between sensors and externals
• Thus, our eyes and ears are remote sensors
• We perform remote sensing essentially all of the
  time

(Virtual Science Centre)
4
OVERVIEW OF REMOTE SENSING Remote Sensing from
Afar

• Remote sensing implies that a sensor not in
  direct contact with objects or events being
  observed
• Information needs a carrier
• Electromagnetic radiation is normally used as
  information carrier
• The output of a remote sensing system is usually
  an image representing the observed scene

(Virtual Science Centre)
5
OVERVIEW OF REMOTE SENSING Remote Sensing
Platforms of the Earth

• Airborne platforms
• Aircraft
• Balloons
• Spaceborne platforms
• Satellites
• The Space Shuttle

(Virtual Science Centre)
6
OVERVIEW OF REMOTE SENSING Remote Sensing from
Space

• Information pertains to all areas of interest,
  such as
• Land
• Oceans
• Atmosphere
• Some practical applications are
• Weather observing
• Mapping and cataloging
• Early warning
• Media coverage
• Extensions of astronomical capabilities, such as
  
• Earthbound telescopes
• Spacecraft carrying visible light sensors
• Addition of radio wave, infrared, ultraviolet,
  x-ray, and gamma ray sensors

7
Energy Interactions with Earth Surface Features

• Solar radiation is electromagnetic energy
  reflected or scattered from the Earth
• Different materials (water, soil, etc.) reflect
  energy in different ways
• Each material has its own spectral reflectance
  signature

(Virtual Science Centre)
8
Electromagnetic Energy

• Electromagnetic energy can be though of as either
  waves or particles, known as photons.
• This energy is propagates through space in form
  of periodic or sinusoidal disturbances of
  electric and magnetic fields
• In free space this is 299,792,458 meters/second
  (exact)
• The waves are characterized by frequency and
  wavelength, related by

• c ??
• where
• c speed of light
• ? frequency
• ? wavelength, usually in ?m (10-6 meters), or
  in nm (10-9 meters)

(Wave Nature of Light)
9
The Electromagnetic Spectrum
(The Wave Nature of Light)
10
Multispectral Images
Red

• One band at a time displayed as gray scale image
• Combination of three bands for color composite
  image.
• Requires knowledge of spectral reflectance for
  composite image interpretation.

Green
Near-IR
(Virtual Science Centre)
11
False Color Composite

• Common false color scheme for SPOT
• R NIR band
• G red band
• B green band

(Virtual Science Centre)
12
Four Views of Crab Nebula from Different
Multispectral Sensing Devices
X-ray
Optical
(rst)
Infrared
Radio
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Electromagnetic Energy

• A photon is quantized energy, or an energy packet
  
• Photons can have different discrete energy values
• The energy of a quantum is given by Planck's
  equation
• Thus photons of shorter wavelengths (?), or
  higher frequency waves (? or f), are more
  energetic than those of longer wavelengths, or
  lower frequencies
• An x-ray photon is more energetic than a light
  photon

14
Electromagnetic Energy

• Radio waves through gamma rays are all
  electromagnetic (EM) waves
• These waves differ only in wavelength
• Visible light is only one form of electromagnetic
  energy
• Ultraviolet, x-rays, and gamma rays are shorter
• Infrared, microwaves, television, and radio waves
  are longer.
• An object of a certain size can scatter EM
  wavelengths on the order of this size or smaller,
  but not larger wavelengths.
• Thus long wavelengths will not identify a small
  object
• Long wavelength radiation can only measure
  distances and objects on the order of the
  wavelength
• Infrared light of micrometer wavelength will
  resolve better than decimeter wavelength radio
  waves

15
Visible Light Bands

• This narrow band of electromagnetic radiation
  extends from about 400 nm (violet) to about 700
  nm (red).
• The various color components of the visible
  spectrum fall roughly within the following
  wavelength regions
• Red 610 - 700 nm
• Orange 590 - 610 nm
• Yellow 570 - 590 nm
• Green 500 - 570 nm
• Blue 450 - 500 nm
• Indigo 430 - 450 nm
• Violet 400 - 430 nm

(Virtual Science Centre)
16
Infrared Bands

• Infrared ranges from 0.7 to 300 µm wavelength.
• This region is further divided into the following
  bands
• Near Infrared (NIR) 0.7 to 1.5 µm.
• Short Wavelength Infrared (SWIR) 1.5 to 3 µm.
• Mid Wavelength Infrared (MWIR) 3 to 8 µm.
• Long Wavelength Infrared (LWIR) 8 to 15 µm.
• Far Infrared (FIR) longer than 15 µm.
• The NIR and SWIR bands are also known as
  reflected infrared, referring to the main
  infrared component of the solar radiation
  reflected from the earth's surface.
• The MWIR and LWIR are known as thermal infrared

(Virtual Science Centre)
17
Electromagnetic Wave Sources

• The Sun at 11,000 F emits most energy in the
  visible spectrum
• Objects reflect EM waves from other sources
• Green leafs reflect green light
• Red flower reflects red light
• Conifers absorb more IR than deciduous plants
• X-rays easily pass through body and create a
  shadowgram of the interior hard parts, thus
  allowing the identification of a broken bone, for
  example

18
Thermal Radiation Principles

• There are two types of temperature Kinetic and
  Radiant temperature
• Kinetic temperature
• Average translational energy of molecules
• Measured by placing sensor in contact with
  material
• Radiant temperature
• Radiation of energy as a function of material
  temperature
• Can be measured remotely
• Basis for thermal scanning

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Thermal Radiation Principles

• Any object having temperature greater than
  absolute zero emits electromagnetic radiation
• Intensity and spectral composition a function of
  material type involved and temperature of object
• High temperature ? Shorter wavelengths
• Lower temperature ? Longer wavelengths
• The energy peak shifts toward shorter wavelengths
  with increased temperature
• An example is when a piece of iron changes color
  from red, to orange, to yellow, and then to white
  when heated at higher temperatures.

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Thermal Radiation Principles (Continued)
(Blackbody)
21
Blackbody, Wien, and Stefan-Boltzmann Summary
(Atmospheric Radiation)
22
Thermal Radiation Principles (Concluded)

• From previous slide, total radiant exitance for
  blackbody varies as fourth power of absolute
  temperature
• Remote measurement of radiant exitance M from a
  surface can be used to infer temperature of
  surface
• This indirect approach to temperature measurement
  used in thermal scanning
• Radiant exitance M measured over discrete
  wavelength range and used to find radiant
  temperature of radiating surface

23
Radiation from Real Materials

• All real materials emit only a fraction of the
  energy emitted by a blackbody at the equivalent
  temperature

• Emissivity can vary with wavelength, viewing
  angle, and somewhat with temperature
• Because of emissivity differences, different
  materials can be at the same temperature, but
  emti at completely different wavelengths.

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Temperature Examples

• Temperature of sun 5700 Kelvin
• ? sun 2900 / 5700 0.51 ?m
• Suns maximum emission - middle of the visible
  spectrum
• Human body temperature 98.6 F 37 C 310 K
  
• ? body 2900 / 310 9.4 ?m
• Human body emits in the thermal infrared region

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Thermal Sensors
(rst)
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Thermal Imagery Uses

• Determining rock type and structure
• Mapping soil type and soil moisture
• Locating irrigation canal leaks
• Determining thermal characteristics of volcanoes
• Studying evaporation from vegetation
• Locating cold-water springs, hot springs, and
  geysers
• Determining the extent and characteristics of
  thermal plumes in lakes and rivers
• Determining extent of active forest fires
• Locating subsurface fires in landfills or coal
  refuse piles.

27
Interpreting Thermal Scanner Imagery

• Darker image tones represent cooler radiant
  temperatures and lighter image tones represent
  warmer radiant temperatures - most commonly used
  representation.
• However, meteorological applications use the
  reverse, so that the light-toned appearance of
  clouds is maintained.

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Interpreting Thermal Scanner Imagery (Cont.)

• Measuring thermal inertia
• Landsat thermal band
• Blackish pattern in Alps represents cooler
  temperatures because of altitude.
• Light tones near bottom are heat from
  Mediterranean Sea.
• Stays warm at night because of thermal capacity.

(rst)
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Interpreting Thermal Scanner Imagery (Cont.)
Contrast between daytime and nighttime thermal
images.
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Interpreting Thermal Scanner Imagery (Cont.)

• Water appears cooler in daytime and warmer at
  night than its surroundings.
• Kinetic temperature has changed little during
  elapsed time between images.
• Surrounding land areas have cooled considerably
  during evening hours.
• Trees generally appear cooler than surroundings
  during daytime hours and warmer at night.
• Tree shadows appear in many places during day
  image but not at night.
• Paved areas appear relatively warm both day and
  night.
• Heats up more during day and loses heat more
  slowly during night.

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Interpreting Thermal Scanner Imagery (Continued)

• Several helicopters parked near hangers
• Thermal shadows left by helicopters not in
  original parked positions.

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Interpreting Thermal Scanner Imagery (Continued)

• Aerial thermal scanning used to study heat loss
  from buildings
• Inadequate or damaged insulation and roof
  material.
• Aerial thermal scanning can be used to estimate
  energy radiated from roofs.
• Emissivitty of roof materials must be known to
  determine kinetic temperature of roof surfaces.

33
Interpreting Thermal Scanner Imagery (Cont.)
34
Microwave Remote Sensing

• Can be passive or active
• Active systems emit pulses of microwave radiation
  to illuminate images of the Earths surfaces
• Images can be acquired day or night
• Wavelengths can penetrate clouds

(Virtual Science Centre)
35
Microwave Bands

• Microwaves are from 1 mm to 1 m wavelength. The
  microwaves are further divided into different
  frequency (wavelength) bands (1 GHz 109 Hz)
• P band 0.3 - 1 GHz (30 - 100 cm)
• L band 1 - 2 GHz (15 - 30 cm)
• S band 2 - 4 GHz (7.5 - 15 cm)
• C band 4 - 8 GHz (3.8 - 7.5 cm)
• X band 8 - 12.5 GHz (2.4 - 3.8 cm)
• Ku band 12.5 - 18 GHz (1.7 - 2.4 cm)
• K band 18 - 26.5 GHz (1.1 - 1.7 cm)
• Ka band 26.5 - 40 GHz (0.75 - 1.1 cm)

(Virtual Science Centre)
36
Microwaves

• Valuable environmental and resource information
  can be acquired in the microwave portion of the
  electromagnetic spectrum, from wavelengths of 1
  mm to 1m
• These wavelengths are about 2,500,000 times
  longer than shortest light waves
• Two distinctive features characterize microwave
  energy from a remote sensing standpoint
• Microwaves penetrate atmosphere under virtually
  all conditions
• Depending on the wavelength -- haze, light rain,
  snow, clouds, and smoke can be penetrated
• Microwave reflections or emissions from Earth
  materials bear no direct relationship to
  counterparts in the visible or thermal portions
  of the spectrum
• Surfaces appearing rough in the visible spectrum
  may appear smooth in the microwave regime
• Microwaves generally give a different view than
  light or thermal spectra

37
Microwaves (Concluded)

• Microwave sensing systems can be active and
  passive
• An active system supplies its source of
  illumination
• The passive system, such as a microwave
  radiometer, responds to the low levels of
  microwave energy that are naturally emitted
  and/or reflected by terrain features
• RADAR is an acronym, and now a proper noun, from
  radio detection and ranging
• Data from radar and passive microwave systems are
  relatively limited compared to photographic or
  scanning systems
• Increasing availability of spaceborne radars may
  allow the microwave database to catch up
• Like RADAR, LIDAR, light detection and ranging,
  use an active source with a sensor
• Lidars use pulses of laser light, rather than
  microwave energy, to illuminate the terrain

38
Range Azimuth Resolution
39
Microwave Resolution
Range Resolution
Azimuth Resolution
(JPL/NASA)
40
Effects of the Atmosphere

• Atmospheric composition causes wavelength
  dependent absorption and scattering
• Atmospheric effects degrade the quality of images
• Some atmospheric effects are correctable before
  an image is analyzed and interpreted

(Virtual Science Centre)
41
Energy Interactions in the Atmosphere Absorption

• Effective loss of energy to atmospheric
  constituents
• Most efficient absorbers water vapor, carbon
  dioxide, and ozone
• Absorption takes place in specific wavelength
  bands
• Concept of atmospheric windows
• Visible range coincides with both an atmospheric
  window and the peak level of energy from the sun
• Emitted heat energy from the Earth is sensed
  through windows at 3 to 5 µm and 8 to 14 µm with
  thermal scanners
• Multispectral scanners sense simultaneously
  through narrow ranges in the visible and thermal
  spectral regions
• Radar and passive microwave operate in the 1mm to
  1m region

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Atmospheric Opaqueness
(The Wave Nature of Light)
43
Atmospheric Transmittance
(Remote Sensing Tutorial)
44
Remote Sensing Images

• Remote sensing images often in the form of
  digital images (pixels)
• Image processing can be used to enhance an image
• Correct
• Restore
• Segmentation and classification used to delineate
  areas into thematic classes

(Virtual Science Centre)
45
Image Processing and Analysis

• Image processing and later analysis is a four
  step process
• Pre-processing
• Image enhancement
• Image classification
• Data storage and use, e.g.
• Geographical Information System (GIS)
• Other storage and usage systems

(Virtual Science Centre)
46
Data Merging and Geographic Information Systems
(GIS) Integration

• Relating information from
• different sources
• Data capture
• Data integration
• Projection and registration
• Data structures
• Data modeling

(rst)
47
Data merging and GIS integration GIS Data
Integration
(rst)
Geographical Information Systems makes it
possible to link, or integrate, information that
is difficult to associate.
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Hyperspectral Image Analysis

• GIS relates information from different sources
• Data capture
• Data integration
• Projection and registration
• Data structures
• Data modeling
• Multisensor image merging often results in a
  composite image product that offers greater
  interpretability
• Can merge multispectral sensor and radar image
  data
• Spectral resolution of multispectral scanner data
• Radiometric and sidelighting characteristics of
  radar data.

49
Hyperspectral Image Analysis
http//satjournal.tcom.ohiou.edu/
50
Supplemental References

• The Wave Nature of Light (Michael Blaber),
  http//wine1.sb.fsu.edu/chm1045/notes/Struct/Wave/
  Struct01.htm
• The Virtual Science Centre Project on Remote
  Sensing, http//www.sci-ctr.edu.sg/ssc/publication
  /remotesense/rms1.htm
• Spectral Reflectance, http//geog.hkbu.edu.hk/GEOG
  3610/Lect-06/sld011.htm
• Everett Infrared and Electro-optic Technology,
  http//www.everettinfrared.com/detectors.htm
• Remote Sensor Tutorials, http//rst.gsfc.nasa.gov
• http//ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/scatterin
  g.rxml?hret/guides/mtr/opt/air/crp.rxml
• Atmospheric Radiation, http//www.public.iastate.e
  du/sege/radiation.html