GE3502GE5502 Geographic and Land Information Systems

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GE3502/GE5502Geographic and LandInformation
Systems
Lecture 16 Remote sensing II Principles
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Lecture Plan

• 1. How does remote sensing work (principles of
  EMR)?
• 2. Aerial photography
• 3. Satellite imagery

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Electromagnetic Radiation

• To produce a remotely-sensed image, need to
  measure a parameter that can be related to the
  scene.
• This parameter must be able to reach the sensor
  from some distance away (000s of kms).
• eg. You can read the whiteboard because visible
  light (from the sun or artificial lights) hits
  the board, and is reflected or scattered into
  your eyes.
• White reflects much of the light
• Black reflects little.
• These signals are transmitted by electromagnetic
  waves.

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Electromagnetic Radiation

• Remote sensing devices measure electromagnetic
  radiation (EMR).
• Spectral resolution determines what is seen by a
  sensor and what features of earth environments
  are recorded.
• Electromagnetic (EM) energy is one of a number of
  forms of energy. Others - chemical (convection),
  electrical (conduction) and mechanical (contact).
• EM energy is transferred by radiation and this
  can occur through empty space. Thus an EM energy
  source can be sensed REMOTELY from a distance
  with a SENSOR.
• The main source of EMR is solar energy (ITR).
  Smaller amounts of EMR are emitted by the earth
  (OTR).

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EMR (cont.)

• EM radiation travels through vacuum at (c) speed
  of light ( 300,000 km/s)
• Waves consist of electric and magnetic fields.
• Always at right angles to each other
• Always at right angles to direction of wave
  travel.
• Wavelength (?) distance between two successive
  peaks (µm).
• Frequency (f) number of peaks that pass a point
  in a second (Hz).
• Amplitude maximum value of the fields. A
  measure of the energy.

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Electromagnetic Radiation (cont.)

• WAVELENGTH is usually measured in microns (?m)
• 1 x 10-6 m 0.000001 m
•  
• Visible light 0.40 - 0.70 ?m blue green -
  red
• Different colours in visible light area of
  spectrum have differing wavelengths e.g. orange
  0.55 - 0.62 ?m

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Electromagnetic Radiation (cont.)

• FREQUENCY is number of cycles (or wave-crests)
  passing a fixed point within a given period of
  time. It is usually measured in Hertz (Hz) ( no.
  of cycles/second

• FREQUENCY is inversely related to WAVELENGTH
  thus
• long wavelength low frequency
• short wavelength high frequency

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Electromagnetic Radiation (cont.)

• Although all EMR travels at the speed of light
  the wavelengths vary.
• High energy forms of EMR eg. X-rays have short
  wavelength and high frequency
• Low energy forms like radio waves have long
  wavelength and low frequency.

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The Electromagnetic Spectrum

• EMR is usually referred to by wavelength and is
  scaled on an EM spectrum. Solar radiation
  comprises the greatest part of the terrestrial EM
  spectrum used in remote sensing.
• The spectrum extends from radio and TV
  frequencies, which are low frequency (1012 - 104
  Hz) and 1mm to 1 km in wavelength, to cosmic rays
  (1022Hz ).
• Such a wide range means frequency and wavelengths
  are plotted on a log scale. 

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EM MODIFICATION

• Physical objects modify electromagnetic energy by
  REFLECTANCE (R), TRANSMISSION (T), ABSORPTION
  (A), or EMITTANCE (E).
• The energy distribution depends on the physical
  (shape, roughness) and chemical structure (what
  it is made of) the object emitting energy.
• BLACK BODY RADIATION THEORY is used to understand
  how EM energy is distributed across the spectrum
  and how differing kinds of physical materials
  interact with EM energy i.e. the relative amounts
  of EM energy () that are R, T, A or E . This
  understanding is crucial to the recognition of
  terrestrial and aquatic features in remote
  sensing.

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BLACK BODY RADIATION

•  A BLACKBODY is a perfect radiator and absorber
  of energy. It is an abstract concept i.e. it does
  not really exist in nature. A blackbody absorbs
  ALL incident radiant energy (and re - emits it at
  longer wavelength as heat). The opposite is a
  whitebody.
•  
• A WHITEBODY is a perfect reflector. None of the
  radiant energy is absorbed at all.
•  
• Most objects are GREYBODIES - some of the
  incident energy is absorbed some is reflected
  and some is transmitted and emitted - the
  relative balance depends on chemical, electrical
  and physical properties of an object.

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e.g. Spectral Reflectance Characteristics of
Vegetation

• Spectral reflectance in vegetation is controlled
  by
• Cell structure of leaf
• Surface properties of the leaf and of the
  vegetation canopy grass fronds bush or tree 
• Vegetation pigmentation
• - esp. chlorophyll content and Water Content
• These have marked effect on spectral properties
  of vegetation in visible - NIR spectrum.

•  

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Spectral Reflectance Curve for Vegetation

• Generally vegetation has high IR reflectance -
  bright in TM Band 4 low visible reflectance
  particularly in TM Band 3 (red) and TM Band 1
  (blue).
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• NIR reflectance can be species dependant, thus
  species of vegetation can be identified from NIR
  BVs.
• Biomass productivity
• Crop estimates can be made and
• Stress detected, i.e. of moisture shortage,
  toxicity, and effect of pollutants affect NIR and
  visible reflectance levels.

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Vegetation reflectance
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Plant disease reflectance
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Spectral resolution of Landsat TM imagery

• Band 1 is for bathymetry Bands 2 and 3 are for
  vegetation discrimination bands 2 and 4 are for
  measuring plant vigour bands 5 and 7 are for
  measuring water stress in plants, and for
  discrimination of rock types. All are 30 m
  spatial resolution.
•  
• Band 6 is for thermal mapping (120 m). There is
  also a Band 8 on LANDSAT 7 which takes black and
  white (panchromatic) imagery with a pixel size of
  15 m.

Thematic Mapper Bands (?m) 1 0.45 -
0.52 (blue) 2 0.52 - 0.60 (green) 3 0.63 - 0.69
(red) 4 0.76 - 0.90 (NIR) 5 1.55 - 1.75 (MIR) 6
10.4 - 12.5 (TIR) 7 2.08 - 2.35 (MIR)
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Seeing Heat
Coincident images of the Brazilian rain forest at
.657 µm (red) and 4.05 µm (thermal infra-red),
respectively. The visible light image shows
only smoke, while the thermal infra-red image
shows details of the fire beneath it.
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Recording EMR

• Low radiant energy levels (Absorbed) are given
  low BVs and high radiant energy levels are given
  high BVs (Reflected).
• The BV is usually recorded as a digital number
  (DN a positive integer) for display and
  analysis in a computer.

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Scanned Data Format

• Brightness values are recorded in a grid, or
  raster, format. This is a matrix of cells or
  pixels organised into an array of rows (lines)
  and columns (samples).

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Unprocessed TM image
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Atmospheric Absorption
Only radiation within certain wavelengths can
pass through the atmosphere to reach the Earth.
Different gases absorb different wavelengths.
Nitrogen Little absorbtion O2 ultraviolet and
at 6.3um CO2 number of wavelengths Water vapour
V important O3 ultraviolet Therefore,
certain wavelengths where most or all EMR gets to
the surface are called atmospheric windows.
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2. Aerial Photography

• Aerial photography has long been a primary source
  of base map data for many common products
• Two major issues
• The ability to obtain required classifications
  from the photography requires the use of an
  interpretation key
• Rectification (scale, relief and tilt
  distortions) and lack of reference grid the
  photo must be georeferenced to a coordinate
  system, which requires the use of ground control
  points (GCPs)

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Distortions on an aerial photograph
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AP of Townsville at 125000
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Why choose aerial photography

• Examine large areas
• Measure things that are difficult to examine on
  the ground
• Measure features that are continuous (change
  gradually across space)
• Examine how a place changes over time
• Finer resolution than satellite imagery (usually)
• You can control when and where images are taken,
  i.e. you are not limited to the position of a
  satellite in orbit
• You can coordinate a ground survey to take place
  at the same time

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Things to consider

• Can be expensive to get all the photos you need
• Can require lots of room to store and use on your
  computer
• Requires processing before you can use in GIS
  can be time consuming
• Includes distortions due to the camera lens, and
  the angle and height of the aircraft

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Sources of aerial photography

• Geoscience Australia
• GBRMPA Great Barrier Reef Marine Park Authority
• AIMS Australian Institute of Marine Science
• TESAG Cartography (Adella Edwards)
• Private companies

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3. Satellite Imagery
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Why use satellite imagery?

• Examine large areas of the Earth at once
• Measure things that are dangerous and difficult
  to examine from the ground
• Measure features that are continuous (change
  gradually across space)
• Examine how a place changes over time

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Active vs. Passive Sensors
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Things to consider

• Cost satellite imagery can be very expensive
• Can require lots of room to store and use on your
  computer
• Requires processing before you can use it in a
  GIS this can be very time consuming
• Spatial and temporal resolution will be limited
  to availability of satellites

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Where to get satellite imagery from

• ACRES Australian Centre for Remote Sensing
• BOM - Australian Bureau of Meteorology
• NOAA National Oceanographic and Atmospheric
  Association
• NASA - National Aeronautics and Space
  Administration
• JCU Dept of Electrical and Computer Engineering
• TESAG Cartography (Adella Edwards)
• Note Always check a few different sources when
  chasing data it is often possible to enter data
  share agreements where agencies provide data at a
  greatly reduced cost.

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Points to remember

• Remote sensing data are not gathered directly
• Electromagnetic signals are received as
  surrogates of what is actually on the ground
• The raw data must be processed by experienced
  interpretation specialist before object
  categories can be properly identified

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Problems stemming from RS data

• The quantisation of space into pixels imposes a
  level of abstraction and simplification on the
  Earths features.
• Objects that are much smaller than the pixels
  cannot be directly identified
• Satellite remotely sensed raw data provides
  little information until it has been analysed
• Two major types of data processing
• Image enhancement
• Image classification

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More problems with RS data

• Different RS data sources may prove to be
  incompatible
• Problems
• Categories may not relate well to each other
• Data may be collected at different scales
• The above are particularly a problem if RS data
  are being used to update historical maps
• Changing atmospheric conditions
• Cloud cover
• Haze
• Sensors age may result in sensor drift

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Getting the data into a GIS

• Data needs to be in a digital format
• This is already the case with satellite imagery,
  however aerial photos will need to be scanned
• The data needs to be in the correct format for
  the particular GIS software you are using
• Most image processing software has tools to
  export and convert data between different formats
• If the image is rectified you will need to know
  the coordinate system, projection and datum
• This is essential for when you are comparing
  different data sets of the same area

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Summary

• Remote sensing is useful for gathering
  information over large areas
• Remotely sensed data needs to be used with
  caution and requires validation by field measures
• Remote sensing is still a rapidly maturing field
  of investigation with much scope for future
  research