Transcript Remote Sensing - DePaul Geography

Remote Sensing
January 20, 2006
Geog 258: Maps and GIS
Outlines
• Principles of remote sensing
• Three key aspects of resolution of
remotely sensed image
• Aerial photo vs. satellite image
• Classifying aerial photograph
• Geometric distortion of aerial photograph
• Classifying satellite image
Principles of remote sensing
• What you see is the amount of electromagnetic energy
reflected by object; needs energy source, object, sensor
• Electromagnetic energy can be divided into different
spectral bands (visible light, NIR, microwave) given its
wavelength
– Electromagnetic spectrum
• Electromagnetic energy interacts with object differently
– Snow reflects most of energy (that’s why it looks bright)
– Dry soil absorbs most of energy (that’s why it looks dark)
• How object interacts with electromagnetic energy is a
function of wavelengths  basis of image interpretation
– Spectral reflectance signature
• Electromagnetic radiation interacts with atmosphere as
well. Some radiations (in visible, NIR and microwave
bands) pass through atmosphere relatively well
– Atmospheric window
Electromagnetic spectrum
Spectral signature
Explain why water looks darkish blue; Explain why vegetation looks
greenish; Explain why sand looks reddish yellow
Atmospheric window
•It shows how much electromagnetic radiation transmits atmosphere
•In which bands, is % transmission high?
•Advantage of using those bands?
•Compare aerial photo and satellite image in terms of spectral bands
Aerial photograph
Optical sensor
Infrared sensor
Radar sensor
Different kinds of image
• Pancromatic image
• True-color image
• False-color image
Pancromatic image
• If airborne cameras use black/white film or
satellite sensors use a single band, it
produces pancromatic image (gray scale
image)
Color composite
• Color primaries: RGB (Red, Green, Blue)
• Many colors are formed by combining color primaries in
various proportions
– Look inside your inkjet color printer. Did you really buy 100
different color cartridges?
• Same principles apply to producing color images taken
from airborne cameras or satellite sensors
True-color image
• See Table 9.1 (p. 182)
• How many bands are used for Landsat 4 and 5?
• What’s the spectral range (and corresponding
colors) for band name 3, 2, 1?
• Each band collects different images (See Figure
9.27 at p. 182)
• To produce color-image, bands 321 are
assigned to RGB respectively
• The color of resulting image resembles what
would be observed by human eyes
• That’s why it’s called true-color image
False-color image
• If three bands are arbitrarily (i.e. doesn’t have to be RGB
bands) assigned to RGB layers, it produces false-color image
SPOT XS1 (green band)
Why does vegetation appear
dark in green band in
pancromatic image?
Why does vegetation appear
bright in NIR band in
pancromatic image?
SPOT XS2 (red band)
SPOT XS3 (Near IR band)
R = XS3 (NIR band)
G = XS2 (red band)
B = XS1 (green band)
Why does vegetation
appear red in this color
composite?
Three key aspects of resolutions
• Spatial: pixel (grain) size; minimum distance
which can be recorded
– Aerial photo has of course higher resolution
• Temporal: how often is the image recorded
– For satellite image, it can be regular (satellites are
orbiting the earth in regular time interval)
• Spectral: how many bands are used
– Satellite sensors use different # bands; most of them
less than 10 (e.g. TM uses 7 bands); sensors with #
band > 10 are called hyperspectral (e.g. MODIS)
• Important in determining the fitness of use
Types of sensor systems
• Passive
– Rely on external energy source
– Optical sensor (reliant on solar energy)
• Active
– Sensor itself sends out radiation (like flash camera)
– Radar sensor (reliant on microwave)
• Which one can record the image of earth at night, and
which one can’t?
• Another reason for the popularity of Radar imaging is the
ability of microwave to penetrate atmosphere (e.g.
clouds); see Figure 9.4 (p. 164)
• So what would be the advantage of Radar imaging
compared to optical imaging?
Aerial photography vs. Satellite imaging
(Airborne RS vs. Spaceborne RS)
• Which began earlier? (with the development of what?)
• Which would have a higher spatial resolution? (in which
altitude?)
• Are they analog or digital? (output format)
• Which is more flexible? (can satellite be launched any
time?)
• Which would provide geographic coverage in a
systematic manner?
• Which is equipped with wider spectral bands? (does air
photo has thermal-infrared sensors?)
• How would post-processing be different?
Classifying aerial photo
• Film emulsion
– {black-and-white, true-color, color-infrared}
• Camera’s height
– {high, mid, low-altitude}
• Camera’s vantage point
– {vertical, oblique}
• When color-infrared films were invented,
and for which purpose?
Geometric distortion of air photo
• Aerial photo gives us perspective view (it distorts
geometry of geographic features)
• Transformation from central to parallel
perspective results in planimetrically correct
photo or orthophoto
Sources of geometric distortion
• Camera tilts
Scale is not uniform
Sources of geometric distortion
• Variations in
terrain
Remember photo is flat (2D)
and objects should be seen
from the point of infinite height
in the planimetrically correct
photo.
In the aerial photo,
a’ is displaced outward to a
b’ is displaced inward to b
due to terrain variations
Classifying satellite image
• Spatial resolution
– {high, low}
• Types of energy source
– {passive, active}
• Types of spectral bands
– {pancromatic, multispectral, hyperspectral}
• Special purposes
– {earth-observing, meterological, thermal-infrared}
• From the list of earth observing satellite,
http://www.ersc.wisc.edu/resources/EOSC.php, place
the sensors in classification schemes above
Review questions
• What is the difference between aerial
photo, orthophoto, and DOQs?
• Discuss advantage and disadvantage of
aerial photo vs. satellite imagery
• What is the advantage of using active
sensor systems?
• What is the advantage of using spectral
bands beyond visible wavelength ranges
(0.4-0.7 micron)?