Transcript Projections and coordinate systems

Remote Sensing
Waves transport energy
Remote Sensing
• According to quantum theory, light may be considered
not only as an electro-magnetic wave but also as a
"stream" of particles called photons which travel at
speed c, the vacuum speed of light.
• Each particle carries one quantum of energy
Q = hf
where:
h is Planck's constant
f is the frequency of the light
• Short wavelenghts  high energy
• Long wavelenghts  low energy
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4μm
7μm
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• Any object above 0K (-273°C) emits electromagnetic energy
• How much is given by the law of Stefan-Boltzmann
M = ε σ T4
where:
σ is a constant
ε is the emissivity coefficient
• For a blackbody, ε is = 1.
A blackbody is an idealized physical body that absorbs all incident
electromagnetic radiation, regardless of frequency or angle of
incidence.
A blackbody in thermal equilibrium (that is, at a constant
temperature) emits electromagnetic radiation called black body
radiation.
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The wavelength at which a black body radiation curve
reaches a maximum is given by the Wien’s wavelength
displacement law
λmax = A / T
where A = ~2898 μm K
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Scattering occurs when particles or large gas molecules
present in the atmosphere interact with and cause the
electromagnetic radiation to be redirected from its original path.
3 types of scattering:
Rayleigh scattering
occurs when particles are very small compared to the wavelength of the
radiation. Rayleigh scattering causes shorter wavelengths of energy to be
scattered much more than longer wavelengths
 sky is blue
Mie scattering
occurs when the particles are just about the same size as the wavelength
of the radiation. Water vapour is a common causes of Mie scattering
 clouds are white
Non selective scattering
this occurs when the particles are much larger than the wavelength of the
radiation. Water droplets and large dust particles can cause this type of
scattering
 haze and smog
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Absorption causes molecules in the atmosphere to absorb
energy at various wavelengths.
Ozone
Absorbs the harmful (to most living things) ultraviolet radiation from the sun. Without this
protective layer in the atmosphere our skin would burn when exposed to sunlight.
Carbon dioxide
Absorbs radiation strongly in the far infrared portion of the spectrum - the area associated with
thermal heating - which serves to trap this heat inside the atmosphere.
Water vapour
Absorbs much of the incoming longwave infrared and shortwave microwave radiation.
The presence of water vapour in the lower atmosphere varies greatly from location to location
and at different times of the year
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Et + Er + Es + Ea = Einc
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Radiance vs. Reflectance
•
Radiance is the variable directly measured by remote sensing instruments.
Radiance most often has units of watt/(steradian/square meter).
•
Reflectance is the ratio of the amount of light leaving a target to the amount
of light striking the target. It has no units.
•
Reflectance is a property of the material being observed.
•
Radiance depends on the illumination (both its intensity and direction), the
orientation and position of the target and the path of the light through the
atmophere.
•
With effort, many of the atmospheric effects and the solar illumination can
be compensated for in remote sensing data. This yields something which is
called "apparent reflectance" and it differs from true reflectance in that
shadows and directional effects on reflectance have not been dealt with.
•
For many applications, radiance and apparent reflectance can be used
interchangibly. However, since reflectance is a property of the target
material itself, you will get the most reliable results using reflectance.
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specular
diffuse
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clean water
turbid water
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Resolutions
• Spectral: refers to the width of spectral bands.
• Radiometric: sensitivity to differences in the
power of the incoming radiation.
• Geometric: minimum distance between objects
perceived as separated (distinguishable detail)
• Temporal: period of time between two
acquisition over the same area.
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8 bits
3 bits
4 bits
2 bits
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10 m
40 m
20 m
80 m
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•
•
LiDAR stands for Light Detection and Ranging, commonly known as Laser
Radar
Each time the laser is pulsed:
– Laser generates an optical pulse
– Pulse is reflected off an object and returns to the system receiver
– High-speed counter measures the time of flight from the start pulse to
the return pulse
– Time measurement is converted to a distance (the distance to the
target and the position of the airplane is then used to determine the
elevation and location)
– Multiple returns can be measured for each pulse
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• High spatial resolution
– Laser footprint on ground ≤ 0.50
meters
– Typical density is 0.5 to 20+
pulses/m2
– 2 to 3 returns/pulse in forest areas
• Large volume of data
– 5,000 to 60,000+ pulses/hectare
– 10,000 to100,000+ returns/hectare
– 0.4 to 5.4+ MB/hectare
• Raw return data is a cloud of XYZ points
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Streams
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Natural
colours
3 0.65µm VIS
2 0.55µm VIS
1 0.48µm VIS
3 0.85µm NIR
3
R
2
G
1
B
2 0.65µm VIS
1 0.55µm VIS
False
colours
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Water
Forest
Urban area
Fields
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Enhancement
• Whenever satellite images are
visualized using the direct
correspondence between the digital
values of pixels and the intensity levels
of primary colors on screen, they appear
to be dark and poorly contrasted.
• Indeed, generally the digital values
associated to the pixels do not cover the
entire range of useable digital values.
The sensibility of the sensor is tuned to
avoid out-of-scale measurements, even
when scanning over lowly or highly
reflective surfaces.
• A typical image histogram shows a
peak of slightly scattered, low values.
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Enhancement
• Contrast stretching is an expansion of the
intensity levels used in visualizing the
image to the entire range of usable
intensity levels, by means of a linear or
non-linear transformation.
• This is obtained using the so called
Lookup Tables (LUT), an array of pairs of
values that map the input values to the
output levels of the display.
• The images very often contain a few very
high values (clouds, small-highly-reflective
surfaces, sensor errors), as well as few
very dark values (shadows, non-reflective
surfaces, sensor errors), thus generally the
“tails” of the histogram that may contain
anomalous values are not taken into
account when building the stretch LUT.
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