Chapter three remote sensing basics
the process
remote sensing systems work by analyzing reflected energy or
energy emitted from the object
sources of energy
the sun
the object itself
the remote sensing system itself
the energy must pass through the atmosphere
atmospheric effects differ depending on wavelength and atmospheric
conditions
once the energy reaches the object the can be:
reflected
transmitted
absorbed
objects emit absorbed energy as heat
the reflected or emitted energy then has to pass back through
the atmosphere to the sensor
the sensor detects the energy using
film
electronic detectors (scanners, digital cameras)
antennae (radar)
the data can then be processed for display or analysis
photographic images or image mosaics or image maps
analysis techniques include:
visual interpretation
digital enhancement
automatic classification
analysis often requires ancillary data
measurements of certain ground locations
existing information about field conditions
geologic maps
soil maps
statistical summaries
all this is commonly stored within a geographic information
system
remote sensing analysis systems provide refined tools
many GIS systems offer basic image processing capabilities
results of interpretation analysis used to make information
products for the user
maps
tabular summaries
images
multimedia presentations
web sites
information must then be placed in the hands of individuals who
can make decisions
energy sources
information that can be extracted, depends on the type of energy
that the remote sensing system detects
principal sources, electromagnetic energy (EM)
gamma rays
< 0.03 nm
x-rays
0.03-240 nm
ultraviolet
0.24-0.38 μm
visible light
0.38-0.7 μm
near infrared
0.7-1 μm
short infrared
1-3 μm
medium infrared 8-14 μm
long infrared
14-1000 μm
microwave
1.0 mm – 100 cm
radio
> 100 cm
EM behaves in a wave like fashion
has electric and magnetic field components
wavelength (λ) is peak to peak distance
frequency (f) is the number of peaks that pass a fixed point per unit
time
wavelength and frequency can be related by
c=fλ
c is the speed of light (3 x108 m/s)
A photon is said to be quantized,
any given photon possesses a certain quantity of energy
Some other photon can have a different energy value.
Photons as quanta thus show a wide range of discrete energies.
The amount of energy characterizing a photon is determined
using Planck's general equation:
E = hν
h is Planck's constant (6.6260... x 10-34 Joules-sec)
ν is the Greek letter, nu, representing frequency
(the letter "f" is sometimes used instead of v).
shorter wavelengths have higher energy
high energy generally means greater penetrating ability
many instruments detect energy in the ultraviolet, visible,
infrared, and microwave, portions of the spectrum
0.30 μm to 30 cm
only wavelengths between 0.30 μm and 15.0 μm can be reflected in
focused by mirrors and lenses
the longer wavelengths in the microwave region are detected using
antennae
different wavelengths, tell us different things about the target
Crab nebula in 4 wavelength bands
X-ray
Visible
Infrared
Radio
not all remote sensing systems depend on EM energy
acoustic energy has better penetration of the earth and water
acoustic energy is defined as the variation in pressure produced by
the vibration of the object within its medium
we hear sounds because of variations in air pressure
sonar bounces sound off objects and water
seismic surveys record the reflections of sound from rock strata within
the earth's crust
electromagnetic energy in the atmosphere
the atmospheric can transmit, absorb or scatter EM energy
the blocking characteristics of the atmosphere protect living
things from damaging high energy radiation
ultraviolet light
these blocking characteristics can also cause problems for
remote sensing systems
of most concern is absorption and scattering
these reduced the amount of energy received by the instrument
the longer the path from the target to the sensor, the greater the
potential for atmospheric effects to degrade the data
atmospheric scattering
redirection of EM energy by particles suspended in the atmosphere
dust and smoke or large molecules like water vapor
for scattering means more energy is redirected away from the path of
travel
the amount of scattering depends on:
the size an abundance of these particles.
The wavelength of the radiation
the distance the energy must travel through the atmosphere to reach the
sensor
scattering causes discarded look blue in the daytime
the shorter blue wavelengths of sunlight are scattered more than longer
green and red wavelengths
we can see detail in general areas because it is illuminated by diffuse
blue light
on the moon or in space the sky looks black, because there's no
atmosphere to scatter the solar radiation
sky is red pink or orange at sunset in sunrise, because the light
travels through a longer path
only the longer orange and red wavelengths can penetrate the
atmosphere without significant scattering
atmospheric scattering tends to mask variations in brightness of
what we are trying to image
scattering not only reduces energy from the target but also directs
energy from outside the sensor's field into the field of view
this reduces image contrast
some scattered light may be excluded by use of filters
atmospheric absorption
absorption occurs when energy is lost to constituents of the
atmosphere
absorbed energy is reradiated at longer wavelengths
if the reradiation occurs in the infrared we since it as heat
three gases account for most of the absorption
water vapor
carbon dioxide
ozone
water vapor is three times the strongest carbon dioxide as an
absorber
a complicating factor is that water vapor tends to vary in space and
time
ranges from trace to 4%
http://www1.cira.colostate.edu/Climate/webloop/nvap_webloop.html
clouds cover about 40% of the earth’s surface
moisture effects are less important over deserts than humid areas
atmospheric windows
the transparency of the atmosphere is wavelength dependent
some wavelengths act like windows, others like doors
atmospheric windows are the wavelengths that are used by remote
sensing systems
the most important windows are
ultraviolet to the near infrared (0.3-1.2 μm)
the mid-infrared bands (3-5 μm)
microwave band (8-14 μm)
narrow windows exist in the microwave region between 1 mm in 1 cm
there's virtually no atmospheric effects of the longer microwave
region
passive and active radar use this region
electromagnetic energy and earth objects
EM radiation interacts with features on the surface much as it
does in the atmosphere
Absorbed, reflected, and transmitted
absorbed energy raises the temperature the leaf and is reemitted as
heat
reflectance and absorbance characteristics give the object its color
The way an object reflects determines the characteristics of the
energy detected by the remote sensing instruments
reflection determined by surface roughness of the object, relative to
the wavelength and incidence angle
specular reflection occurs when energy strikes a smooth surface
wavelength is longer than the surface height variation or particle size of
the surface
energy is redirected away from the object in a single direction
diffuse reflection is when a surface is relatively rough
wavelength is shorter than the surface height variation
energy is redirected uniformly in all directions
fine-grained sand would appear smooth when illuminated by
microwave energy used in radar
because it has wavelength of several centimeters
fine-grained sand would appear rough when illuminated by visible
light
it has wavelengths of 400-700 nm
most things are not perfectly diffuse or perfectly specular
diffuse reflection is useful for remote sensing, because wavelengths
absorbed or transmitted by the object will be reduced in intensity
specular reflection provides little information about the reflecting object
all the energy just bounces back out
emission of electromagnetic radiation
we may also look at energy emitted by or features
all objects above absolute zero continuously admit electromagnetic
radiation
broad range of intensities and wavelengths
warmer objects emit more energy, and at shorter wavelengths than
cooler ones
quantity of energy emitted by an object depends on
temperature
emissivity
a measure of the object’s efficiency in radiating energy
from 0-1, one is a perfect emitter and absorber of EM energy
also called a blackbody
the emissivity of most objects has not been quantified
water is an exception 0.98
most often the only featuring the scene with known emissivity
this is the reason we can measure see surface temperature from space within
less than a degree centigrade
infrared –more than your eyes can see
http://www.jpl.nasa.gov/multimedia/
spectral properties of objects
Objects selectively absorb and reflect EM energy
Due differences of molecular composition at their surface
Sunlight has almost equal energy in all visible wavelengths
Vegetation looks green because of the portion of the spectrum that is
reflected
Tomatoes are red because they reflect those wavelengths
preferentially
Colors
Blue 0.4-0.5 μm
Green 0.5-0.6 μm
Red 0.6-0.7 μm
Spectral response pattern is called the spectral signature
Can be described for a particular material
we can use this to define things from remote sensing
while some things have distinctive signatures – water and vegetation
others are very similar in the visible range – pinewoods and
grassland
they are different in the infrared
such differences in the infrared is why often imagery is shown as IR
ideally we want all objects to have different spectral signatures
this is often not the case
a wheat field may have different appearance in different parts of the field
plant age, health, amount of sun received by individualleaves, soil moisture
combination of analysis methods used to distinguish features that are
confused
multi-temporal images can distinguish early vs late season plants
ancillary elevation data on mountainous terrain can separated species that grow
at high and low altitude
different kinds of remote sensing might be applied
use of Lidar and MSS
can get more spectral bands – hyperspectral
data volume grows very large
Commonly used spectral bands
Green, red, near-IR, mid-IR
Sometimes also use blue and thermal-IR
RS in the UV
Most UV is absorbed or scattered in the atmosphere
Some material fluoresce (glow) when UV shines on them.
UV-laser fluorosensors are especially good for oil spill detection
RS in the visible – what we see
We are most familiar with this range of light
It also has the most solar energy of the EM spectrum
Blue has the most water penetrating power
But it is most subject to atmospherics scattering and absorption
Used for water depth and detection of subsurface features
Also used for soil and vegetation discrimination, forest type, geology, cultural
feaures
Green
Used for vegetation discrimination and vigor
Also cultural feautures and urban land
Only moderate water penetration
Less affected by atmospheric scattering
These last two make it useful for measuring suspended sediment and chlorophyll in water
A proxy for nutrient load
Red
The chlorophyll absorption band for healthy plants
Good for discriminating types of vegetation, assessing plant condition
Good for soil and geologic boundaries
Identifying cultural features
Least affected by the atmosphere.
Good contrast
Not good for water penetration, but OK near surface
Panchromatic (B&W)
Covers 0.5-0.9 μm broad band of wavelengths
Generally collected at high spatial resolution
Using image fusion can combine with other lower resolution bands to get higher
resulting image