Ch-1-Introduction to Remote Sensing_7.pdf

Ch-1- Introduction to Remote
Sensing
Inst. Biruk G. (MSc)
March 2020

Introduction
• Earth Observation is the gathering of information about
planet Earth’s physical, chemical and biological systems via
remote sensing technologies supplemented by earth
surveying techniques, encompassing the collection, analysis
and presentation of data.

• Natural and human-induced disasters;
• Environmental factors affecting human health and well-being;
• Improving the management of energy resources;
• Understanding, assessing, predicting, mitigating, and adapting to
climate variability and change;
Introduction
The nine societal benefit areas of RS

Introduction
• Improving water resource management through better
understanding of the water cycle;
• Improving weather information, forecasting and warning;
• Improving the management and protection of terrestrial,
coastal and marine ecosystems;
• Supporting sustainable agriculture and combating
desertification;
• Understanding, monitoring and conserving biodiversity.

Remote Sensing:
• Art, science and technology of observing an object, scene or
phenomenon by instrument-based techniques without
physical contact
• Remote sensing is the observation of an object from a
distance.
• This is done by sensing and recording reflected or emitted
energy and processing, analyzing, and applying that
information.
• Examples are Aerial Photography and the use of satellites to
observe the Earth.
Introduction

Introduction
Key Words While defining RS
• Information is collected by a device (sensor) that is not in
contact with the objects being measured.
• Information transfer is accomplished by use of electromagnetic
radiation (EMR).
• Concerned not only with data collection but also extracting
information via different techniques.
Q#1
• Which humans sense organ is remote sensor and which is not?

Introduction
Why Earth Observation (EO) by RS
• Synoptic view- observations that give a broad view of a
subject at a particular time.
• Large areas, dense data
• Synergy with in situ measurements
• High repeatability (temporal resolution)
• Global, inaccessible/secure areas
• Multi purpose
• Cost effective (can be)
• Data collection without disturbance

Introduction: History of RS
• 1826 The invention of photography
• 1850’s Photography from balloons
• 1873 Theory of electromagnetic energy by J. C. Maxwell
• 1909 Photography from airplanes
• 1910’s World War I: aerial reconnaissance
• 1920’s Development and applications of aerial
photography and photogrammetry
• 1930’s Development of radar in Germany, USA, and UK
• 1940’s World War II: application of Infrared and microwave
regions
• 1950’s Military Research and Development

Introduction: History of RS
• 1960’s The satellite era: Space race between USA and USSR.
• 1960 The first meteorological satellite (TIROS-1)
• 1960’s First use of term “remote sensing”
• 1960’s Skylab remote sensing observations from the space
• 1972 Launch of the first earth resource satellite (Landsat-1)
• 1970’s Rapid advances in digital image processing
• 1980’s Landsat-4: new generation of Landsat sensors
• 1986 Launch of French earth observation satellite (SPOT-1)
• 1980’s Development of hyperspectral sensors
• 1990’s Launch of earth resource satellites by national space
agencies and commercial companies

Major Components Remote Sensing Technology
• Energy Source
– Passive System: sun, irradiance from earth's materials;
– Active System: irradiance from artificially generated energy
sources such as radar.
• Platforms: (Vehicle to carry the sensor) (truck, aircraft, space
shuttle, satellite, etc.)
• Sensors: (Device to detect electro-magnetic radiation) (camera,
scanner, etc.)
• Detectors: (Handling signal data) (photographic, digital, etc.)
• Processing: (Handling Signal data) (photographic, digital etc.)
• Institutionalization: (Organization for execution at all stages of
remote-sensing technology: international and national
organizations, centers, universities, etc.).

Remote Sensing Process
Energy Source or Illumination (A)
–the first requirement for remote
sensing is to have an energy
source which illuminates or
provides electromagnetic energy
to the target of interest.
Radiation and the Atmosphere (B)
– as the energy travels from its
source to the target, it will come in
contact with and interact with the
atmosphere it passes through.
This interaction may take place a
second time as the energy travels
from the target to the sensor.

Interaction with the Target (C) -
once the energy makes its way to
the target through the
atmosphere, it interacts with the
target depending on the
properties of both the target and
the radiation.
Recording of Energy by the Sensor
(D) - after the energy has been
scattered by, or emitted from the
target, we require a sensor
(remote - not in contact with the
target) to collect and record the
electromagnetic radiation

Reception, and Processing (E) - the
energy recorded by the sensor has to
be transmitted, often in electronic
form, to a receiving and processing
station where the data are processed
into an image (hardcopy and/or
digital).
Interpretation and Analysis (F) - the
processed image is interpreted,
visually and/or digitally or
electronically, to extract information
about the target which was
illuminated.

Application (G) - the final element of the
remote sensing process is achieved
when we apply the information we have
been able to extract from the imagery
about the target in order to better
understand it, reveal some new
information, or assist in solving a
particular problem.

Electromagnetic radiation
• EMR is a form of energy that reveals its presence by the
observable effects it produces when it strikes the matter.
• The sun is the most obvious source of EMR for remote sensing,
• Light is Electromagnetic (EM) radiation
• Electromagnetic energy can be discussed in terms of its energy
distribution, or the spread of energy over a range of
wavelengths.

• This distribution of energy is also known as the spectral
distribution.
• EM-Spectrum is the entire range of wavelengths of
electromagnetic radiation.
• The electromagnetic spectrum extends from below the low
frequencies used for modern radio communication to gamma
radiation at the short-wavelength (high-frequency)
• Most sensors operate in the visible, infrared, and microwave
regions of the spectrum.

Solar Spectrum
• The range of electromagnetic energy emitted by the sun is
known as the solar spectrum, and lies mainly in three regions:
– ultraviolet,
– visible, and
– infrared.
• The solar spectrum extends from about 0.29 μm (or 290 nm)
in the longer wavelengths of the ultraviolet region, to over 3.2
μm (3,200 nm) in the far infrared.

Ultraviolet:
• Has short wavelengths (0.3 to 0.4 μm) and high frequency.
• Used in geologic and atmospheric science applications (tracking
changes in the ozone layer).
Visible Light
• Radiation detected by human eyes.
• The only portion of the spectrum that can be perceived as colors.
• Ranges from approximately 0.4 to 0.7μm.
• Applicable in manmade and natural feature identification and
study.

Infrared:
• Ranges from approximately 0.7 to 1000 μm.
• It is 100 times as wide as the visible portion.
• Based on their radiation properties, IR can be the reflected IR
(0.7 to 3.0 μm) and the emitted or thermal IR (3.0 to 1000 μm)
Reflected Infrared
• Shares radiation properties exhibited by the visible portion.
• Valuable for delineating healthy verses unhealthy or fallow
vegetation, and
• For distinguishing among vegetation, soil, and rocks.

Thermal Infrared
• Radiation that is emitted from the Earth’s surface in the form of
thermal energy.
• Useful for monitoring temperature variations in land, water and
ice
Microwave
• Ranging on the spectrum from 1000μm to 1 m.
• The longest wavelength used for remote sensing.
• Used in the studies of meteorology, hydrology, oceans, geology,
agriculture, forest and ice and for topographic mapping.
• Microwave emission is influenced by moisture content, it is
useful for mapping soil moisture, sea ice, currents, and surface
winds.

Examples:
Different molecules absorb different regions of electromagnetic
energy preferentially.
Water molecule preferentially absorbs certain wavelengths in the
microwave region of the electromagnetic spectrum. This
preference is the basis of the efficient cooking of food by
microwave ovens.
Calcium, a primary constituent of bones, absorbs energy in the x-
ray region more strongly than do the water or carbon in the cells of
ordinary tissue, allowing for the use of x-rays to generate images
that show unevenness such as broken bones or tumors.
The chlorophyll molecule in green plants absorbs mostly
ultraviolet (and also some blue violet, and red light) and uses this
energy for photosynthesis. Most of the green light in sunlight is
reflected by leaves, making them appear green to our eyes.

Interaction
• The interaction could be surface and/or volume phenomena.
• The surface and volume interactions with matter can produce a
number of changes in the incident EMR:
– magnitude,
– direction,
– wavelength,
– polarization and phase.
• RS technology detects and records these changes.
• The resulting images and data are interpreted to identify
remotely the characteristics of the matter that produced the
changes in the recorded EMR.

A. EM Interaction with Atmosphere
• Absorption, transmission, and scattering.
• The transmitted radiation is then either absorbed by the surface
material or reflected.
• The reflected radiation is also subject to scattering and
absorption in the atmosphere before reaching a remote sensor
• Gases mainly absorb EM energy
– known concentrations and location enable to predict influence
• Aerosols mainly scatter EM energy
– variable and difficult to model (human and natural changing influence)
• Either way sensor sense less than what reached the Earth’s
atmosphere

A. EM Interaction with Atmosphere…contd

Absorption
• As it moves through the atmosphere, EM radiation is partly
absorbed by various molecules. The most efficient absorbers of
solar radiation in the atmosphere are ozone (O3), water vapour
(H2O) and carbon dioxide (CO2).
• From below fig. it can be seen that many of the wavelengths
are not useful for remote sensing of the Earth’s surface, simply
because the corresponding radiation cannot penetrate the
atmosphere. Only the wavelengths outside the main
atmospheric transmission absorption ranges of the
atmospheric gases can be used for remote sensing ES.

• Absorption-Global Warming
• Most of the atmosphere consists of nitrogen (78%) and oxygen
(21%), other gases account for the remaining 1%.
• Water vapor (can vary highly in concentration), carbon dioxide
(0.035%), methane, and nitrous oxide (N2O) are called greenhouse
gases.
• These gases (mainly water vapor and carbon dioxide) have the
ability to absorb long wave radiation emitted by the Earth and
reemit it from much colder levels to the outer space. They are
able to store heat in the atmosphere and keep the Earth warm. It
looks like the Earth is surrounded by a thermal blanket. This
phenomenon is called the greenhouse effect.

• What is negative is the enhanced greenhouse effect. As
Temperature of atm > Temperature earth, it leads to the
increase in the Earth temperature. The more the concentration
of greenhouse gases, the higher the temperature of the
atmosphere.
• Atmospheric Window-Good for Sensing Ground Surface
• The EM-Spectrum for which the atmosphere is relatively
transparent.
• The atmosphere selectively transmits energy of certain
wavelengths.

Scattering (Many Direction reflection)
• Atmospheric scattering occurs when particles or gaseous
molecules present in the atmosphere cause EM radiation to be
redirected from its original path.
• The amount of scattering depends on several factors
– The wavelength of the radiation
– The size and amount of particles and
– The distance the radiation travels through the atmosphere.
• Three types of scattering occurs in the atmosphere depending on
the size of particles in the atmosphere causing it. They are of
different relevance to RS.
• Rayleigh Scattering, Mie scattering and Non-selective scattering

• Rayleigh scattering dominates where electromagnetic radiation
interacts with particles that are smaller than the wavelengths of
light. Eg. of such particles are tiny Rayleigh scattering specks of
dust, and nitrogen (NO2) and oxygen (O2) molecules. Light of
shorter wavelength(e.g. blue) is scattered more than light of a
longer wavelength (e.g. red)

Mie scattering occurs when the wavelength
of EM radiation is similar in size to particles
in the atmosphere. The most important
cause of Mie scattering is aerosols: a mixture
of gases, water vapour and dust.
• Mie scattering is generally restricted to
the lower atmosphere, where larger
particles are more abundant, and it
dominates under overcast, cloudy
conditions.
• Mie scattering influences the spectral
range from the near-UV up to mid-IR, and
has a greater effect on radiation of longer
wavelengths than Rayleigh scattering.

Non-selective scattering is
independent of the wavelength within
the optical range. The most prominent
example of non-selective scattering is
that we see clouds as white bodies. A
cloud consists of water droplets; since
they scatter light of every wavelength
equally, a cloud appears white.
Moreover, clouds have a further
limiting effect on optical RS
Non-selective scattering occurs when
particle sizes are much larger than the
radiation wavelength. Typical particles
responsible for this effect are water
droplets and larger dust particles.

B. EM Interaction with surface (Earth)

B. EM Interaction with surface…contd
• In “land and water applications” of remote sensing, we are
most interested in reflected solar radiation because this tells
us much about surface characteristics.
• Reflection occurs when radiation bounces off the target and is
thus redirected.
• A target is defied as an object on the Earth surface, being
detected or sensed.
• Not all solar radiation is reflected by water and land surfaces;
some of it may be absorbed and some even transmitted
(through a water body, for example, the sea floor).
• The proportion of reflected-absorbed-transmitted radiation
will vary with wavelength and material type

• Two types of reflection by a target: “specular reflection” and
“diffuse reflection”.
• In the real world, usually a combination of both types is found.

• Various materials of the earth’s surface have different spectral
reflectance characteristics (used to discriminate different
features)
• It is unique for different features/objects regardless of the
sum of the incoming radiation.
• The spectral reflectance is dependent on wavelength, it has
different values at different wavelengths for a given terrain
feature.
• Unique objects reflect differently, while similar objects only
reflect differently if there has been a physical or chemical
change

Spectral reflectance curves
• Surface feature can be characterized by the percentage of
incoming EME it reflects at each wavelength across the
electromagnetic spectrum.
• This is said to be spectral reflectance curve or “spectral
signature”
• SRC: is a curve which shows the portion of the incident
radiation that is reflected as a function of wavelength
• Each type of material of interest has its own reflectance
property/ curve.
• Reflectance measurements can be carried out in a laboratory
measurements or in the field using a field spectrometer.

Reflectance of vegetation
• The spectral characteristics of vegetation vary with wavelength.
• Plant pigment in leaves called chlorophyll strongly absorbs
radiation in the red and blue wavelengths but reflects green
wavelength.
• The internal structure of healthy leaves acts as diffuse reflector
of near infrared wavelengths.
• Measuring and monitoring the near infrared reflectance is one
way that scientists determine how healthy particular vegetation
may be.

Reflectance of water
• Majority of the radiation incident upon water is not reflected
but is either absorbed or transmitted.
• Longer visible wavelengths and near infrared radiation is
absorbed more by water than by the visible wavelengths.
• Thus water looks blue or blue green due to stronger
reflectance at these shorter wavelengths and darker if viewed
at red or near infrared wavelengths.
• The factors that affect the variability in reflectance of a water
body are depth of water, materials within water and surface
roughness of water.

Digital image preprocessing and visual
interpretation
• Interpretation and analysis of remote sensing imagery involves the
identification and/or measurement of various targets in an image
in order to extract useful information about them.
• Targets in remote sensing images may be any feature or object
which can be observed in an image, and have the following
characteristics:
• Targets may be a point, line, or area feature.
• This means that they can have any form, from a bus in a parking
lot or plane on a runway, to a bridge or roadway, to a large area of
water or a field.
• The target must be distinguishable
• It must contrast with other features around it in the image.

• Much interpretation and identification of
targets in remote sensing imagery is
performed manually or visually, i.e. by a
human interpreter.
• Remote sensing images can also be
represented in a computer as arrays of
pixels, with each pixel corresponding to a
digital number, representing the
brightness level of that pixel in the
image.
– In this case, the data are in a digital
format.
Digital image preprocessing…cont

• Both analogue and digital imagery can be displayed as black and
white (also called monochrome) images, or as colour images by
combining different channels or bands representing different
wavelengths.
• When remote sensing data are available in digital format, digital
processing and analysis may be performed using a computer.
• Digital processing may be used to enhance data as a preface to visual
interpretation.
• Digital processing and analysis may also be carried out to
automatically identify targets and extract information completely
without manual intervention by a computer interpreter.

• However, rarely is digital processing and analysis carried out as a
complete replacement for manual interpretation.
• Often, it is done to supplement and assist the human analyst.
• Manual interpretation requires little, if any, specialized equipment,
while digital analysis requires specialized, and often expensive,
equipment.
• Manual interpretation is often limited to analyzing only a single
channel of data or a single image at a time due to the difficulty in
performing visual interpretation with multiple images.

• The computer environment is more amenable to handling complex
images of several or many channels or from several dates
• Manual interpretation is a subjective process, meaning that the results
will vary with different interpreters.
• Digital analysis is based on the manipulation of digital numbers in a
computer and is thus more objective, generally resulting in more
consistent results.
• However, determining the validity and accuracy of the results from
digital processing can be difficult.
• Both methods have their merits. In most cases, a mix of both methods
is usually employed when analyzing imagery

Elements of Visual/Manual/ Interpretation
• Recognizing targets is the key to interpretation and information extraction.
• Observing the differences between targets and their backgrounds involves
comparing different targets based on any, or all, of the visual elements of:
• texture,
• shadow,
• association.
• Association takes into account the relationship between other
recognizable objects or features in proximity to the target of interest.
– E.g commercial properties may be associated with proximity to major
transportation routes, whereas residential areas would be associated with
schools, playgrounds
• shape,
• size,
• pattern,

Ch-1-Introduction to Remote Sensing_7.pdf

Ch-1-Introduction to Remote Sensing_7.pdf

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