Remote Sensing Fundamentals L2x - FTP

murmerlastUrban and Civil

Nov 16, 2013 (3 years and 6 months ago)

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Brief history and definitions of remote sensing



Physical basis of remote sensing


Electromagnetic spectrum and radiation


Spectral region definitions and wavelengths



The remote sensing signal


Interactions with surface materials and the atmosphere

o
Reflect

ion

o
Absorption

o
Transmission

o
Emission


Interactions in the atmosphere

o
Scattering

o
Absorption

o
Refraction


Source: modis.nasa.gov

A general understanding of physical basis and
fundamental concepts of remote sensing is
required to understand how to interpret, analyze,
and apply the information for various
environmental applications.


Dates to 1960



Evelyn Pruitt, Office of Naval Research



Term used as a surrogate for the words “air
photo interpretation”



Nebraska Remote Sensing Center > Center
for Advanced Land Management
Information Technologies

(created in 1972)


long history of remote sensing at UNL


Reconnaissance

at

a

distance

(Colwell,

1966
)



Study

of

remote

objects

(earth,

lunar,



planetary

surfaces

and

atmospheres

[etc
.
])



from

great

distances

(Nat
.

Acad
.

of

Sciences,

1970
)



Acquisition

of

physical

data

of

an

object

without

touch

or

contact

(Lintz

&

Simonett,

1976
)



Observation

of

a

target

by

a

device

separated

from

it

by

some

distance

(Barrett

&

Curtis,

1976
)



Science

of

deriving

information

about

an

object

from

measurements

made

at

a

distance

from

the

object,

i
.
e
.
,

without

actually

coming

in

contact

with

it

(Landgrebe,

1978
)


Basically,

it

is

the

science

and

art

of

collecting

information

about

an

object

or

feature,

without

physical

contact
.


The American Society of Photogrammetry and Remote
Sensing (ASPRS) adopted a combined formal definition of
remote sensing as:


“the
art
,
science
, and
technology

of obtaining reliable information
about physical objects and the environment, through
the process of
recording, measuring, and interpreting imagery and digital
representations of energy patterns derived from non
-
contact sensor
system
.” (Colwell, 1997)

Civil War

WWI

WWII

Cold War

Iraq / Afghanistan

1858

1900

1950

1990

1970

Balloon

Plane

Space

Program

Landsat
-
4

Space

Shuttle

Landsat
-
1

Sputnik

Meteorological

Satellites

Space Station

2010

ERS
-
1

U

n

i

t

e

d

S

s

a

t

t

e

Commercial

Satellites

Pigeon camera

Remote sensing is part of the ‘mapping sciences’ that

inter
-
relate with mathematics and the biological, physical,
and social sciences.

Mathematical

S
tatistical algorithms
applied to r.s. data for
information extraction.

Social

-
Land use change

-
Urban sprawl

-

Monitoring policy



and management



plans

Physical

-

Geologic exploration

-

Elevation mapping

-

Natural hazard assessments (e.g. drought, floods, hurricanes, and wildfire)

Biological

-

Land cover mapping

-

Ecosystem assessment

-

Water quality monitoring

-

Agriculture monitoring

GIS/Cartography

R.s.
-
derived information
input into a GIS and utilized
in mapping activities.


Figure 1.3, Jensen

Remote sensing
applications are at the
intersection of
multiple disciplines.


Remote sensing can encompass observations made at distances
ranging from 1
-
meter or less to thousands of meters.



Platforms for a remote sensing instrument can be:

Satellite
-
based

Airborne

Close
-
range (proximal)

The p
latform

of the instrument will determine:

1) geographic extent

of information collected,



-

few meters
2
or a few km
2

to 1,000+

km
2



2) spatial information in the image,



-

individual houses, roads, and fields


to broad
-
scale landscape features


(e.g., Sandhills)


3)

appropriate application of


remotely s
ensed data



-

mapping individual urban


structures vs. global land cover


patterns

1
-
kilometer

AVHRR image


continental U.S.

0.5 meter

IKONOS image


Washington, DC

A remote sensing instrument collects
information about an object or
phenomenon within the
instantaneous
-
field
-
of
-
view
(IFOV)
of the sensor system:


1)
without
being in direct physical




contact
with
it (unobtrusive),


2) ‘systematic’ fashion (e.g., frame of



aerial photo or a pixels of a large



pixel array comprising an image)



that is objective and repeatable,


3) sensing
electromagnetic (EM)



radiation


Advantages of
Collecting
RS
Data



Bird’s eye” view (complete spatial
coverage over large areas)


Observation beyond the visible
portion of the electro
-
magnetic
spectrum (EMS)


Multi
-
temporal (frequent revisit)
for monitoring and change
detection


Non
-
destructive sampling

Result:


can
often provide answers
to questions such as
“what”, “where
”,
“how much”, and “how
severe”.

DIGITAL PROCESSING

Energy Source

Receiving System

End
-
users

Earth
´
s cover



Atmosphere

VISUAL INTERPRETATION

Remote Sensing

Platform


The Science

Requires general
understanding of
basic remote
sensing principles.

The Application

Based on this
understanding,
image processing
and analysis
techniques can
be applied to
translate digital
image data to use
information.

Physical Basis for Remote Sensing


The Basic Process

Basic remote sensing process
:
Incident solar radiation
(also referred to as

electromagnetic radiation (EMR)/energy”
) from the Sun that is
reflected or
emitted from a land surface object
and
‘sensed’ by a remote sensing instrument

expresses information about that object (e.g., size, structure, and condition).

Electromagnetic Spectrum

12

10

9

8

7

6

5

4

3

2

13

11

10

10

10

10

10

10

10

10

10

10

10

10

10

14

0.01

0.1

1

10

100

1

1

10

10

100

10

0.1

0.1

1

0.4

0.5

0.6

0.7

µ

m

VISIBLE SPECTRUM

X RAYS

GAMMA

RAYS

Wavelength (

l

)

Frequency (MHz)

Angstroms

Micrometers

Centimetres

Meters

UHF

VHF

MICROWAVES

BLUE

GREEN

RED

RADAR

MIDDLE

NEAR

ULTRA
-
VIOLET

THERMAL

RADIO, TV.

INFRARED

This applications course will focus primarily on remotely sensed
multi
-
spectral data

in the visible, near
-
infrared (NIR), middle infrared (MIR),
and thermal infrared (TIR) regions.

Electromagnetic Spectrum

The human eye can visually observe different wavelengths of
EMR in the visible part of the EM spectrum using a prism.


Interactions of incident EMR across the visible spectrum with
objects (or surface materials) we visually observe give them their
color. Every color we see is a combination of reflected EMR of
various visible wavelengths.

Color vision of the human eye
is ‘trichromatic’, having 3
independent channels for
conveying color information.

RGB Color Model

Electromagnetic Spectrum

Human eye can only detect a small
portion of the EM spectrum within
the visible range.


Remote sensing instruments can
sense (or detect) EMR across a much
larger range of the EM spectrum that
convey important information about
the Earth’s surface (e.g., plant health
or water quality/quantity) that are of
interest to agricultural and natural
resource applications.

Different land cover types can be discriminated based on
their unique spectral responses (signatures), which are a
function of each cover type’s physical characteristics and
their interaction with the incident radiant flux in different
regions of the EM spectrum.

Dominant factors
controlling reflectance

of plants.

The multi
-
spectral response of a target (e.g., plants) is a function of ‘known’
interactions between various parts of the EM spectrum and the physical
properties of the target (or surface).

Changes in multi
-
spectral

response of target (e.g., vegetation)

provide valuable information for monitoring
(e.g., plant health or phenology).

Physical Principles of Remote Sensing


Key Physical Processes and Interactions of EMR

with the Atmosphere and Terrestrial Surface

n


=

frequency

Electric field

Transmission

direction

Amplitude

Speed of Light (
c
)

Electromagnetic Radiation (EMR)

How Does EMR Travel?

Part 1
-

The Wave Model

EMR travels
through
space at the speed of light
(186,282 miles s
-
1
)
as an
electromagnetic wave
consisting of 2 orthogonal
(at right angle) fields:


1)
Electric field

2)
Magnetic field

Examples of
electromagnetic waves of
the
blue
,
green
, and
red

portions of the visible
spectrum.

Longer wavelength

Shorter wavelength

Electromagnetic Radiation (EMR)

How Does EMR Travel?

Part 2
-

The Particle Model

Quantum theory of EMR



energy is transferred in
‘discrete’ packets called
‘photons’

or
‘quanta’
(particle
-
like units of light). Photons carry particle
-
like
properties such as energy and momentum.

Wave

Photons (or Quanta)

Electromagnetic Radiation (EMR)

Putting it all together……

The energy of a photon is inversely proportional to
its wavelength.

All objects above absolute
z
ero
(
-
273
o

C or 0
Kelvin (K)) emit EM energy (e.g., Sun, soil,
water, and vegetation).


‘Blackbody’

is a theoretical construct of an
object that absorbs and emits ALL the
energy it receives (emissivity = 1).


The amount of radiant energy emitted by an
object is a function of its temperature.
The
greater the temperature, the greater the
amount of energy exiting the object.


The wavelength of maximum radiant energy
shifts to SHORTER wavelengths as an
object’s temperature increases.





Stefan
-
Boltzmann Law

Wien’s Displacement Law

‘Whitebody’

(also theoretical) does not absorb any of the incident
energy, reflecting all the energy received (emissivity =
o
).


‘Graybody’

absorbs and emits a fixed proportion of energy equally
across all wavelengths. (emissivity > 0 and < 1).


The Real World:

Most objects in nature have emissivity values that vary with wavelength
and referred to as
‘selective radiating bodies
’.


The amount and spectral distribution of energy radiated by an object
varies by:


1) the temperature of the object



2) the nature of object’s material; as depicted by ‘
emissivity
’.


Emissivity



the ratio between the radiant energy flux exiting a ‘real
-
world’ selective radiating body (
M
r
) and a theoretical blackbody at the
same temperature (
M
b
).


Radiation Budget Equation


E
I
(
l
) =
E
R
(
l
) +
E
A
(
l
) +
E
T
(
l
)

Law of Conservation of Energy


The sum of reflectance
(r

or R

),

absorptance
(a
or A
),

and
transmittance
(t

or T
)

is equal to
1
.

Radiation Budget Equation


E
I
(
l
) =
E
R
(
l
) +
E
A
(
l
) +
E
T
(
l
)

Law of Conservation of Energy


The sum of reflectance
(r

or R
),

absorptance
(a

or A
),

and
transmittance
(t

or T
)

is equal to
1
.

The proportions of incident EMR that is
reflected, absorbed, and transmitted is a
function of the unique characteristics of the
surface and can vary by wavelength and changes
to the surface conditions.

This unique spectral behavior
of EMR allows us to infer
information about the type,
state, and condition of the
surface (target).


Basis for
Remote Sensing

Incidence

angle

Reflectance

angle

Specular reflector

Lambertian reflector

The nature of reflection depends on sizes of surface irregularities
(roughness or smoothness) in relation to the wavelength of the EMR.

The
surface is
smooth

relative to the
wavelength
and redirects all (or nearly all)
incident radiation in a
single direction
.



Ex.
-

Mirror, smooth metal, or calm water

The
surface is
rough

relative to the
wavelength and radiation is
scattered
equally in all directions (“
Lambertian
surface
”)
.



Ex.


white paper

Most real
-
world surfaces exhibit complex patterns of reflection
, which are
neither perfectly specular or Lambertian, that are determined by details of
surface geometry (e.g., size, shape, and orientation of plant leaves).

Diffuse Reflection

Some surfaces may approximate Lambertian behavior at some incidence
angles, but have non
-
Lambertian properties (anisotropic) at other angles
and at different wavelengths.

Lambertian Surface

Anisotropic (or non
-
Lambertian) Affects

Spruce



Soybeans

Appearance of an object varies according to the position of the observer
(sensor) in relation to the incident EMR.


Should be considered for applications requiring detailed computations of
reflectance and precise calculations.

Thermal Infrared Region

Emitted Energy Signal

Signals in thermal region (8 to 14
m
m)

of the EM spectrum results from
the
Earth’s emitted energy
and not from the Sun’s reflected energy.


When a material is ‘opaque’ to incident radiation, a portion of the EMR is
absorbed by the material and converted to heat, which is then emitted
(re
-
radiated) and can detected by thermal sensors.

Water content of crop fields

Heat loss for buildings

Warm current of Gulf Stream

white body

Emissivity

(continued)

Emissivity
(e)

values of common objects.

Kirchoff’s Law



spectral emissivity of
an object generally equals its spectral
absorptance (thermal equilibrium).


“Good absorbers are good emitters,
good reflectors are poor emitters.”

If emissivity increases, then reflectance
(reflected energy) must decrease and
vice versa.

EMR Interactions with the Atmosphere

The
atmosphere interacts with both the incoming and outgoing
EMR modifying the signal (speed, wavelength, intensity, and
spectral distribution of the radiation)
that comes from the Earth’s
surface observed by a remote sensing instrument.


The remotely sensed signal can be modified (or “
attenuated
”) by:


1)
several
gases

in the atmosphere: oxygen (O
2
), ozone (O
3
), water
vapor (H
2
O), and carbon dioxide (CO
2
)


2)
Aerosols



solid particles (e.g., dust and pollutants)

Influence of EMR Interactions with the
Atmosphere

Combined effects of several physical processes in the atmosphere on
EMR reduces the amount of solar radiation reaching the Earth’s
surface (the target on most remote sensing applications).

As EMR radiation passes through the atmosphere,

it can be subjected to several physical processes:


1)
Scattering



reflection of EMR in



unpredictable directions by gases,



aerosols, and water vapor


2)
Absorption



EMR is absorbed by



gases in atmosphere (absorbed radiation



can also be emitted)


3)
Refraction



bending of light when it



passes from one medium (or material)



to another because of their different



densities and speed to EMR travels.


4)
Transmission



EMR that pass through the


atmosphere and is incident on the Earth’s



surface


Processes in the Atmosphere that Modify EMR

Atmospheric Scattering

The amount of scattering depends on the:

1)
sizes and abundance of particles in atmosphere

2)
wavelength of the radiation

3)
depth of the atmosphere the radiation traveled (‘path length’)


Types of Atmospheric Scattering

1.
Rayleigh scattering (‘molecular scattering’)

occurs
when the
diameter of the particles is many times smaller
than the wavelength of the incident EMR.



Very dependent on wavelength


and mainly affects the shorter



wavelengths (UV and blue)



Occurs in upper 4.5 km of the



atmosphere


A function of: 1) size of the wavelength of the incident EMR
and 2) the size (diameter) of the particles encountered in the
atmosphere.

Atmospheric Scattering
Diameter
Rayleigh Scattering
Mie Scattering
Non-Selective Scattering

Gas molecule

Smoke, dust

l

Water

vapor

P
hoton of electromagnetic

energy modeled as a wave
a.
c.
b.
Rayleigh scattering of blue
wavelengths on clear day
results in a clear, blue sky
appearance.

Types of Scattering

Atmospheric Scattering
Diameter
Rayleigh Scattering
Mie Scattering
Non-Selective Scattering

Gas molecule

Smoke, dust

l

Water

vapor

P
hoton of electromagnetic

energy modeled as a wave
a.
c.
b.
2.
Mie scattering (non
-
molecular scattering)



occurs
when the
diameter of the particles are of equal size of the
wavelength
of the incident EMR



Takes places in lower 4.5 km of


atmosphere



Particle sizes range from 0.1 to 10


times the wavelength of the EMR


(dust and pollution)



Amount of scatter is greater than



for Rayleigh scatter and longer



wavelengths are scattered

Dust and smoke particles can add scattering of
longer wavelengths beyond the blue to increase
red sky appearance at sunset and sunrise.
Sunset/sunrise appearance due to greater
distance through the atmosphere the EMR has
to travel, resulting in more scattering.

Types of Atmospheric Scattering

Atmospheric Scattering
Diameter
Rayleigh Scattering
Mie Scattering
Non-Selective Scattering

Gas molecule

Smoke, dust

l

Water

vapor

P
hoton of electromagnetic

energy modeled as a wave
a.
c.
b.
3
.
Non
-
selective scattering



occurs when the particles are
larger than the wavelengths of the incident EMR.



Takes place in lowest portion of



the atmosphere



Particles greater than 10 times the



wavelength (water droplets and


ice crystals of clouds and fog)



All wavelengths of EMR are



scattered

Atmospheric Absorption

The atmosphere behaves as a ‘selective’ filter at different
wavelengths and remote observations cannot be conducted
over certain spectral regions.


Atmospheric windows


portions of the EM spectrum where
the transmittance of the atmosphere is sufficiently high and
suitable to remote observation in cloud
-
free conditions.

Visible and NIR
(0.13


1.35
m
m)

M
IR

(1.5


1.8
m
m)

(2.0
-

2.4
m


(2.9
-

4.2
m


(4.5
-

5.5
m


TIR

(8
-

14
m
m)


Spectral bands of remote
sensing instruments,
particularly on satellites,
designed for terrestrial
applications collect
spectral data within these
atmospheric windows.

Incoming Solar Radiation (
l
x
)

Absorption in the atmosphere is
primarily due to three gases:

1) Ozone


absorbs shorter, high energy
UV wavelengths

2) Carbon Dioxide


absorbs middle and
far (thermal) infrared radiation

3) Water Vapor


absorbs middle infrared
radiation; also secondary absorption
across the visible and near
-

and middle
-
infrared wavelengths.


Atmospheric Absorption

Certain wavelengths of radiation are affected far more by
absorption than scattering
in the atmosphere.


Result
:

Reduces the amount of radiation in those
wavelengths reaching the Earth’s surface.

Absorption
attenuates
the
incident light

Land Surface

Absorbed
radiation is
transformed and
re
-
radiated
(emitted) at a
longer wavelength
(thermal IR).

Reduced incident radiation (energy)

Spectral (EMR) Signal Detected by Remote
Sensing Instrument

Review
: Function of many processes
and interactions that EMR encounters
both in the atmosphere and on the
Earth’s surface before reaching the
remote sensing instrument.

Must be considered in preparing and
analyzing remote sensing data for an
application.
Atmospheric corrections
typically applied to calculate a spectral
signal representative of the Earth’s
surface
(interest of most ag. and natural
resource applications). Discussed later
in this course.