Introduction to Remote Sensing - Home | PEOPLE AT ... › nurulhazrina › files › 2015 › 04 › L...What is Remote Sensing? • Remote Sensing –science of acquiring, processing

Introduction to Remote Sensing

Introduction to Photogrammetry and Remote Sensing (SGHG 1473)

Dr. Muhammad Zulkarnain Abdul Rahman

Spatial data acquisition

• The need for spatial data:

• Agronomist

– forecasting the overall agricultural production of a large area

– Require data on area planted with diff crops

– Data on biomass production to estimate yield

• Urban Planner

– Needs to identify areas with illegal dwelling

– Future landuse plan

– The info should be in format that allows integration with socio-economic info.


• Engineer

– Determine the suitable location for power plan, relay station for telecommunication and etc.

– Op1mal area → terrain, landuse and etc.

• Geologist

– Map of surface mineralogy, lineament and etc.

– Landslides potential areas

• Climatologist

– Understanding on the causes of El nino phenomenon which requires: Sea Surface Temp. (SST), sea level, meteorological parameters, etc.


• All these applications deal with spatial phenomena

• Additional element – spatio-temporal phenomena (time is also an important dimension)

• How to acquire this info.? – interviews, land surveying, labs measurements of samples, interpretation of satellite images, measurements using in-situ sensors and etc.

• Spatial data acquisition methods – 1. Ground based and 2. Remote Sensing methods

• Ground-based – field observation, taking in-situ sample/measurements


• Remote Sensing methods – using remotely sensed data acquired using sensors e.g. aerial cameras, laser scanner, radiometer and etc.

• These sensors are attached on the remote platform (airborne, spaceborne, or remote instruments (laser scanner etc.))

Satellite Image Point clouds from

laser scanner

What is Remote Sensing?

• Remote Sensing – science of acquiring, processing and interpreting remotely sensed energy / images that record the interaction between EMR and matter

• Remote Sensing – is the science and art of obtaininginformation about an object, area, or phenomenon through the analysis of data acquired by using device that is not in contact with the object, area, or phenomenon under investigation

• Remote Sensing - the instrumentation, techniques and methods to observe the Earth's surface at a distance and to interpret the images or numerical values obtained in order to acquire meaningful information of particular objects on Earth


• Common definition - data on characteristics of the Earth's surface are acquired by a device (sensor) that is not in contact with the objects being measured

• Not necessarily, stored as image data

• The characteristics measured by a sensor - the electromagnetic energy reflected or emitted by the Earth's surface

• Energy spectrum - visible, infrared, and radio waves.

• RS requires ground data – why?

– RS data can be interpreted without ground data, BUT the bestresults are obtained by linking with ground data


Field data


Advantageous of RS

• RS provides synoptic (general), i.e. area covering data

– For example, SST can be obtained by interpolating in-situ measurements by buoys. However the number and distribution of data from bouys most likely inadequate to enable mapping of SST

– The distance between buoys – 2000 km (x-direction), 300km (y-direction)

– Meteorological satellite – SST can be obtained up to 1km

• RS provides surface and some subsurface characteristics

• RS provides multipurpose data

• RS is cost effective

Electromagnetic energy and remote

sensing

• RS relies on the measurement of electromagnetic (EM) energy

• Several sources of EM – sun (visible, UV), emitted by earth, and provided by the sensor (active sensor)

Electromagnetic energy

• Electromagnetic (EM) energy can be modelled – 1) waves 2) Energy bearing particles called photons

• Wave model – Propagation of EM energy in the form of sine waves

• The waves are characterized by: 1) Electrical (E) and Magnetic (M) fields – perpendicular to each other


• Vibrations of both fields – perpendicular to direction of travel

• Both fields propagate at speed of light 3 x 108 m/s

• Wavelength, λ - distance between successive wave crests

• Frequency v, is the synoptic passing a fixed point over specific period of time

• Frequency - hertz (Hz) – one cycle per second

c is the speed of light (3.108 m/s), λ is the wavelength (m), vis the frequency (cycles per second,Hz).


• λ C


Electromagnetic spectrum


Superposition principle and wave analysis:

– The wave motion produced in a region where 2 separate waves cross each other –the amplitude is the sum of the amplitudes of the 2 separate waves


• Jean Baptiste Fourier (French mathematician) –mathematically any complicated wave form could be constructed by the superposition of an infinite of these sinusoidal waves as components, if the waves had the proper amplitudes, frequencies, and phases

• A set of sinusoidal waves that generate the complicated waves are called the spectral component

• The component sinusoidal can be reconstructed back from the complicated wave through the use of Fourier analysis

• Possible to predict the kind of wave which will result when an incidence wave reflects or penetrated the matter (material) – if we know the sinusoidal component waves (that compose the incident wave) & how each component reflect and penetrate separately


• Reflected or penetrating wave will be the superposition of reflecting or penetrating component waves

• Spectral properties - the material properties that specify the response of material to sinusoidal component waves at every frequency or wavelength

• Spectrographic instrument – to measure amount of radiant flux carried by each of the sinusoidal components of any wave forms (complicated wave)

Electromagnetic energy: Polarization• EMR consists: 1) electric force field 2) magnetic force field

• The direction of these force fields is important to determine the response of matter intercepted with EMR

• Both fields are at the right angles to each other

• We only have to know the direction and magnitude one of these fields (i.e. electric field) – we can determine the other

• Plane polarized – EMR that produces electric field in a fixed direction

• If 2 EM waves having the same frequency and direction, but different direc1on of polariza1on →superpose→theresultant EM field equal to the vector sum of the two component electric fields

Electromagnetic energy: Polarization

• The direction of polarization of each individual wave = the direction in which the electric field is vibrating (lie along the y axis)

• Because all directions of vibration from a wave source are possible, the resultant electromagnetic wave is a superposition of waves vibrating in many different directions - The result is an unpolarizedlight beam

The arrows show a few possible

directions of the electric

field vectors for the individual waves

making up the resultant beam


• The plane formed by E and the direction of propagation is called the plane of polarization of the wave

• In this case, the plane of polarization is the xy plane

• A linearly polarized (or plane-polarized) beam can be obtained from an unpolarized beam by removing all waves from the beam except those whose electric field vectors oscillate in a single plane


The twist of the yellow

arrow plane keeps

changing randomly

The electric force field in any

plane of light can be

separated into a vertical and

horizontal component

Polarized Light

Unpolarized Light


• Radiation from some sources (mostly in the optical region) does not have any clearly defined polarization – the electric field is assumed at randomly different directions as the waved is received (known as randomly polarized)

• In another case – the electric field has some direction at every moment – except when the field has zero magnitude

• Plane polarization:

– Horizontal polarization – polarization direction that is parallel to plane surface (incident surface)

– Vertical polarization – the plane polarization which is at the right angle to the horizontally polarized direction


• In the literature of optics – different reference plane is used

• Plane of incidence – the plane formed by the normal to the

surface and the direction of propagation of the incident wave

• Parallel polarization – polarization direction that is parallel to

the plane of incidence

• Perpendicular polarization –

polarization direction perpendicular

to the plane of incidence

Electromagnetic energy: Coherent and

incoherent radiation

• Coherent waves – if there is a regular, or systematic, relationship between their amplitudes (a receiving detector will indicate more power at some locations and less power at other locations)

• Incoherent waves – their amplitudes are related in random fashion (a receiving detector will always indicate an average power)

Quantization of radiation

Electromagnetic energy: monochromatic

radiation

• Monochromatic refers to electromagnetic radiation of a single frequency

• Microwave radar and lasers normally utilize monochromatic waves

• The reflected waves from two distant objects close together are highly coherent – so image produced by these objects either 1) indicate no power received (dark) OR 2) four times the average power of one object

• For this reason – we have speckled or grainy effect


• Some purposes - EM energy is modelled by the particle theory

Q is the energy of a photon (J), h is Planck's constant (6.625 x

10-34 Js), v is the frequency (hertz)

• EM energy is composed of discrete units called “photons”

• The longer the wavelength – the lower the energy content

• Gamma rays (around 10-9 m) – most energetic – shorter λ

• Radio waves (> 1 m) - the least energetic

• In remote sensing - more difficult to measure the energy emitted in longer wavelengths than in shorter wavelengths.

Radiant energy described by

Maxwell

Source of EM energy: Thermal emission

• All matter with a temp above absolute zero (0 K) radiates EM energy due to molecular agitation (the movement of the molecule)

• The sun and the earth radiate energy in the form of wave

• Blackbody - Matter that is capable of absorbing and re-emitting all EM energy it receives

• Blackbody – emissivity (ϵ) = absorptivity (α) = maximum value (1.0)

• The amount of energy radiated by an object (not blackbodies) depends on its absolute temp. and its emissivity – a function of wavelength

Blackbodies

• Ideal thermal emitter – transform heat energy into radiant energy with the max. rate permitted by thermodynamic laws

• Must absorb and convert all incident radiant energy into heatenergy regardless of the spectral band

• Incident radiant energy → abs by blackbodies → trans. to heat energy → trans. to radiant energy

• Spectral exitance - power per unit area per unit wavelength• Planck’s formula:

c1 = 3.74 x 10-16, c2 = 1.44 x 10-2, λ= wavelength (meters), T = absolute temp in degrees Kelvin.

• Spectral exitance of blackbodies at a given Temp – not the same at all λ

( )15

1 2exp 1M c c Tλ λ λ−−= −

Blackbodies

• Wavelength in between – spectral exitance reaches a

max value depending on temp. The wavelength is

(Wien displacement law):

C – 2.898 x 10-3

• Spectral exitance of blackbodies of all wavelength

(Stefan–Boltzmann law)

mC Tλ =

( )4

blackbody at TM Tσ=

Spectral radiant exitance of blackbodies

Very long and very short

wavelength – spectral

exitance is low !!!!

Spectral emissivity, ϵ (λ)

• Any body of material – at a specific temp emits radiation with its own characteristics (unique)

• This is the capability to emit radiation due to thermal→energyconversion

• ϵ (λ) – ratio of spectral exitance of material to the spectral exitanceof blackbody at the same temp

• ϵ (λ) – is nearly independent of temp. for most common materials and for temp. of terrestrial environment

• For the condition where ϵ (λ) does not change significantly with temp - we can calculate the spectral exitance of any material if we know: 1) emissivity of that object, 2) Spectral exitance of blackbodies (refer to the previous graph)

ϵ (λ) = M λ (material, °K) / M λ (blackbody, °K)

Interaction mechanism

Interaction mechanism

• EMR traveling from its initial source to the remote sensorwill undergo 1) absorption, 2) re-rediation, 3) reflection, 4) scattering, 5) polarization, and 6) spectrum re-distribution

• Changes of EMR from source to sensor depend on interaction with the material

• These changes – supply info to interpreter on the matter with which the EMR interacts

• It gives info about – temperature, texture, moisture, and electrical properties

Interaction mechanism:

Energy interaction in the atmosphere

• Three fundamental interaction in the atmosphere – absorption, transmission, and scattering.

• Abs and transmission:

– EMR is partly absorbed by various molecules

– Most efficient absorbers – Ozone (O3), water vapour (H2O) and carbon dioxide (CO2)



• About half of the spectrum between 0-22µm – not useful for RS of earth surface – can’t penetrate atmosphere

• Region outside the main absorption bands is called “Atmospheric Transmission Windows”:

– A window in the visible and reflected infrared (0.4-2 µm). Window for optical RS operates

– Three windows in the thermal infrared; two narrow windows, 3 and 5 µm, one broad window, 8-14 µm

• Atmospheric moisture – strong abs for EMR at longer wavelengths

• Hardly any transmission between 22 µm – 1mm.

• > 1mm more transparent – microwave region

EM spectrum of the sun with and without the influence

of atmosphere

Radiation

produced

by the sun

Radiation

observed on

the earth

surface



• Atmospheric scattering – particle or gaseous molecules present in the atmosphere re-directs EM waves from their original path

• Amount of scattering depends on – 1) wavelength, 2) amount of particle and gases, 3) distance the radiation travels through the atmosphere

• Visible wavelengths - 100% (in case of cloud cover) to 5% (in case of a clear atmosphere) of the energy received by the sensor is directly contributed by the atmosphere

• Three types of scattering: Rayleigh scattering, Mie Scattering and Non-selective Scattering.



• Rayleigh scattering – Electromagnetic radiation interacts with particles that

are smaller than the wavelength

– Examples: tiny specks of dust, and nitrogen (N02) and oxygen (02) molecules

– Effect of Rayleigh scattering

– Shorter wavelengths are scattered more than longer wavelengths

– In the absence of particles and scattering, the sky would appear black

4

1

λ

Always occur at

short wavelength


Energy interaction in the atmosphere– Daytime - the Sun rays travel the shortest distance through the

atmosphere, Rayleigh scattering causes a clear sky to be observed as blue (scattering in blue wavelength)

– At sunrise and sunset - sun rays travel a longer distance - All the shorter wavelengths are scattered after some distance - only the longer wavelengths reach the Earth's surface - the sky appears orange or red

– Rayleigh scattering - most important type of scattering - distortion of spectral characteristics of the reflected light vs measurements taken on the ground - shorter wavelengths are overestimated

Interaction mechanism: Energy interaction in the atmosphere

• Mie scattering

– the wavelength of the incoming radiation is similar in size to the atmospheric particles

– Aerosols: a mixture of gases, water vapour and dust

– Generally restricted to the lower atmosphere where largerparticles are more abundant, and dominates under overcast cloud conditions

– Mie scattering influences the entire spectral region from the near-ultraviolet up to and including the near-infrared

– Greater effect on the larger wavelengths than the Rayleigh scattering



• Non-selective scattering– The particle size is much larger than the radiation

wavelength

– Particles responsible - water droplets and larger dust particles

– Independent of wavelength,

with all wavelengths scattered

about equally

– Prominent example - the effect of

clouds (clouds consist of water

droplets). All wavelengths are

scattered equally - a cloud

appears white

– Cloud – shadow effect


Energy interaction with earth’s surface• Reflection occurs when radiation 'bounces' off the target and is then

redirected

• Reflectance (%) = ER / EI

• 2 types of reflection: Specular and diffuse reflection

• Specular reflection –

– Mirror-like reflection - typically occurs when a surface is smooth and all (almost all) of the energy is directed away from the surface in a single direction

– The sun is high in the sky

– Water/glass surface

– Very bight spot in the image

Reflected

EnergyIncidence

Energy

Absorbed

EnergyTransmitted

Energy


Energy interaction with earth’s surface



• Diffuse reflection:

– Surface is rough and the energy is reflected almost uniform in all directions

• Neither perfectly specular nor diffuse reflectors – in between

• Reflection is sensitive/a function of: 1)wavelength,2)angle of incidence,3)polarization,and 4)electrical properties of the substance

• Intensity of a reflected EM wave from a particular substance – can vary greatly throughout the EM spectrum

• EMR interactions are detected by remote sensor – designed to selectively respond to different parts of the EM spectrum



• Energy reaching the earth’s surface – irradiance

• Energy reflected by the surface – radiance

• Irradiance and radiance – expressed in

• For each material - specific reflectance curve (show the fraction of the incident radiation that is reflected as a function of wavelength)

• Store collections of typical curves in spectral libraries

• Reflectance measurements - laboratory or in the field using spectrometer


Energy interaction with earth’s surface• Spectral curve of vegetation

– Reflectance characteristics of veg. – depends on the properties of leaf (e.g. Chlorophyll, orientation and structure of leaf canopy)

– The proportion of the radiation reflected in the diff parts of the spectrum – depends on leaf pigmentation, leaf thickness and composition, amount of water in the leaf tissue

Photosynthesis

Interaction mechanism: Energy interaction with earth’s surface

• Vegetation reflects relatively more green light

• Reflentance in near infrared – the highest – the amount depends on the leaf development and cell structure

• Middle infrared - reflectance is determined by the free water in the leaf tissue – more free water will result less reflectance

• Middle infrared – water absorption bands

• Dried leaf - the plant may change colour (for example, to yellow) -no photosynthesis, reflectance in the red portion is higher – water content is low, higher MIR reflectance - lower NIR reflectance

• Optical remote sensing – vegetation health


• Bare soil

– Reflectance from soil is dependent on so many factors that it is difficult to give one typical soil reflectance curve

– Main factors – soil color, moisture content, the presence of carbonate and iron oxide content


• Water

– Compared to vegetation and soils, water has the lower reflectance

– Vegetation may reflect up to 50%, soils up to 30-40%, water at most 10%

– Water reflects EM energy in the visible up to NIR

– Beyond 1.2 µm all energy is absorbed

– The highest reflectance is given by turbid (silt loaded) water

– Water containing plants with a chlorophyll - peak at the green wavelength


Basic concept: Digital Number (DN) and

Radiance

• DNs are the scaled integers from quantization- not a physical quantity

• Most quantization systems in remote sensing are linear

• A larger Q leads to a higher radiometric precision

• DNs should be converted to physical quantities for estimating land surface variables such as radiance measured in the energy per area per solid angle

• Spectral radiance - the energy per area per solid angle per unit wavelength (Wcm-2Sr-1μm-1)


Radiance• Radiant energy (Q) – the energy carried by EMR - is a

measure of the capacity of the radiation

– to do physical work by moving something by a force

– to heat an object

– to change state of matter

• Radiant flux (Ф) – the time rate with which radiant energy passes a spatial position (flow rate), J s-1 or Watt

• Radiant intensity (I) – Radiant flux per unit solid angle leaving the source in that direction, W sr -1

• Radiance (L) – Radiant flux per unit solid angle leaving an extended source in a given direction per unit projected source area in that direction, Wm-2sr -1

2I=Φ/(A/R )

ΦL= AcosθΩ


Radiance

• Lambertian source - the plane source where the radiance does not change value as a function of angle of view

• Lambertian source example – a piece of white paper illuminated by diffuse sky skylight – the visual brightness does not change with angle of view

Basic concept: Solid angle

• The angle β between two radii of a circle of

radius r is:

Basic concept : Solid angle

• Understanding the angular dependence is very important since most sensors are targeting the Earth surface in a specific direction

• A solid angle is often represented by the zenith and azimuth angles in polar coordinates

• θ represents the zenith angle (the angle measured from the vertical or from the horizontal to a surface), Ф represents the azimuth angle


Radiance

• Average Radiant Flux Density (E) – when radiant flux intercepted divided by the area of a plane, Wm-2

• Each small segment of the plane – intercepting a small part of the flux

• The flux density of each segment = amount of flux intercepted of the segment / area of the segment

• IF the segment is significantly small – the ratio can be taken as the radiant flux density

• The radiant flux density for flux incident upon a surface is called irradiance, Wm-2


Radiance• Normally, DNs are linearly related to radiance, and most remote

sensing data providers produce the conversion coefficients for the users

• These coefficients are usually included in the image data header file (or metadata)

• Sensor calibration - procedure that determines these conversion coefficients

• Sensor calibration – post & pre-flight

• Sensor records intensity level (correspond to radiances reflected or emitted energy from the target area ) – Electronic sensor measures the “intensity” by detecting photons, and convert photons to electrons – convert the collected charge to electrical signal

• The analogue electrical signal is sampled and converted to a Digital Number (DN)

• Radiometric resolution – number of bits for DN

Basic concept: Digital Number (DN)

and Radiance

• Older remote sensors have used 8 bits recording (DNs in the range of 0-255)

• Higher radiometric resolution – requires more storage capacity and offers higher information content

• The array of DNs represent an image in terms of discrete picture elements called pixels

• The DN correspond to the radiance of the light reflected from the small ground area viewed by the sensor (spatial resolution)

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