BASICS OF REMOTE SENSING

Developed ByDr. Mohamed A. Mohamed

With assistance fromMs. Sungmi Park

Pixoneer Geomatics Inc.Phone: (703) 852 2162

E-mail: mmohamed@pixoneer.com

Summer 2003

LECTURE 1

Introduction to Remote Sensing

FROM IMAGE TO INFORMATION

Film

CD-ROM

Ground Reference

The selection tools ordinarily make handedged selections, as if the selections were cut with a razor-sharp knife. Thus,when selections made with these tools are cut or pasted into an image, the individual pixels along the border cna be seen very clearly. This often results in an image that appears unatural. By defining a feather edge around a selection, you can cut and paste the selection without making it stand out dramatically from its surroundings. In this section you will define a feather edge, or border, around a lassoed selection using the Lasso Options dialog box. Note that you can also define a feather edge for rectangular and elliptical marquee selections using the Feather command in the Select menu.

The selection tools ordinarily make handedged selections, as if the selections were cut with a razor-sharp knife. Thus,when selections made with these tools are cut or pasted into an image, the individual pixels along the border cna be seen very clearly. This often results in an image that appears unatural. By defining a feather edge around a selection, you can cut and paste the selection without making it stand out dramatically from its surroundings. In this section you will define a feather edge, or border, around a lassoed selection using the Lasso Options dialog box. Note that you can also define a feather edge for rectangular and elliptical marquee selections using the Feather command in the Select menu.

Photograph

Computer

Energy Source

Data Acquisition

Data Products

andStorage

Image Interpretation

andAnalysis

Productsand

InformationExtraction

Users and

Decision Makers

Maps

Reports

Geographic Information

Systems

Earth Surface Features

ReceiverImage

Tape

Atmosphere

Science & Software

DEFINITION OF REMOTE SENSING

The science and art of obtaining information about features or phenomena from data acquired by a device that records reflected, emitted, or diffracted electromagnetic energy, and is not in direct contact with the features or phenomena under investigation.

Partially adapted from Lillesand and Keifer, 2000

Science & Software

HISTORY OF REMOTE SENSING• Born in 1839, photography was first used in topographic

surveying in 1840’s• First aerial photograph was taken from a balloon in 1858• Three-Color photographic process was developed in 1861• Invented in 1903, airplane was first used as a camera

platform in 1909.• Aerial photography was extensively used for reconnaissance

during World War I. • Photo interpretation and photogrammetric mapping

techniques and instruments were greatly developed during World War II

• The lunar missions in 1960’s marked the era of space imaging

• First imaging satellites were launched in early 1970’s

Adapted from Lillesand and Keifer, 2000

Science & Software

RADIATION PRINCIPLES

•Basic Wave Theory

• Electromagnetic Spectrum• Particle Theory

• Sources of Electromagnetic Energy

• Stephan Boltzmann Law

• Blackbody Radiator

• Wien’s Displacement Law

Science & Software

ELECTROMAGNETIC WAVE

V = Frequency

Science & Software

BASIC WAVE THEORY

• Electromagnetic energy travels at the speed of light in a harmonic sinusoidal fashion

• The wavelength () is the distance between two successive Peeks

• Wave frequency (v) is the number of peaks passing a point in space per unit time

Science & Software

WAVELENGTH AND FREQUENCY

• Wavelength:₋ Distance between two

successive peaks

• Frequency:₋ Number of peaks (crests)

that pass a given point in space per unit time

• Amplitude:₋ Height of peak

Wavelength

1 SecondFrequency

8 cycles

4 cycles

Frequency1 Second

Wavelength

8 cycles

4 cycles

Amplitude

Science & Software

WAVELENGTH MEASUREMENT UNITS

• Angstrom (Å) = 10-10 m or one 10 billionth of a meter

• Nanometer (nm) = 10-9 m or one billionth of a meter

• Micrometer (µm) = 10-6 m or one millionth of a meter

• Millimeter (mm) = 10-3 m or one thousandth of a meter

• Centimeter (cm) = 10-2 m or one hundredth of a meter

• Decimeter (dm) = 10-1 m or one tenth of a meter

• Meters (m) = 100 m or one meter

• Kilometer (dm) = 103 m or one thousand meter

Science & Software

FREQUENCY MEASUREMENT UNITS

• Hertz (Hz) =one cycle per second

• Kilohertz (KHz) =1000 cycles per second

• Megahertz (MHz) = 106 Hz or million Hz

• Gigahertz (GHz) = 109 Hz or billion Hz

Science & Software

BASIC WAVE EQUATION

The longer the wavelength the lower the frequency

v is inversely related to

Where: C = Speed of light = Wavelength v = Wave frequency

C = v

Science & Software

ELECTROMAGNETIC SPECTRUM

Science & Software

PARTICLE THEORY

Where: Q = Energy of a photon h= Planck’s constant v = Wave frequency

Electromagnetic radiation is composed of manydiscrete units called photons or quanta

Q = hv

Science & Software

ENERGY/WAVELENGTH RELATIONSHIP

The longer the wavelength the lower its energy content

The photon energy is inversely related to

From Equation 1 & 2 Q = hC

Q = hv ------ 2C = v ------ 1

Science & Software

SOURCES OF ELECTROMAGNETIC ENERGY

• The Sun

Examples are terrestrial objects

• All matter at temperature above absolute zero (zero degree K or -273 degree C)

Science & Software

STEPHAN BOLTZMANN LAW

Total energy increases very rapidly with increase in temperature

Where: M = Total radiant from the surface= Boltzmann constant T = Absolute temperature

M = T4

Science & Software

BLACKBODY RADIATOR

• A hypothetical ideal radiator that totally absorbs and re-emits all energy incident upon it

• All earth surface features are not ideal radiators

Science & Software

WIEN’S DISPLACEMENT LAW

Wavelength and temperature are inversely related

Where: m = Wavelength of maximum spectral radiantA= ConstantT = Absolute temperature

m =A/T

Science & Software

GRAPHICAL REPRESENTATION OF WIEN’S DISPLACEMENT LAW

Science & Software

LECTURE 2

Energy Interaction with the Atmosphere and Earth Surface Features (Objects)

ENERGY INTERACTION WITH THE ATMOSPHERE & EARTH FEATURES

Scattered Radiation

Em

itted

Rad

iatio

n

Reflect

ed R

adiation

Absorbed

Radiatio

n

RadiationAbsorbed

Inci

dent

Rad

iatio

n

Science & Software

REFRACTION

• The bending of light when it passes from one medium to another due to differing densities

Where: c = speed of light in vacuum

• The index of refraction (n) is a measure of the optical density of a substance

n = c / cn

cn = speed of light in a substance

Science & Software

SCATTERING

• Unpredicted diffusion of radiation by particles in the atmosphere

- Mie scatter

• Three types of scatter:

- Rayleigh scatter

- Non-selective scatter

Science & Software

RAYLEIGH SCATTER

• Atmospheric molecules and tiny particles are much smaller in diameter than wavelength of the interacting radiation

- Example is a blue sky

Science & Software

MIE SCATTER

• Atmospheric molecule and particle diameters are equal to the wavelength of the interacting radiation

• Water vapor and dust are major causes

Science & Software

NON-SELECTIVE SCATTER

• Atmospheric molecule and particle diameters are much larger than the wavelength of the interacting radiation

• Water droplets scatter all visible and near-to-mid infrared wavelengths equally

- Examples are fog and white clouds

Science & Software

ABSORPTION

• Effective loss of energy to atmospheric constituents

• Most efficient absorbers are:

- Water vapor

- Ozone

- Carbon dioxide

• Absorption band is a range of wavelengths in the electromagnetic spectrum within which radiant energy is absorbed by a substance

Science & Software

ATMOSPHERIC WINDOWS

Courtesy of NASA Goddard Space Flight Center

Science & Software

ENERGY INTERACTION WITHEARTH SURFACE FEATURES

Where: EI () = Incident energy ER () = Reflected energy EA () = Absorbed energy ET () = Transmitted energy

Energy incident on an element are reflected,absorbed, and/or transmitted

EI () = ER () +EA () + ET ()

Science & Software

ENERGY INTERACTION WITHEARTH SURFACE FEATURES

EI () = Incident energy

ET () = Transmitted energyEA () = Absorbed energy

ER () = Reflected energy

Science & Software

ENERGY REFLECTION BYEARTH SURFACE FEATURES

The geometric manner in which objects reflect energy is a function of surface roughness

ER () = EI () - [EA () + ET ()]

• Specular reflector

• Diffuse (Lambertian) reflector

• In-between (near specular, spread, near diffuse)

- Examples are earth surface features

Science & Software

SURFACE REFLECTANCE

Science & Software

IDEAL SPECULAR REFLECTOR

Angle of Incidence

Angle of Reflection

ir

i = rFlat Surface that Manifest Mirror-like Reflection

Science & Software

LAMBERTIAN SURFACE

Uniform reflectance in all directionsScience & Software

DIFFUSE REFLECTION

It contains information on the “color” of the reflecting surface

In remote sensing, we are most often interestedin measuring the diffuse reflectance properties

of terrain (earth surface) features

Science & Software

SPECTRAL REFLECTANCE

Where:

= Spectral reflectance ER () = Reflected energy EI () = Incident energy

• The portion of incident energy that is reflected

= ER () / EI () * 100

• It is often expressed as a percentage

Science & Software

SPECTRAL REFLECTANCE CURVE

Is a graph of spectral reflectance of an objectas a function of wavelength

Science & Software

SPECTRAL REFLECTANCE CURVE

Wavelength

% R

efle

ctan

ce

10

20

30

40

50

60

0.5 0.6 0.7 0.8 1.1

Near IRBlue Green Red

MSS 4 MSS 5 MSS 6MSS 7

Dead grass

Dry bare soil

Green grass

Science & Software

ALBEDO

• % reflection off surfaces at particular wavelengths

BLUE GREEN RED INFRARED

50

40

30

20

10

0

Visible Light

0.4 0.5 0.6 0.7 0.8 0.9Microns

Natural Grass

Artificial TurfP

erc

en

t R

efle

cta

nce

• Artificial turf has a low albedo in the Near Infrared (IR) region

• Healthy natural grass has a high albedo in the Near Infrared (IR) region

Science & Software

SPECTRAL SIGNATURE CURVE

Is a spectral response measured to assess the typeand/or condition of the feature

Tends to imply an absolute or unique pattern

Main characteristic

Science & Software

SPECTRAL RESPONSE PATTERN

Is the spectral reflectance or emittance of a terrain feature

• Quantitative but not absolute

• Distinctive but not unique

Main characteristics

Science & Software

SPATIAL EFFECT

Factors that cause the same type of feature at a given point of time to have different spectral

characteristics at different locations

• Same crop in different fields

Example

Science & Software

TEMPORAL EFFECT

Factors that change the spectral characteristicsof a feature over time

• Vegetation in different seasons

Example

Science & Software

IDEAL REMOTE SENSING SYSTEM

• Uniform energy source

• Non-interfering atmosphere

• A super sensor

• Series of unique energy/matter interactions at the earth’s surface

• Multiple data users.

• A real-time data handling system

Science & Software

REAL REMOTE SENSING SYSTEM

•Variable and non-uniform energy sources

• Interfering atmosphere

• Sensors have limited spectral sensitivity

• Energy/matter interactions at the earth’s surface are not unique

• Concerns and issues about multiple data usage

• Data handling system have limited capabilities

Science & Software

LECTURE 5

• Electromagnetic Energy Detection with Optical and Thermal Imaging Systems

• Concepts of resolution

SCANNER SYSTEMS

Build up two-dimensional images of the terrain for a swath beneath the plane using either

across-track (whiskbroom) scanningor

along-track (pushbroom) scanning

Science & Software

WHISKBROOM SCANNING

• Uses a set of detectors, each of which is designed to have its peak sensitivity at a specific wavelength

• Uses a rotating mirror to scan the terrain along scan lines perpendicular to flight line₋ The scanner repeatedly measure energy on both

sides of the platform

• Successive scan lines (contiguous) compose a two-dimensional image

Science & Software

WHISKBROOM SCANNING

Scanning Direction

Scanning Mirror Detectors

Flight Direction

Science & Software

RESOLUTION CELL SIZE

• At Nadir:

• At a Scan Angle of

Where: D = Diameter of resolution cell H’ = Flying height above the terrain IFOV

D = H’

D = (H’ sec

Science & Software

RESOLUTION CELL SIZE VARIATION

Flight Line

H’

Scan Line

H’ θ = H

’ sec

θ

H’ß

ß

ß

θ

(H’ s

ec θ)

ß

(H’ sec2 θ) ß

Adapted from Lillesand and Keifer, 2000

Science & Software

SWATH WIDTH

Where: W = Swath width H’ = Flying height above the terrain Half the total field of view

W = 2H’ tan

Science & Software

INSTANTANEOUS FIELD OF VIEW

• Commonly referred to as IFOV

• IFOV is expressed as the angle within which incident energy is focussed on the detector

• The ground area covered by the IFOV is often expressed as a circle and called resolution cell

• At any instant, the scanner sense the energy within the IFOV

Science & Software

PUSHBROOM SCANNING

• Uses linear arrays of detectors to scan in The direction perpendicular to flight line

• Linear arrays normally consist of charged-coupled devices (CCDs)

• A single array may contain > 10000 CCD

• Each detector is dedicated to sensing the radiation in a single resolution cell

• All scan lines are viewed by all arrays simultaneously

Science & Software

PUSHBROOM SCANNING

Scanning

Directi

on

Detectors

Flight Direction

Science & Software

ADVANTAGES OF PUSHBROOM SCANNING

• Each detector has a longer dwell time, over which to measure energy from the res. Cell₋ Better spatial and radiometric resolution

• Greater geometric integrity because of the fixed relationship among detectors

• CCDs are smaller in size, lighter in weight, and require less power for their operation

• Having no moving parts, a linear array system has higher reliability & longer life

Science & Software

DISADVANTAGES OF PUSHBROOM SCANNING

• Need to calibrate more detectors

• Current CCDs have a relatively limited range of spectral sensitivity

• Commercially available CCDs are not sensitive to wavelength longer than the Near Infrared

Science & Software

MULTISPECTRAL SCANNERS

• Sensitive to the region from 0.3 - 14 m

• Bands are relatively broader in range

• Three or more bands

• Different bands may have different spatial resolution

Science & Software

ATLAS SENSOR

Courtesy of NASA Stennis Space Center

Science & Software

ATLAS COLOR-IR IMAGE OF ATLANTA

Courtesy of NASA Stennis Space Center

Science & Software

HYPERSPECTRAL SCANNERS

• Sensitive to the region from 0.3 - 2.5 m

• Bands are very narrow in range

• Many to several hundred bands

• Normally all bands have the same spatial resolution

Science & Software

AVIRIS SENSOR

Courtesy of NASA Jet Propulsion Laboratory

Science & Software

FOREST FIRE IN BRAZIL (AVIRIS)

Courtesy of NASA Jet Propulsion Laboratory

Science & Software

THERMAL SCANNERS

• Sensitive to the region from 3 - 14 m

• Band ranges are variable

• Few to several bands

• Normally all bands have the same spatial resolution

Science & Software

Science & Software

THERMAL INFRARED SCANNER

RadiationSource

ScanningOptics

Dewar

(liquidnitrogen)

ElectricalSignal

AmplifierTapeTapeRecorderRecorder

Glow Tube

7070mmmmFilmFilm

RecorderRecorder

Lens MotorMotor

OscillatingMirror

VOLCANOLOGY STUDIES

TIMS image draped over 1:4000 DEM

Courtesy of NASA Ames Research Center

Science & Software

SPATIAL RESOLUTION

A measure of the smallest angular or linear separation between two objects that can be resolved by the sensor

The smaller the object the higher the resolution

i.e. It is the limit on how small an object can be and still be

“seen” by a sensor as being separate from its surroundings

Science & Software

SPATIAL RESOLUTION

4 meter x 4 meter 16 meter x 16 meter

Science & Software

± < 0.2 ± 0.4 ± 61:500 1:1,000 1:2,500 1:25,000

± 5 ± 121:50,000

Standard Error in Position (RMSE meters) SCALE

± 1

Developed Area Regional Area

Urban Rural Undeveloped

Urban - Distr ibution Cross Country Transmission

Facilities Lease Holdings Dev. Area Lease H old Undeveloped

Urban Area & Construction State - M ulti State R egion

Row Crops, Or chard, Small Fields Field Crops, Large Fields

Regional Sys.

Grazing

LAND MANAGEMENTCITY

COUNTY

STATE

INFRASTRUCTUREUTILITIES

TRANSPORTATION

RESOURCE MANAGEMENTAGRICULTURE

MINERALS & PETROL

1:5,000± 2

Map

Usa

ge

1:10,000

IKONOS( 1 ~ 4 m)

IRS/Kompsat( 5 ~ 10 m)

SPOT/Landsat10 ~ 30 m

Typical Users

SPATIAL RESOLUTION & APPLICATIONS

Science & Software

SPECTRAL RESOLUTION

i.e.The ability to discriminate fine spectral differences

The higher the number of bands the higher the resolution

The number and dimensions of specific wavelength intervals in the electromagnetic spectrum to which a

remote sensing instrument is sensitive.

Science & Software

SPECTRAL RESOLUTION

Red

Green

Blue

Grey

Science & Software

RADIOMETRIC RESOLUTION

The sensitivity of a remote sensing detector to differences in signal strength as it records the radiance flux reflected

or emitted from the terrain.

The higher the number of bits,the wider the range of values, and the higher the resolution

i.e.The ability to discriminate very slight energy differences

Science & Software

RADIOMETRIC RESOLUTION

8 bit (0 ~ 255 ) 11 bit (0 ~ 2047 )

In 2 bit case, target A and target B has brightness value of 1(can not be recognized as different objects in the image). However, in 4 bit case , target A

has value of 3 and target b has value of 2.

Science & Software

TEMPORAL RESOLUTION

Frequency of data acquisition over the area

Terms implying temporal resolution:• Revisit capability• Global cycle• Global coverage

Science & Software

CHANGE DETECTION

The ability to measure temporal effectsor

The ability to quantify change over a period of time

Science & Software

METHODS OF SCANNERS CALIBRATION

• In laboratories

• Fly over natural and/or man-made targets

• On-Board

Science & Software

CALIBRATION OF SCANNERS

• Spatial resolution

• Radiometric values

• Spectral bands (ranges & co-registration)

• Signal-to-noise ratio

Science & Software

IMAGERY

Merits:

Recorded by electronic sensors that generate electrical signals that corresponds to

energy variations in the scene

- Improved calibration potential

- Broader spectral range sensitivity

- Electronic transmittal of data

Science & Software

LECTURE 7

Microwave Remote Sensing

PASSIVE MICROWAVE SENSORS

• Radiometers or scanners that operate similar to their optical counterparts

• Use antenna to detect naturally emitted microwave energy (atmosphere, surface features, and sub-surface transmittance)

• Characterized by low spatial resolution (large field of view) due to the relatively small magnitude of emitted energy

• The emitted microwave energy is related to the temperature and moisture properties of the object

• Data from such sensors is typical used by:₋ Meteorologists (atmospheric profile, ozone and water

content)₋ Hydrologists (soil moisture content)₋ Oceanographers (mapping sea ice, currents, winds, and pollutants)

Science & Software

ACTIVE MICROWAVE SENSORS

• Non-Imaging sensors₋ Profiling devices such as altimeters and scatterometers

that take measurements in one linear dimension

• Imaging sensors: ₋ Radar instruments that record two-dimensional images

of surfaces beneath the platforms₋ In general, image acquisition is not affected by

weather (Clouds, Haze, Dust, etc.)₋ Images can be acquired at day and/or night time

Science & Software

NON-IMAGING SENSORS• Radar Altimeters:

₋ Look straight down at nadir and transmits short microwave pulses to determine distances of targets through measurement of round trip time delays

₋ Often used for determination of aerial platforms’ altitudes, as well as topographic mapping and sea surface height estimation.

• Scatterometers:₋ Used to precisely measure the amount of energy

backscattered from a target, which depends on surface roughness and the incident angle at which the energy contacts the target

₋ Typically used in oceanographic applications for estimation of wind speeds, and identification of materials and characterization of surfaces in land applications.

Science & Software

PRINCIPLES OF IMAGING RADAR

• Radar is a range or distance measuring device which consists of a transmitter, a receiver, an antenna, and a data processing and recording system

• The transmitter generates short successive pulses of microwave energy at regular intervals which are focused by the antenna into a beam

• The beam obliquely illuminates a surface at a right angle to the direction of the platform

• Targets (objects) reflects the signal back to the receiver (echo or backscatter)

• The location of an object (based on its distance from the radar) is determined by measuring the time delay between transmission of a pulse and reception of the echo backscattered by that object.

• The forward motion of the platform and the continued processing and recording of the backscattered echo builds up a two-dimensional image of the surface

Science & Software

MICROWAVE BANDS

Letter Code Frequency Range (MHz) Wavelength Range (cm)

P 220 – 390 136 – 77UHF 300 – 1000 100 – 30

L 1000 – 2000 30 – 15S 2000 – 4000 15 – 7.5

C 4000 – 8000 7.5 – 3.75

X 8000 – 12500 3.75 – 2.4Ku 12500 – 18000 2.4 – 1.67K 18000 – 26500 1.67 – 1.18

Ka 26500 – 40000 1.18 – 0.75

Wavelength (λ) in cm = 30000 / frequency in MHz

Adapted from Henderson and Lewis, 1998

Science & Software

EXAMPLES OF RADAR IMAGERY

L-Band X-Band

Science & Software

POLARIZATION

Transmission of energy in either a horizontal (H) or vertical (V) plane

Horizontal (H) Vertical (V)

Science & Software

PARALLEL POLARIZED SYSTEM

Sends and receives signal in same polarization (HH) or (VV)

Horizontal (HH) Vertical (VV)

Science & Software

CROSS-POLARIZED SYSTEM

• Sends in one polarity and receives in another polarity (VH) or (HV)

• Requires a second antenna

• With a second antenna two images can be recorded simultaneously (HH-HV) or (VV-VH)

• These pairs of images (dual polarization images) can be analyzed for differences

Science & Software

CROSS-POLARIZED SYSTEM

Vertical Horizontal (VH) Horizontal Vertical (HV)

Science & Software

POLARITY EFFECT ON IMAGERY

HH Polarization HV Polarization

Science & Software

MULTI-CHANNEL RADAR

• Records four images simultaneously

• Two polarities and two different wavelengths₋ Band X HH₋ Band X HV₋ Band L HH₋ Band L HV

Science & Software

SIDE-LOOKING AIRBORNE RADAR (SLAR) VIEWING GEOMETRY

Image SwathImage Swath

Look AngleLook Angle

90˚90˚

90˚90˚

Radar PulseRadar Pulse

Depression AngleDepression Angle

Nadir Line orNadir Line orGround TrackGround Track

Radar BeamRadar Beam

IncidenceIncidenceAngleAngle

Radar GroundRadar GroundContactContact

Ground RangeGround Range

Across TrackAcross Track

Along TrackAlong Track

Science & Software

RADAR PARAMETERS

• Azimuth direction Direction of flight

• Range (look) direction Direction of illumination which is at right angle to azimuth direction₋ Significantly impact feature interpretation₋ Enhancement or suppression of linear features depends on their

relative orientation to range direction

• Depression angle (γ) Angle between the range direction and the electromagnetic pulse from the radar antenna to a point in the ground

• Incident angle (θ) Angle between the radar pulse and a line perpendicular to the surface it contacts

Science & Software

RANGE RESOLUTION

• The resolution in the across-track direction which is proportional to the length of the microwave pulse₋ The shorter the pulse length, the finer the resolution

• Range resolution (Rr) = τ.c/2cosγ₋ τ = duration of transmission₋ c = speed of light₋ γ = depression angle

Science & Software

SLANT RANGE RESOLUTION

AA BB CC DD

All objectsAll objectsdistinguishabledistinguishable

Radar PulseRadar Pulse

AA BB CC DD

Radar PulseRadar Pulse

Cannot distinguishCannot distinguishbetween A & Bbetween A & B

Science & Software

AZIMUTH RESOLUTION

• Determined by computing the width of the terrain strip illuminated by the radar beam₋ The angular beam width is directly proportional to

the wavelength of the transmitted pulse₋ The beam width is inversely proportional to

antenna length

• Azimuth resolution (Ra) = S x γ / L₋ S = slant range₋ γ = depression angle₋ L = antenna length

Science & Software

AZIMUTH RESOLUTION

A & B areresolvable C & D are not

resolvable

IlluminatedArea

Antenna

AB

CD

Beam Width

Science & Software

GEOMETRIC DISTORTION OFRADAR IMAGERY

• Slant-range scale distortion

• Relief displacement₋ Foreshortening₋ Layover

• Shadowing

Inherent geometric distortions in radar imagery are caused by the following:

Science & Software

SLANT-RANGE SCALE DISTORTION

• Occurs because radar measure distance to objects in slant ranges (not horizontal along the ground distances)

• Objects in the near range are more distorted (compressed) than those in the far range

• Can be easily corrected by using trigonometry to calculate ground-range distances to objects

Science & Software

RELEIF DISPLACEMENT

• A one-dimensional displacement along the range (look) direction

• Higher Objects are displaced towards the sensor

• Radar foreshortening and layover are typical consequences of relief displacement

Science & Software

FORESHORTENING OF SLOPE• Occurs when the radar beam reaches the base of a high object

before it reaches the top

• Slopes are compressed. Severity of compression depends on angle of slope in relation to incident angle of radar beam

• Maximum compression occurs when the base and top are imaged simultaneously (radar beam is perpendicular to slope)

BA

A’ B’

Science & Software

SEVERITY OF FORESHORTENING

Very severe slope compression

Maximum slope compression

Science & Software

FORESHORTENING OF SLOPES

Courtesy of RADARSAT Corporation

Science & Software

LAYOVER OF SLOPE

• Occurs when the radar beam reaches the top of a high object before it reaches the base

• Very severe at the near range with small incident angles

Science & Software

LAYOVER OF SLOPE

Courtesy of RADARSAT Corporation

Science & Software

RADAR SHADOWS

Radar Image

Photo Image Shadow

• Result from foreshortening and layover

• Occur in the down range direction behind vertical features and steep slopes

• Radar beam does not illuminate the surface

• Shadows appear dark

• Objects in shadows are obscured

Science & Software

RADAR SHADOWS

Courtesy of RADARSAT Corporation

Science & Software

RADAR IMAGE INTERPRETATION

• Images are a result of different factors than aerial photos₋ degree of reflectance in one wavelength₋ angle of depression₋ negative terrain: absence of data in the shadows

Science & Software

TONES ON A RADAR IMAGE

• Measure of backscatter strength

• The stronger the return the brighter the area on the image

• Light tones: prominent cultural and topographic features

• Varying tones: cultivated fields and most terrain surfaces

• Dark tones: calm water bodies, smooth ice, and some depositional landforms

• Uniform tones: relatively homogeneous feature

• Grainy or speckled tones: rough surfaces

Science & Software

SUPERIOR, WISCONSIN

StorageTanks

City

Water

Sandy Deposits

Science & Software

ALBERTA, CANADA

Road

Lakes

SelectiveClearcut

Drumlins

Science & Software

DETRMINING FLIGHT DIRECTION

• What direction was the aircraft flying if the SLAR was mounted on the port (left) side?

A

BC D

Choices…

Science & Software

FEATURES IDENTIFICATION

PowerLines

ErodedAnticlines

CultivatedFields

Science & Software