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Kenai GIS User Group LiDAR Workshop 1/35

A LiDAR Primer

Richard L CollinsGeophysical Institute and Department

of Atmospheric SciencesUniversity of Alaska Fairbanks

February 15, 2011

Kenai GIS User Group


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Lidar studies in 1930’s using

search lights. Use of lasers

since the 1960’s.

Used in a wide variety of both

civilian and military

applications;• Biohazards

• Fisheries

• Forestry

•

Fire

•

Glaciology

•

Mapping

• Meteorology

• Pollution

• Space Surveillance

LiDAR – Light Detection And Ranging

Lidar Firsts – From “Airborne andSpaceborne Lidar” M. P. McCormick,2005.

CALIPSO 2007


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LITE – LiDAR In-space Technology Experiment


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Contemporary LiDAR System Concept

650 m widescan swath

30°scanangle

1200 mflight

altitude

1.5 m

1.5 m

IMU

GPSgroundstation

DifferentialGPS

navigation

GPS satellites

Optech ALTM

Airborne LiDAR

± 15 cm

vertical

accuracy

30 cm widelaser

footprint

Topographic Laser Ranging and Scanning:Principles and Processing Shan, J, and C.K. Toth, CRC Press, 2009.


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Optech’s Gemini Airborne Laser Terrain Mapper System

Range 150-4000 mWavelength 1064 nmElevation accuracy 5-35 cm

PRF 33 - 167 kHzPosition GPS and GLONASS

Scan Width 0-50°San Rate 0 - 70 Hz

Range Capture < 4 returnsDivergence 0.25/0.8 mrad


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Hand-held Laser Scanners - Helimap

Heigel laser scanning engine withHasselblad digital frame camera.Rigid carbon fiber frame withhandles.System IMU in box below thecamera.

Helimap system being operated from side ofAlouette III helicopter. System allowsaccurate and high resolution mapping ofsteep and narrow terrain.


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LiDAR Comes of Age

Lidar systems were developed as research systems since the 1960’s.

Terrestrial lidar systems come of age in the 1990’s as several enabling

technologies mature;1.

Fully solid-state lasers where solid-state lasers (e.g., Nd:YAG,

Nd:YLF) pumped by laser diodes

2.

High-speed electronics and computers

3.

A mature Global Positioning System

The following elements are found in contemporary lidar systems

1.

Laser Ranging Unit

2.

Optical Scanning Mechanism

3.

Electronic and Computer Unit4. Position and Orientation Unit

5. Software6.

Imaging System


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Basic LiDAR System

Laser(s)

Telescope

Beam Expander

Photodiode

Optics -Collimation,

FOV and

BandwidthPhotodiode

BS

Lens

Electronics -

Counting/

Timing and

Threshold

Computer

Basic system composed of;• Laser-based transmitter• Telescope-based receiver• High-speed electronics• Optics•

Computer

Lasers providehigh-intensitysmall-footprintnarrow-bandfrequency-stable

beams for probing the environment.


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Echo Detection and Ranging

Knowing the speed oflight, the range fromthe LiDAR to a targetis determined by theround trip time.

The echo timing isdefined by the

leading edge of thelaser pulse and theecho pulse.

Timing is everything,!R = (!t"v + t"!v)/2

as !v/v is very small.

The thresholddetection can dependon signal amplitude.

Time

R a n g e

R1

R2

t

1

t

2

R = v x t

v x t1 = 2 x R

1v x t

2= 2 x R

2

Time

R e c e i v e d

S i g n a

l

t1

T r a n s m i t t e d

S i g n a l

t0

Threshold

Threshold

!t = t1 - t0

T p

T p


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d1 d2

= d1 + R x !R

Laser beams diverge with distance. Anyfinite beam inherently expands as it

propagates.

The Field-Of-View (FOV) of the receivermust match (or exceed) the divergence ofthe transmitter.

• The FOV determines the amount of

background signal.• The FOV places mechanical stability

requirements on the lidar system.

Beam Divergence and Reflection

Specular Lambertian Diffuse Complex

Surfaces found in nature are rarely a smoothtransition between two homogenous media.


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Energy Link Budget - 1

Ground

dR

AircraftET

Eig Erg

ER

Rg

T T

!

Consider a single laserpulse transmitted froman aircraft.

Follow the round trip.

1. ET

2. Eig = ET " T

3. Erg = Eig " #

4. ER = Erg " T " P

where,

P = " # $ # (dR /2)

2

2# $ # Rg2


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Energy Link Budget-2

The analysis yields a form of the Lidar Equation

ER = " # $ # (dR /2)2

2# $ # Rg2

#T 2%

&

' '

(

)

* *

#ET

Consider an airborne lidar, Rg = 1000 m (3281 ft, 0.6214 mile)

dR = 11.3 cm (4.44 in)

# = 0.5

T = 0.8

$ ER = 5.1 x 10-10 " ET

Consider a satellite lidar (CALIPSO)

Rg = 705 kmdR = 1 m

# = 0.5

T = 0.8

$ ER = 1.6 x 10-13 " ET

ET = 110 mJPRF = 20HzT p = 10 ns% = 532 nm &

1064 nm& = 130 µrad

ET = 20 µJPRF = 100 KHz

T p = 10 ns% = 1064 nm,

1550 nm& = 0.2 – 1 mrad


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Energy Link Budget-3

80-90%

~50%

F r e s h S n o w

G r a s s , L i g h t s o i l

~10% ~15%~25%

L a k e

C o n c r e t e

A s p h a l t

Reflectivities

at 900 nm

~60%

~30%

D e c i d u o u s T r e e s

C o n i f e r o u s T r e e s

L i m e s t o n e , C l a y


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Profiling

0

Rg

R

a n g e

Aircraft

Ground

H o r i z o n t a l D i s t a n c e

0

xf

2xf

3x

fdg

0

va x T PRF

va x 2 xT PRF

v

a

x 3 xT

PRF

T i

m e

dg

Cessna 337 at 1000 m

and 67 m/s (130 knots).

Profiling (not scanning)

PRF < 150 kHz.

Divergence

& = 0.2 mrad

Footprintdg = 0.2 m

xf > 0.5 mm

Given divergence non-overlap occurs at

xf > dg

PRF = 335 Hz


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Scanning

The scanner extends the scope

of the profiling system to yieldswaths in the transversedirection. A variety of scanningmethods have been implementedusing oscillating and rotatingmirrors.

A Palmer scanner uses a nutatingmotion to yield a scan that yields an elliptical scan. Most ofthe measurement points arescanned twice, once in theforward and once in thebackward view. This redundancycan be used to calibrate thescanner and the position andorientation system.

Topographic Laser Ranging and Scanning:

Principles and Processing Shan, J, and C.

K. Toth, CRC Press, 2009.


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Range Resolution and Range Discrimination-1

The finite rise time of thelaser pulse, tR, determines theprecision with which thethreshold detector operates.

Ideally tR should be as smallas ossible.

The finite width of thelaser pulse, T P,determines theprecision with which

the LiDAR candistinguish between

closely spaced objects.

Ideally T P should be assmall as possible.

Time

R e c e i v e d

S

i g n a l

t1

Threshold

t2

RangeR2R1

t2 - t1 = < T pR2 - R12 x v

!R > 2 x v x T p

0

0

Time

R e c e i v

e d

S i g n a

l

Threshold

0

tR

T P

The need to minimize tR and T P pushes designers to employ high-

speed (i.e., large bandwidth) analog circuits.


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Range Resolution and Range Discrimination–2

Threshold1

Time

!t1

Threshold2

!t2

R e c e i v e d

S i g n a l

Time

R e c e i v e

d

S i g n a l

0 tS

t

R

The slope of the leading

edge may vary due to• signal noise,• pulse amplitude

variations• pulse spreading due to

elevation variations in

the footprint.

Full-waveform samplingcaptures the shape of theentire signal, not just theleading edge.

The approach supports more

information at GREAT costin both (analog and digital)circuitry and data handling.


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Multiple Returns-1


Principles and Processing Shan, J, and C.

K. Toth, CRC Press, 2009.


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Multiple Returns-2

The lidar echo can contain much more information than is revealed by the multiple

returns detected using a single threshold level.

Full-waveform sampling captures the complete echo but increases the hardware

data handling costs of the system.


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Terrain Mapping-1

The first returns yield the Digital Surface Model (DSM). In vegetated areas the DSM isthe Canopy Top.

The Digital terrain Map (DTM, or bare Earth Surface) is inferred from a spatial filteringthat identifies the lowest returns that define a continuous surface.


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Terrain Mapping-2 Commercial survey in Puget Sound. Lidarfootprint sub-meter diameter, with two

foot rints er s uare meter, and four returns


Principles and Processing Shan, J, and C.K. Toth, CRC Press, 2009.


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Increasing PRF

Multiple Pulses in Air "MPiA"

t0

Time , t0

The maximum range

(i.e., the range of thelast return) and the

PRF of the LiDAR

system are related

as,

T PRF

=

1

PRF

=

2"Rmax

v

Thus as an aircraft

flies at higher

altitude for large

area surveys, the PRF(and hence the

sampling density)must decrease.Announced in 2006 and commercially fielded in 2007,

MPiA is now a mainstream LiDAR technology.


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Interleaving Pulses-1

Region of

Interest

Region of ?

0

Rc

Rg

R

a n g e

Aircraft

For an aircraft high above theground at an altitude of ~1000

m may only be interested in

the ~100 m above the ground

“the region of interest”.

The air between the ground

and the region of interest

yields no significant echoes.

Is it possible to transmit

multiple pulses spaced suchthat their echoes do not

interfere with each other.


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Interleaving Pulses-2

0 tc tg

Recieved

Signal

0 tc tg

Recieved

Signal

T P R F

t c + T P R F

Pulse #1

Pulse #1 Pulse #2 Pulse #3

t g+ T P R F

2

x T P

R F

t c + 2

x T P R F

t g+ 2

x T P R F

The time from 0 to tc is“dead time”, while the

signal of interest is foundbetween tc and tg.

In a single pulse system thesecond pulse must await thereturn of the first pulse.

In an interleaved systemthe second pulse istransmitted “early” so thatit can return just after thefirst pulse without waiting

for tc.

Thus the PRF can bedoubled and possibly betripled or more.

0 tc tg

Recieved

Signal

Pulse #1 Pulse #2 Pulse #3

T P R F

t c

+ T P R F

t g

+ T P R F

2

x T P R F

t c + 2

x T P R F

t g+ 2

x T P R F

Pulse #4

3

x T P R F

t c

+ 3

x T P R F

t g

+ 3

x

T P R F 4

x T P R F

t c + 4

x T P R F

t g + 4

x T P R F

Pulse #5


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MPiA Performance

The relationship between PRF and flightheight for the Leica ALS60 system.Very high PRFs can exceed thecapabilities of current lasers. Thus

multiple laser systems are used toachieve very high PRFs (e.g., ~400 kHz).

The point density at a PRF of 400 KHz isshown as a function of ground speed andflight height for the Reigl LMS-Q680iAt 400 kHz the maximum single pulse

range is 375 m. Thus there are 3 pulsesin the air when flying at 800 m.


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Current Airborne LiDAR Systems

Toth, C. K., LARS, 2010.


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Kenai LiDAR Mappingin 2008-1


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Kenai LiDAR Mapping in 2008-2

LiDAR mapping of the 4550 square miles (11,780 km2) of the Kenai Peninsula.

Kenai LiDAR Mapping

Location Western Lowlands Eastern Kenai Watershed

Area 3295 sq miles (8530 km2) 1255 sq miles (3250 km2)

Post Spacing 1.4 m 3.0 m

Horizontal Accuracy 1 m 2.0

BareEarth RMSE 18.5 cm 50 cm

• Contract to Aero-Metric for LiDAR data collection and processing.• Kenai Watershed Forum contract to Alaska Satellite Facility (ASF) at the Geophysical

Institute-University of Alaska Fairbanks (GI-UAF) to provide Quality Assurance.

ASF Quality Assurance included;1. Review of formatting and completeness of data deliverables2. Review of the completeness, clarity, and compliance of the metadata3. Review of the contractor-provided quality assurance reports4. Evaluation of the planimetric accuracy of the LiDAR data5. Evaluation of the height accuracy of the LiDAR data6. Identification and characterization of any systematic errors observed in the data


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Planimetric Assessment of Road Intersection from GI-UAF Geodetic Control

Courtesy Rick Guritz, ASF, GI-UAF.


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Planimetric Assessment of Lake Point from GI-UAFGeodetic Control

Courtesy Rick Guritz, ASF, GI-UAF.


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Alaskan LiDAR Mapping

Region of

Interest

0

Rg

R a n g e

GroundGround

Snow

Ground

Leaves

Early-April Late-April Early-May

In wooded terrain the trees may shadow the ground. A better ground map will be obtainedif the survey is conducted AFTER snow has melted and BEFORE trees have leafed out.


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LiDAR Researchers atUniversity of Alaska Fairbanks

Researcher Primary Focus Contact

Anthony Arendt Glacier Mapping [email protected] Collins Atmospheric Sciences [email protected] Cunningham Terrestrial Mapping [email protected]

Javier Fochesatto Atmospheric Sciences [email protected]

Rick Guritz Terrestrial Mapping [email protected] Sassen Atmospheric Sciences [email protected]


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Eruption of Mount Augustine in 2006

PUFF Model Forecast

Lidar Data


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Acknowledgements

LiDAR research at the University of Alaska Fairbanks has been conducted with

support from the following;• State of Alaska

• US Department of Agriculture

• US Department of Defense

• US Geological Survey

• US National Aeronautics and Space Administration

•

US National Oceanic and Atmospheric Administration• US National Science Foundation

• Government of Japan

• Fulbright Commission of Germany

LiDAR research at the University of Alaska Fairbanks depends on the active

participation of undergraduate and graduate students.

Thanks to Rick Guritz and Keith Cunningham for helpful discussions.

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