Airborne Laser Scanning: Remote Sensing with LiDAR

Airborne Laser Scanning:
Remote Sensing with LiDAR
Aaron Smith
Forest Resources
Remote Sensing and
GIS Lab
University of Idaho

• Laser remote sensing background
• Basic components of an ALS/LIDAR system
• Two distinct families of ALS systems
• Waveform
ALS / LiDAR outline
• Discrete Return (small footprint)
• Working with the data, processing requirements
• Current applications and developing applications
• Advancing ALS / LiDAR technology

Airborne Laser Scanning
• ALS/LiDAR is an active remote
sensing technology that
measures distance with reflected
laser light
• 1 st developed in 1960 by
Hughes Aircraft inc.
• Modern computers and DGPS
make it practical
• Typically used in very accurate
mapping of topography
• New technologies and
applications are currently being
developed

Why use a Laser?
ALS systems take advantage of two of the unique properties of
laser light:
1. The laser is monochromatic. It is one specific wavelength of
light. The wavelength of light is determined by the lasing material
used.
Advantage: We know how specific wavelengths interact with the
atmosphere and with materials.
2. The light is very directional. A laser has a very narrow beam
which remains concentrated over long distances. A flashlight (or
Radar) on the other hand, releases energy in many directions,
and the energy is weakened by diffusion.
Advantage: The beam maintains its strength over long distances.
3mrad divergence = 30 cm at 1 km and 1.5m at 5 km
For an easy to understand discussion of how lasers work check out:
http://www.howstuffworks.com/laser.htm

ALS mapping concepts
• The position of the aircraft is known
(from DGPS and IMU)
• Measures distance to surfaces by
timing the outgoing laser pulse and
the corresponding return(s)
• Distance = time*(speed of light)/2
• By keeping track of the angle at
which the laser was fired: you can
calculate the X, Y, Z position of each
“return”
• Requires extremely accurate timing
and a very fast computer

Basic ALS System Components
• Laser (usually NIR –1064nm)
mounted in an aircraft
• Scanning assembly – precisely
controlled rotating mirror
• Receiver for recording reflected
energy “Returns”
• Aircraft location system
incorporating Differential
GPS and Inertial
Navigation System
• A very fast computer to
synchronize and control
the whole thing

Two distinct families of ALS systems
Waveform systems (a.k.a. large-footprint)
Records the COMPLETE range of the energy pulse (intensity)
reflected by surfaces in the vertical dimension
Samples transects in the horizontal (X,Y) plane
Waveform systems designed to capture vegetation information are
not widely available
Waveform systems include SHOALS, SLICER, LVIS, ICESat
Discrete-return systems (a.k.a. small-footprint or topographic)
SAMPLES the returned energy from each outgoing laser pulse in
the vertical plane (Z) (if the return reflection is strong enough)
Most commercial lidar systems are discrete return, many different
types are available

Waveform LiDAR
This is the “waveform” graph of energy intensity
Notice that it follows the “shape” of the tree biomass

Waveform LiDAR: SLICER
Waveform systems have been used to accurately measure
(r 2 >.90) vertical distribution of tropical and temperate forests
(Lefsky 2001)

Spaceborne LiDAR: ICEsat and VCL
Spaceborne waveform
LiDAR systems
ICEsat launched in 2003 for
survey of topography and
polar ice
VCL Launch date???
(pending funding)
VCL Designed to sample 5
separate widely spaced
tracks with 25m footprint
waveforms
Do not provide “wall to wall”
mapping

Discrete-return LiDAR
• Records data as X,Y,Z points
• Spatial resolution is expressed
in terms of “post spacing” which
is the avg. horizontal distance
between points
• Returns are “triggered” if the
laser reflects from a surface large
enough to exceed a pre-set
energy threshold
• Minimum vertical distance
between returns is ~ 5 meters
• New capability to record the
intensity of point returns.

Discrete return data: Millions of X,Y,Z points
Area is approximately: 1 X 0.75mi.
includes ~ 440,000 returns

From point clouds to 3-D surface models
• Points are used to create 3D
surface models for applications
• Triangular Irregular Networks
(TIN)s are used to classify the
points and to develop Digital
Elevation Models (DEM)s
• Points must be classified
before use: “bare earth” points
hit the “ground”; other point
categories include tree canopy
and buildings
• Correct identification of “bare
earth” is critical for any lidar
mapping application

LiDAR surfacing

Working with Commercial LiDAR
Discrete return lidar data requires several complex processing
steps to develop useable products, these steps are usually
divided into two phases:
Pre-processing is the preparation of the raw data, correcting for
errors induced by flight geometry, removing overlaps
between flight lines, surfacing, and accuracy assessment of
the raw point locations
Post-processing is the development of usable information from
the point clouds including development of TINs, DEMs, and
other products like canopy height maps
Pre-processing is always done by the lidar contractor

Discrete Return LiDAR Accuracy
• Average reported point location accuracies for a hard flat
reflective surface (e.g. airport runway):
2-15 cm vertical
25-100 cm horizontal
• Reported measurement accuracies: (forestry research, best cases)
Height prediction r 2 = 0.93 (Means 2000)
Volume prediction r 2 = 0.92 (Maclean and Krabill 1992)
Comparing field measurements to laser measurements
• Timing system accuracy is critical: 1 nanosecond = 30cm
• Accuracy is also affected by errors in GPS, surfacing and data
analysis
• Measurements of object height requires accurate surfacing to
provide accurate base height

Applications for Discrete-Return LiDAR
Digital Elevation Model (DEM)
4m DEM of PREF Subset

Geomorphology
Mapping geomorphic features (Puget Sound LiDAR Consortium)

Geology
Mapping Faults (Puget Sound LiDAR Consortium)

Other Applications
Measuring volume change
in open pit mines
Utilities use lidar to map
power transmission line
curvature and clearance

Discrete Return LiDAR for Forestry
Can be used to measure
forest characteristics
including stand height,
biomass distribution,
volume.
DEMs are used in
planning for erosion and
engineering projects
including roads
Mapping of forest stand
structure characteristics
and fire fuels conditions is
under development

Forest Engineering

LiDAR mapping of forest structure

LiDAR captures 3D structure information

LiDAR products for forest management

Working with LiDAR
If you want to use commercial ALS data for a specific application:
• Educate yourself prior to seeking a contractor
• Clearly communicate early and often through out the
acquisition process, be specific about deliverables
• Get involved in the flight planning process, stay involved
through accuracy assessment, QC, and post processing
• Most forestry applications for discrete return lidar (other
than developing contours/DEMs) are relatively new and
untested: rigorous quantitative accuracy assessment needs
to be done on any “product”
• If you plan to do post-processing “in house” realize that it is
an intensive and complex process requiring substantial
GIS/Remote sensing expertise and computer support

The future of ALS
ALS/LiDAR technology is evolving rapidly, the next 10
years will bring many new capabilities and applications
• The PREF data we are working with (slide 12) was flown at
15,000 pulses per second. Systems are now available that
record 50,000 pps. Expect to see 100,000 pps systems
soon
• Automatic merging of multi-spectral imagery with LiDAR is
being developed now
• Laser induced fluorescence and measurement of changes in
laser coherence and polarization will provide more
information about the surfaces that the laser light interacts
with
• Look for multi-spectral LiDAR using multiple lasers in different
frequencies recording intensity to spectrally identify
surface materials

New ALS technologies
This is an example of a new application that merges lidar and
multi-spectral imagery to automatically classify features
• Expect higher data density: current systems operate at 15,000hz;
• New systems operate at 50,000hz,
• Next generation will be 100,000hz +

Gamba and Houshmand 1999

Concepts to remember about ALS
1. ALS/LiDAR is an active remote sensing technology designed
to take advantage of the unique properties of laser light
2. Two basic families of lidar systems exist: waveform and
discrete-return
3. Commercially available lidar is discrete-return, it is used in for
a number of applications
4. Accuracy of ALS is dependant on a number of highly complex
interacting factors
5. Complex processing steps are required to use lidar and
accurate surfacing is critical for applications measuring object
heights
6. Development of lidar technology is occurring at a rapid pace,
new applications will develop

Case study on use of small-
footprint lidar for generating
crown structure
• Akira Kato, PhD candidate, UW
• L.Monika Moskal (UW)
• Werner Stuetzle (UW)
• Mark E. Swanson (WSU)
• Peter Schiess (UW)
• Donna Calhoun (UW)

Surface wrapping using spatial
interpolation methods (Kato et al. 2007)
Point cloud Wrapped surface

Ground truthing tree
crown parameters
• Nikon total station set up 4-5 tree heights
from tree
• Measured precisely:
– Distance to tree
– Angles to base of crown, top of tree
– Horizontal angles to crown edge every 5º vertical

LIDAR VS. FIELD
Slide: Akira Kato
(Google map)
Conical shape case
Giant Sequoia
(Sequoiadendron giganteum)

VALIDATION
The total station (Nikon DTM -450)
SE
E W
NE
N
S
NW
SW
Slide: Akira Kato

CROWN VOLUME
Parameters Traditional
The Total Station
8 profiles ISR
CV (m 3 ) 2422 2825 2894
RE (%) 19 2 NA
Slide: Akira Katoc
LID A R data C one
Integration of
the crow n profiles W rapped surface

LIDAR VS. FIELD
Round shape case
Cedarwood Atlas
(Cedrus atlantica)

CROWN VOLUME
Parameters
The Total Station
8 profiles ISR
CV (m 3 Method
) 4534 4780
RE (%) 0.05 NA
Slide: Akira Kato

GROUND VS. AIRBORNE LIDAR
knobcone pine (Pinus attenuata)
Slide: Akira Kato
Airbone LiDAR *Ground-based LiDAR Red: airborne
Blue: Ground-based
Airborne LiDAR is captured at March 2005 and ground-based LiDAR is captured
at Sept. 2007. The seasonality does not matter (because it is coniferous), but
there are two years difference.
*Ground-based LiDAR data was captured from two vantage points separated around
180 degrees each other in terms of the stem. So it covers all aspects of a single tree.

GROUND VS. AIRBORNE LIDAR
Slide: Akira Kato
Crown Volume:
859.49 m 3
Crown Volume:
1058.15 m 3
Crown volume estimated by the total station is 1029.76 m 3 , which is close to
the crown volume estimated by the ground-based LiDAR through the wrapped surface.