FOR 474: Forest Inventory

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FOR 474 Forest Inventory

• Introduction to LiDAR
• What is it?
• How does it work?
• LiDAR Jargon and Terms
• Natural Resource Applications
• Data Acquisition Standards
• Readings
• Western Forester April 2008

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Lidar What is it?
Light Detection and Ranging Essentially a laser
rangefinder that has been strapped to the belly
of an airplane.
The time for the light to travel to and from the
target is used to determine distance Distance
Speed x time This distance and the position of
the airplane is used to get elevation and
location.
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Lidar What is it?
Lidar Light Detection and Ranging The Basic
Lidar Concept Speed distance/time d ct d
Distance (meters) t time (seconds) c speed of
light
Sensor/Detector
D
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Lidar What is it?
Creation of the Lidar pulse
Sensor/Detector
D
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Lidar What is it?
Steps in the Lidar Process Creation of the
Lidar pulse Lidar pulse travels to the target
Sensor/Detector
D
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Lidar What is it?
Steps in the Lidar Process Creation of the
Lidar pulse Lidar pulse travels to the
target Interaction with the target
Sensor/Detector
D
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Lidar What is it?
Steps in the Lidar Process Creation of the
Lidar pulse Lidar pulse travels to the
target Interaction with the target Lidar pulse
travels back to sensor
Sensor/Detector
D
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Lidar What is it?
Steps in the Lidar Process Creation of the
Lidar pulse Lidar pulse travels to the
target Interaction with the target Lidar pulse
travels back to sensor Sensor processes return
signal
Sensor/Detector
D
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Lidar What is it?
Lidar uses LASER light LASER Light
Amplification by Stimulated Emission of Radiation
Properties of LASERS Monochoromatic The light
emitted by a laser occupies a very small range of
the EM spectrum. Intensity The intensity of a
laser can exceed the sun Directionality A laser
travels in a straight line. (The laser light
spreads out at lt 1mm per m) Coherence A physics
terms stating that the laser light is in phase
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LASER Properties
Monochoromatic The light emitted by a laser
occupies a very small range of the EM spectrum.
The color of the LASER light depends on its
wavelength
SOURCE http//repairfaq.ece.drexel.edu/sam/CORD/l
eot/course01_mod01/mod01-01.html
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LASER Properties
Directionality A laser travels in a straight
line. (The laser light spreads out at lt 1mm per m)
LASER
NOTE Perfectly parallel (collimated) light
cannot be produced. The SUN emits light in an
unidirectional manner The light spreads out
equally in all directions.
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LASER Properties
Coherence A physics terms stating that the
laser light is in phase
Incoherent Light Waves They Dont Match Up
Coherent Light Waves They Match Up
SOURCE http//repairfaq.ece.drexel.edu/sam/CORD/l
eot/course01_mod01/mod01-01.html
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How to Produce Laser Light
LASER Properties
Atomic Physics Atoms can hold several different
quantities of energy. However, if the hold to
much they are unstable and tend to shake-off
the extra energy until they only have the lowest
amount of that they can have. This lowest energy
level is called the Atomic Ground
State Exciting the Atom By deliberately giving
the atom too much energy you can make it unstable
and effectively force it to shake-off the extra
energy. In atoms of certain materials this
energy, which is in the form of photons of light,
exhibit predictable wavelengths.
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LASER Properties
In this example Atom it can only have 3 possible
energies E1, E2, E3. When in E1 it cannot
release energy. If in E2 or E3 the energy can
fall to a lower level and release a photon. If
this occurs naturally, it is called spontaneous
emission of light.
Energy Level Diagrams
SOURCE http//repairfaq.ece.drexel.edu/sam/CORD/l
eot/course01_mod01/mod01-01.html
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LASER Properties
By intentionally giving the atoms energy, they
can be forced to have a particular energy (i.e.,
have a certain energy level) This is achieved by
hitting the atoms with a photon that has an
energy exactly equal to that needed to force it
into the next energy level
This makes the atom unstable. It shakes off the
extra energy by emitting another photon that is
identical to the one that first hit it
EP E3-E2
This is stimulated emission.
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LASER Properties
Source Lefsky (2005)
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Lidar What is it?
How does a LASER produce light?
http//repairfaq.ece.drexel.edu/sam/CORD/leot/cour
se01_mod01/mod01-01.html How do we produce the
Intense LASER light? http//www.micro.magnet.fsu.
edu/primer/java/lasers/heliumneonlaser/
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Lidar The Pulse
As the lidar pulse travels to the target the
light fans out (as the distance from the target
can be several kilometers)
Lidar footprint Height x divergence The
footprint is the effective area that the laser
light encompasses Divergence is the degree by
which the light fans out from a straight line
(measured in radians 1 rad 57.3
degrees) Typical divergence 0.25-4 mradians per
1000m
Source Lefsky (2005)
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Lidar The Pulse
Source Lefsky (2005)
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Lidar The Pulse
Low Divergence Canopy penetration and some
pulses will reach the ground High
Divergence Reduced canopy penetration and low
percentage of pulses hitting and RETURNING from
the ground
Source Lefsky (2005)
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Lidar The Pulse
Low Spacing Canopy penetration and some pulses
will reach the ground High spacing Less pulses
hitting and RETURNING from the ground
Source Lefsky (2005)
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Lidar The Main Kinds
Two main types Waveform Sampling Discrete Return
Source Lefsky (2005)
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Lidar What is it?
Each pulse of laser light contains a large number
of photons. A few of these photons return to the
sensor The 1st return might be a tree top, while
the last return could be from the ground. It is
important to note that The 1st could also be
the last return. The Last return might not be the
ground.
1 return
2 return
3 return
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Lidar The Main Kinds
Source Blair and Harding NASA GSFC
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Lidar The Main Kinds
Source David Harding NASA GSFC
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Lidar The Main Kinds

• Advantages of Waveform Lidar
• No signal processing errors
• Enhanced ability to characterize canopy
  information over large areas
• Global satellite datasets available
• Compatible with other RS global datasets

• Advantages of Discrete Return Lidar
• High spatial resolution (0.05-2.00 m)
• Small diameter footprint
• Flexibility in available data processing methods
• Highly available

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Lidar What the Data Looks Like
Area is approximately 1 X 0.75mi. includes
440,000 returns
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Lidar Geomorphologic Applications
Volume change in open pit mines
Landslide Detection
Utilities map power lines for signs of damage
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Lidar Mapping Fault Lines
Image Source Puget Sound LiDAR Consortium
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Lidar Riparian and Coastal Ecology
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Lidar Underwater DEMs for Coastal Mapping
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Lidar Forestry
Image source H-E Anderson
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Lidar Forestry
Image source MJ Falkowski
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LiDAR Data Acquisition Standards
Although the use of LiDAR is widespread in
forestry people are inconsistent on how they
collect the data
If we want to compare measurements between
different areas we need the data to be collected
using standard properties
One day you may be asked to get a LiDAR
acquisition for your forest so its important
that you know what to ask for!
Source Evans et al PERS (in review)
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LiDAR Data Acquisition Standards for Forestry
Pulse Repetition Frequency (number of pulses per
second) This should be high enough so that the
pulses are well-distributed vertically throughout
the canopy
1 return
Number of Returns When using Discrete Return
LiDAR ask for at least 3 returns per laser pulse
2 return
3 return
Source Evans et al PERS (in review)
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LiDAR Data Acquisition Standards for Forestry
Post-Spacing Average horizontal spacing between
pulses (may be multiple returns per pulse). Ask
for a maximum post spacing of 70 cm If after
shrubs or seedlings, ask for a post spacing
closer to 15cm
Source Evans et al PERS (in review)
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LiDAR Data Acquisition Standards for Forestry
Scan-Angle The maximum off-nadir angle the
sensor head swings to. High scan angles can
distort the LiDAR footprint (worse on
slope). Ask for a maximum scan-angle of
12 The total view angle is then 24
Source Evans et al PERS (in review)
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LiDAR Data Acquisition Standards for Forestry
Flight Line Overlap This ensures features are
well sampled. Ask for a flight line overlap
of 50
When to Collect Data Avoid bad weather or snow
(unless you are snow modeling). Do you want leaf
on or leaf-off data?
Source Evans et al PERS (in review)
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LiDAR Data Acquisition Standards for Forestry
Accuracy Standards Vertical Root Mean Square
Error lt 15cm Horizontal Root Mean Square Error lt
55cm
The vendor should calculate errors using real
time geodetic surveys (GPS and total stations)
Source Evans et al PERS (in review)
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LiDAR Data Acquisition Standards for Forestry
Typical Lidar Products to ask for Ground Surface
Model (Digital Elevation Model) Digital Surface
Model (surface of all non ground
returns) Intensity ( aerial Photograph) Point
Heights (DSM DEM) Canopy Height Density
Source Evans et al PERS (in review)
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FOR 474 Forest Inventory
Next Time Using LiDAR data to produce DEMs