LiDAR Basics
By Christopher Butson
Forest Analysis & Inventory Branch
June 28, 2016
Presentation Outline
1.
2.
3.
4.
5.
6.
7.
What is LiDAR?
How Does LiDAR Work?
Terrestrial vs. Aerial
Discrete vs. Full Waveform
Point clouds & Density
LiDAR Enhanced Forest Inventory
Sensor Specifications and Acquisition
Parameters
8. Summary
What is(n’t) LiDAR?
RAdio Detection And Ranging-> RADAR
• 1886 – Heinrich Hertz showed that radio waves
could be reflected by solid objects.
SOund Navigation And Ranging -> SONAR
• After the Titanic disaster (1912) the world’s first
patent for an underwater echo ranging device was
filed by English meteorologist Lewis Richardson.
Light Detection And Ranging or Light RADAR -> LiDAR
• 1971 - First LiDAR system used during the Apollo15
mission to map the surface of the moon.
What is LiDAR con’t?
It is an ACTIVE remote sensing technology that uses a laser
to measure distances to target points.
Because laser light has a much shorter wavelength it is possible to
accurately measure much smaller objects, such as aerosols, cloud
particles and molecules
1980’s - Stuttgart University proved the high geometric accuracy of a
laser profiling system HOWEVER, the lack of a reliable commercial
GPS/IMU for sensor positioning presented a significant roadblock
Topographic tools up to 1990…
Stereo Photogrammetry (passive)- inferential technology, the
features must be “seen” to be mapped using parallax
(displacement in apparent position of an object when viewed
from two different lines of sight).
RADAR (active)
• Longer wavelength microwave radiation can
penetrate through cloud cover, haze, dust, and all
but the heaviest rainfall
• certain limitations for measuring ground
elevations beneath forest canopy and,
• peculiar artifacts in very steep terrain.
© Department of Natural Resources Canada.
All rights reserved
1990’s…
• Demand for GPS/IMU systems for use in aerial photogrammetry spurred
rapid development
• The US-DOD GPS satellite constellation reached full configuration needed
for widespread operations.
• High-accuracy inertial measurement units became available as certain
military missile guidance systems were declassified
By the mid-1990s, laser
scanner manufacturers were
delivering LiDAR sensors
capable of 2,000 to 25,000
pulses per second.
By 2005, laser scanner
manufacturers were
delivering LiDAR sensors
capable of 250,000 pulses
per second.
http://brianjohnsonaviation.weebly.com/brians-blog
Four (not so) basic needs…
1. Sensor
3. Inertial Measurement Unit (IMU)
4. High precision Clocks
2. GPS
LiDAR Key Benefits…
1. Acquisition similar to aerial photography- The ground coverage of an airborne lidar
sensor is very similar to that of a traditional
aerial camera, so photogrammetric methods of
flight planning could be directly applied to
lidar.
2. Lidar is capable of "seeing" between trees in forested areas Similar data processing, the development of end products
from the dense lidar mass points is much like
photogrammetric data processing.
3. Lidar presented fast, accurate, and direct (not inferential) data collection
Generating 3-dimensional data point clouds. As the cost of
instruments and services stabilized, it quickly became a very
attractive mapping solution.
Light Detection & Ranging (LiDAR)
LiDAR (Light Detection And Ranging) has become an established
method for collecting very dense and accurate elevation values. This
active remote sensing technique is similar to radar but uses light
pulses instead of radio waves.
The laser uses its own energy
and surveys can be carried out
at any time of day or night.
Haze, cloud or foggy conditions
are not ideal and lead to data
anomalies.
ACTIVE vs PASSIVE
Remote Sensing
The laser transmitter emits a short pulse of coherent light in a
very narrow (monochromatic) wavelength band that travels to
the target and is reflected back. A very accurate clock is used
to measure the time difference between the transmitted
pulse and the return echo.
Distance=(Speed of light × Time taken for light to reflect) / 2.
Scanning the target by moving the laser records the threedimensional surface of the target as a mass or cloud of
individual points.
Two types of LiDAR used in forestry:
1. Airborne LiDAR (ALS)
- Data acquired using fixed-wing,
rotary aircraft or satellite.
- Point data is used to derive models.
2. Terrestrial LiDAR (TLS)
- Ground-based system used for forest inventory
work.
- Point data reflect direct measurements (basal area,
tree height and stem density) on the ground.
LiDAR pulse
• Small-footprint vs. largefootprint system;
• Refers to the size (diameter) of the
light beam. Small is usually 0.2m1.0m in diameter. Large can be up to
70m (ICEsat).
• “Continuous waveform”
vs. “discrete return”
systems;
Discrete Vs. Waveform
The transmitted lidar pulse is a coherent
waveform that could hit a solid object and be
reflected back in one coherent return. – PULSE A.
The waveform could also, be partially reflected
by leaves and branches near the top of a tree,
again be partially reflected by understory
vegetation, and finally be reflected by the ground
at the base of the tree – PULSE B.
DISCRETE - Commercial mapping lidar systems
are most often of the discrete-return type,
recording up to five reflections per transmitted
pulse.
WAVEFORM – records entire waveform of the
reflected pulse. Requires much more storage
and more complex data processing, most often
used in research applications to measure
detailed structure of vegetation canopy.
LiDAR Point Cloud Interpretation
Reporting minimum and maximum for all LAS point record
entries ...
X
136017726 136957032
Y
50580735 51236785
Z
97779 180239
intensity
0
255
return_number
1
5
number_of_returns 1
5
edge_of_flight_line 0
0
scan_direction_flag 0
1
classification 2
5
scan_angle_rank -28
33
number of first returns:
110,490,691
number of intermediate returns: 3,818,770
number of last returns:
110,449,287
number of single returns:
95,401,927
histogram of classification of points:
23,581,527 ground (2)
16,320,499 low vegetation (3)
4,453,570 medium vegetation (4)
Point Cloud-> A set of vertices in a three85,001,225 high vegetation (5)
dimensional coordinate system
Point densities…how high: 1,2…100?
Tree
3 points per m2:
8 points per
12 points per
m 2:
m 2:
Tree
Tree
Digital Elevation Model…
ALS DEM
TRIM DEM
Canopy Height Model…
1m CHM from 3pts/m2
2m CHM from 3pts/m2
LiDAR Enhanced Forest Inventory
• Provides a quantified description of 3-D canopy
structure
• Used to improve canopy heights, density, vertical
structure and for sampling applications
• Acquisition->Calibration->Modelling->Mapping
LiDAR Enhanced Forest Inventory Flow Chart
Lastools
Lastools/FUSION
R
ArcGIS
TFL18: Calibration
data – observed (field)
vs. predicted (ALS)
Forest Attribution…
Bimodal
Top loaded
LiDAR
Vertical Plot
Profiles
Stand Delineation & Species
Multi-resolution inventory systems both at the stand level
and sub-stand level are typical when using LiDAR outputs
With higher point densities & different ALS
wavelengths, software is available to identify
tree species based on crown 3D architecture
leading to individual tree inventories.
Inventory Enhancement
Height 5m
Height 10m
Canopy Height 1m
Inventory Enhancement:
• Consistent and measured
high-resolution data
• Scalability and,
• Data Preservation.
Height 15m
Canopy Height 25m
LiDAR Specifications
The following minimum specifications for forestry-related LiDAR acquisition have been
assembled using information from various sources (White et al. 2013; Reutebuch and
McGaughey 2008; Hopkinson 2007).
Acquisition Parameter
1.Laser beam divergence
Specification for Coast Forests
Narrow beam divergence
between 0.1-0.6 mrad. Influences
the size of the laser footprint on
the ground.
Specification for Interior Forests
Narrow beam divergence
between 0.1-0.6 mrad. Wider
beam reduces peak pulse power.
2.Scan angle
+/- 12 degrees from nadir.
+/- 15 degrees from nadir–
greater scan angles are permitted
in open canopies.
3.Pulse Repetition Frequency
Frequencies between 150kHz250kHz. Higher frequency results
in increases in noise and height
range data.
Frequencies between 50kHz150kHz are satisfactory. 500khz
are common in 2015.
4.Pulse Density
A minimum of 8 pulses per square
metre for stand-level canopy
models. Greater densities provide
an improved description of the
forest canopy
A minimum of 4 pulses per square
metre for stand-level canopy
models. Multiply X3 for individual
tree analysis.
5.Returns per pulse
Minimum four returns per pulse.
6.Swath overlap
Greater than 50% sidelap is
required to ensure the area is
covered with appropriate pulse
densities and multiple look angles.
Minimum two returns per pulse
(first and last return).
Greater than 50% sidelap.
Fixed-wing Acquisition Specifications
Merritt 2016 example…
Operating altitude
1,200m AGL
Target Groundspeed
160 knots
Total Line Length
57,145km
Field of View (FOV)
50 degrees
Pulse Rate
444.4kHz
Sidelap
50%
Average Point Density (w/ overlap)
~8.0/m²
Fundamental Vertical Accuracy (FVA)
15cm at 2 sigma
Cost is $1.64 per hectare
LiDAR Summary
• ACTIVE remote sensing technology that uses a
laser to measure distances to target points.
• Necessary requirements include; GPS, IMU
and high-precision clocks.
• LiDAR is an established & direct method for
collecting very dense and accurate 3dimensional point cloud information.
Technological mashup…2016
2007-house of cards
The making:
https://www.youtube.com/watch?v=cyQoTGdQ
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Video:
https://www.youtube.com/watch?v=8nTFjVm9s
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References
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https://www.e-education.psu.edu/geog481/l1_p4.html
http://www.lidar-uk.com/a-brief-history-of-lidar/
http://www.lasermap.com/laserM/en/doc02.htm
http://brianjohnsonaviation.weebly.com/brians-blog