1 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 Li ght D etection a nd R anging Essentially a laser rangefinder that has been strapped to the belly of an airplane. The time for the light to travel to Lidar: What is it? 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. Lidar – Light Detection and Ranging The Basic Lidar Concept: Speed = distance/time d *t Sensor/ Detector D Lidar: What is it? d = c*t d = Distance (meters) t = time (seconds) c = speed of light D
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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
Light Detection and Ranging
Essentially a laser rangefinder that has beenstrapped to the belly of an airplane.
The time for the light to travel to
Lidar: What is it?
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 ofthe airplane is used to getelevation and location.
Lidar – Light Detection and Ranging
The Basic Lidar Concept:
Speed = distance/time
d *t
Sensor/Detector
D
Lidar: What is it?
d = c*t
d = Distance (meters)
t = time (seconds)
c = speed of light
D
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Creation of the Lidar pulseSensor/Detector
D
Lidar: What is it?
D
Steps in the Lidar Process:
Creation of the Lidar pulse
Lidar pulse travels to the target
Sensor/Detector
D
Lidar: What is it?
D
Steps in the Lidar Process:
Creation of the Lidar pulse
Lidar pulse travels to the target
I t ti ith th t t
Sensor/Detector
D
Lidar: What is it?
Interaction with the target D
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Steps in the Lidar Process:
Creation of the Lidar pulse
Lidar pulse travels to the target
I t ti ith th t t
Sensor/Detector
D
Lidar: What is it?
Interaction with the target
Lidar pulse travels back to sensor
D
Steps in the Lidar Process:
Creation of the Lidar pulse
Lidar pulse travels to the target
I t ti ith th t t
Sensor/Detector
D
Lidar: What is it?
Interaction with the target
Lidar pulse travels back to sensor
Sensor processes return signal
D
Lidar uses LASER light
LASER: Light Amplification by Stimulated Emission of Radiation
Properties of LASERS:
Lidar: What is it?
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 < 1mm per m)
Coherence – A physics terms stating that the laser light is in phase
Although the use of LiDAR is widespreadin forestry people are inconsistent on howthey collect the data
If we want to compare measurementsbetween different areas we need the databetween different areas we need the datato be collected using standard properties
One day you may be asked to get a LiDARacquisition for your forest: so its importantthat you know what to ask for!
Source: Evans et al PE&RS (in review)
Pulse Repetition Frequency (number of pulses per second):
This should be high enough so that the pulses are well-distributed vertically throughout the canopy
LiDAR: Data Acquisition Standards for Forestry
1 return
2 returnNumber of Returns:
3 return
Number of Returns:
When using Discrete Return LiDAR ask for at least 3 returns per laser pulse
Source: Evans et al PE&RS (in review)
Post-Spacing
Average horizontal spacing between pulses (may be multiple returns per pulse).
Ask for a maximum post spacing of 70 cm
LiDAR: Data Acquisition Standards for Forestry
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 PE&RS (in review)
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Scan-Angle:
The maximum off-nadir angle the sensor head swings to. High scan angles can distort the LiDAR footprint (worse on slope).
LiDAR: Data Acquisition Standards for Forestry
Ask for a maximum scan-angle of 12°
The total view angle is then 24°
Source: Evans et al PE&RS (in review)
Flight Line Overlap:
This ensures features are well “sampled”.
Ask for a flight line overlap of 50%
LiDAR: Data Acquisition Standards for Forestry
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 PE&RS (in review)
Accuracy Standards:
Vertical Root Mean Square Error < 15cm
Horizontal Root Mean Square Error < 55cm
LiDAR: Data Acquisition Standards for Forestry
The vendor should calculate errors using real timeThe vendor should calculate errors using real time geodetic surveys (GPS and total stations)
Source: Evans et al PE&RS (in review)
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Typical Lidar Products to ask for:
Ground Surface Model (Digital Elevation Model)
Digital Surface Model (surface of all non ground returns)