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FOR 274: Forest Measurements and Inventory
Lecture 24: Introduction to Lidar
• Overview
• Digital Elevation Models
• Canopy Heights
• Sources of Uncertainty
Lidar stands for Light Detection and Ranging
A Lidar system is essentially a Laser Rangefinder that hasbeen strapped to the belly of an airplane. This laser shootspulses of light towards the ground.
Lidar: Overview
The time for the light to travel toand from the target is used todetermine distance:
Distance = Speed x time
This distance and the position ofthe airplane is used to getelevation and location.
Footprint fromlaser pulse
1 return
2 return
Each pulse of laser light contains a largenumber of light pulses.
Of those sent down by the laser systemvery few make it all the way back to thesensor. Those that do are called returns.
Lidar: Overview
3 return
The 1st return to make it back to thesensor might be a tree top, while the lastreturn could represent the ground.
IMPORTANT NOTES:
Where no tree exists, the 1st could alsobe the LAST return.
The Last return might NOT be theground.
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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
Lidar: Overview
Lidar footprint Height x divergenceThe 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)
Lidar: Overview
Low Divergence:
Canopy penetration and some pulses will reach the ground
Lidar: Overview
Source Lefsky (2005)
High Divergence:
Reduced canopy penetration and low percentage of pulses hitting and RETURNING from the ground
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Low Spacing:
Canopy penetration and some pulses will reach the ground
Lidar: Overview
Source Lefsky (2005)
High spacing:
Less pulses hitting and RETURNING from the ground
What Lidar data Looks LikeWhat Lidar data Looks Like
Millions of X,Y,Z points called a Lidar Point Cloud
Lidar: Overview
Area is approximately: 1 X 0.75mi. Area is approximately: 1 X 0.75mi. includes ~ 440,000 returnsincludes ~ 440,000 returns
Delete all points except the last return, which hopefully is from the ground (or near to it).
There are a couple of ways people do this. The easiest is with the Block Minimum method
Th Lid i t di id d i t id ll d th l t
Lidar: Digital Elevation Models
The Lidar points are divided into grid cells and the lowest point is chosen as the ground.
z
Image source: Jeffery Evans (RMRS)
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Slope thresholdparameter based on slope
z
Lidar: Digital Elevation Models
Block MinimumMinimum Z in bin
z
zMorphologyParameter based on surface morphology
Image source: Jeffery Evans (RMRS)
This process is repeated until the surface stops changing:
Lidar: Digital Elevation Models
Image source: Jeffery Evans (RMRS)
From Vosselman, (2000
Lidar: Digital Elevation Models
Filter in urban area
Building Footprint Remnant vegetation
Image source: Jeffery Evans (RMRS)
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Block Minimum:In high biomass forest fail to address areas with no ground returns
Lidar: Digital Elevation Models
Block Minimum 6 meter bin size
g
Disconnects the scale of ground from the scale of vegetation
Image source: Jeffery Evans (RMRS)
From Zang et.al, (2000
Lidar: Digital Elevation Models
Filter in forested areaShrub and understory
Image source: Jeffery Evans (RMRS)
Lidar: Digital Elevation Models
PCF in high biomass setting Image source: Jeffery Evans (RMRS)
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C
Subtract the surface produced from the ground returns from a surface produced from the non-ground returns
Lidar: Canopy Heights
Ground Returns
Canopy Returns
‘minus’
Lidar: Canopy Heights
Image source: H-E Anderson (USDA FS-PNW)
Interpolation error(ground)
Minimal error(ground)
Lidar: Sources of Uncertainty
Scale error(over smoothing ground) Density, geometry, and measuremen
(Non-ground)
Image source: Jeffery Evans (RMRS)
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High scan angles can distort theLiDAR footprint:
Scan angle interacts with slope toadd error by increasing distance
Lidar: Sources of Uncertainty
Scan Angle
FootprintFootprint
Image source: Jeffery Evans (RMRS)
> 15•
Subset data to< 15• scan angle
< 15•
Footprint distortion with high scan angles
Lidar: Sources of Uncertainty