for 274: forest measurements and inventory · 2013-02-20 · image source: jeffery evans (rmrs)...

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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