for 474: forest inventory introduction to lidar what is it? how does it work? lidar jargon and terms...

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 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.

Lidar: What is it?

Lidar – Light Detection and Ranging

The Basic Lidar Concept:

Speed = distance/time

d = c*t

d = Distance (meters)

t = time (seconds)

c = speed of light

Sensor/Detector

D

Lidar: What is it?

Creation of the Lidar pulseSensor/Detector

D

Lidar: What is it?

Steps in the Lidar Process:

Creation of the Lidar pulse

Lidar pulse travels to the target

Sensor/Detector

D

Lidar: What is it?




Interaction with the target

Sensor/Detector

D

Lidar: What is it?





Lidar pulse travels back to sensor

Sensor/Detector

D

Lidar: What is it?





Lidar pulse travels back to sensor

Sensor processes return signal

Sensor/Detector

D

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 < 1mm per m)

Coherence – A physics terms stating that the laser light is in phase

Lidar: What is it?

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/leot/course01_mod01/mod01-01.html

LASER: Properties

Directionality – A laser travels in a straight line. (The laser light spreads out at < 1mm per m)

NOTE: Perfectly parallel (collimated) light cannot be produced.

The SUN emits light in an unidirectional manner – The light spreads out equally in all directions.

LASER

LASER: Properties

Coherence – A physics terms stating that the laser light is in phase


Incoherent Light Waves:

They Don’t Match Up

Coherent Light Waves:

They Match Up

LASER: Properties

How to Produce Laser Light

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.

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


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:

EP = E3-E2

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:

This is stimulated emission.

LASER: Properties

Source Lefsky (2005)

LASER: Properties

How does a LASER produce light?

http://repairfaq.ece.drexel.edu/sam/CORD/leot/course01_mod01/mod01-01.html

How do we produce the Intense LASER light?

http://www.micro.magnet.fsu.edu/primer/java/lasers/heliumneonlaser/

Lidar: What is it?





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


Lidar: The Pulse


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

Lidar: The Pulse


Low Spacing:

Canopy penetration and some pulses will reach the ground

High spacing:

Less pulses hitting and RETURNING from the ground

Lidar: The Pulse


Two main types:

Waveform Sampling

Discrete Return

Lidar: The Main Kinds

1 return

2 return

3 return

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.

Lidar: What is it?

Source Blair and Harding NASA GSFC


Source David Harding NASA GSFC

Sometimes the last return can be a ‘false summit’ in the signal.

This results in noise in the resultant height data (seen as dips and peaks within the lidar image)


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


Area is approximately: 1 X 0.75mi. Area is approximately: 1 X 0.75mi. includes ~ 440,000 returnsincludes ~ 440,000 returns

Lidar: What the Data Looks Like

Lidar: Geomorphologic Applications

Utilities map power lines for signs of damage:Utilities map power lines for signs of damage:

Volume change in open pit minesVolume change in open pit minesLandslide DetectionLandslide Detection

Lidar: Mapping Fault Lines

Image Source: Image Source: Puget Sound LiDAR ConsortiumPuget Sound LiDAR Consortium

Lidar: Riparian and Coastal Ecology

Lidar: Underwater DEMs for Coastal Mapping

Image source: H-E Anderson

Lidar: Forestry

Image source: MJ Falkowski

Lidar: Forestry

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 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 return

3 return

Number of Returns:

When using Discrete Return LiDAR ask for at least 3 returns per laser pulse


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



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°



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?


Accuracy Standards:

Vertical Root Mean Square Error < 15cm

Horizontal Root Mean Square Error < 55cm


The vendor should calculate errors using real time geodetic surveys (GPS and total stations)


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



FOR 474: Forest Inventory

Next Time …

Using LiDAR data to produce DEMs

for 474: forest inventory introduction to lidar what is it? how does it work? lidar jargon and terms...

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