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

2. 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?

3. 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?

4. Creation of the Lidar pulse Sensor/ Detector D Lidar: What is it?

5. Steps in the Lidar Process: Creation of the Lidar pulse Lidar pulse travels to the target Sensor/ Detector D Lidar: What is it?

6. Steps in the Lidar Process: Creation of the Lidar pulse Lidar pulse travels to the target Interaction with the target Sensor/ Detector D Lidar: What is it?

7. Steps in the Lidar Process: Creation of the Lidar pulse Lidar pulse travels to the target Interaction with the target Lidar pulse travels back to sensor Sensor/ Detector D Lidar: What is it?

8. Steps in the Lidar Process: Creation of the Lidar pulse Lidar pulse travels to the target Interaction with the target Lidar pulse travels back to sensor Sensor processes return signal Sensor/ Detector D Lidar: What is it?

9. 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?

10. 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

11. 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

12. Coherence – A physics terms stating that the laser light is in phase SOURCE: http://repairfaq.ece.drexel.edu/sam/CORD/leot/course01_mod01/mod01-01.html Incoherent Light Waves: They Don’t Match Up Coherent Light Waves: They Match Up LASER: Properties

13. 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

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

15. 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: E P = E 3 -E 2 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

16. Source Lefsky (2005) LASER: Properties

17. How does a LASER produce light? http://repairfaq.ece.drexel.edu/sam/CORD/leot/course01_mo d01/mod01-01.html http://repairfaq.ece.drexel.edu/sam/CORD/leot/course01_mo d01/mod01-01.html How do we produce the Intense LASER light? http://www.micro.magnet.fsu.edu/primer/java/lasers/heliumne onlaser/ Lidar: What is it?

18. 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 Source Lefsky (2005) Lidar: The Pulse

19. Source Lefsky (2005) Lidar: The Pulse

20. Source Lefsky (2005) 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

21. Source Lefsky (2005) Low Spacing: Canopy penetration and some pulses will reach the ground High spacing: Less pulses hitting and RETURNING from the ground Lidar: The Pulse

22. Source Lefsky (2005) Two main types: Waveform Sampling Discrete Return Lidar: The Main Kinds

23. 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 1 st return might be a tree top, while the last return could be from the ground. It is important to note that: The 1 st could also be the last return. The Last return might not be the ground. Lidar: What is it?

24. Source Blair and Harding NASA GSFC Lidar: The Main Kinds

25. 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) Lidar: The Main Kinds

26. 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 Lidar: The Main Kinds

27. Area is approximately: 1 X 0.75mi. includes ~ 440,000 returns Lidar: What the Data Looks Like

28. Lidar: Geomorphologic Applications Utilities map power lines for signs of damage: Volume change in open pit mines Landslide Detection

29. Lidar: Mapping Fault Lines Image Source: Puget Sound LiDAR Consortium

30. Lidar: Riparian and Coastal Ecology

31. Lidar: Underwater DEMs for Coastal Mapping

32. Image source: H-E Anderson Lidar: Forestry

33. Image source: MJ Falkowski Lidar: Forestry

34. 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)

35. 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 Source: Evans et al PE&RS (in review)

36. 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 LiDAR: Data Acquisition Standards for Forestry Source: Evans et al PE&RS (in review)

37. 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° LiDAR: Data Acquisition Standards for Forestry Source: Evans et al PE&RS (in review)

38. Flight Line Overlap: This ensures features are well “sampled”. Ask for a flight line overlap of 50% LiDAR: Data Acquisition Standards for Forestry 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)

39. 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 time geodetic surveys (GPS and total stations) Source: Evans et al PE&RS (in review)

40. 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 LiDAR: Data Acquisition Standards for Forestry Source: Evans et al PE&RS (in review)

41. FOR 474: Forest Inventory Next Time … Using LiDAR data to produce DEMs