LiDAR Basics By Christopher Butson Forest Analysis & Inventory Branch June 28, 2016

Presentation Outline What is LiDAR? How Does LiDAR Work? Terrestrial vs. Aerial Discrete vs. Full Waveform Point clouds & Density LiDAR Enhanced Forest Inventory Sensor Specifications and Acquisition Parameters Summary

What is(n’t) LiDAR? RAdio Detection And Ranging-> RADAR 1886 – Heinrich Hertz showed that radio waves could be reflected by solid objects. SOund Navigation And Ranging -> SONAR After the Titanic disaster (1912) the world’s first patent for an underwater echo ranging device was filed by English meteorologist Lewis Richardson. During the 70’s lidar was used to measure clouds and aerosols in the amtosphere 19080’s lidar used to measure distance to the moon Light Detection And Ranging or Light RADAR -> LiDAR 1971 - First LiDAR system used during the Apollo15 mission to map the surface of the moon.

What is LiDAR con’t? It is an ACTIVE remote sensing technology that uses a laser to measure distances to target points. Because laser light has a much shorter wavelength it is possible to accurately measure much smaller objects, such as aerosols, cloud particles and molecules 1980’s - Stuttgart University proved the high geometric accuracy of a laser profiling system HOWEVER, the lack of a reliable commercial GPS/IMU for sensor positioning presented a significant roadblock

Topographic tools up to 1990… Stereo Photogrammetry (passive)- inferential technology, the features must be “seen” to be mapped using parallax (displacement in apparent position of an object when viewed from two different lines of sight). RADAR (active) Longer wavelength microwave radiation can penetrate through cloud cover, haze, dust, and all but the heaviest rainfall certain limitations for measuring ground elevations beneath forest canopy and, peculiar artifacts in very steep terrain. © Department of Natural Resources Canada. All rights reserved

1990’s… Demand for GPS/IMU systems for use in aerial photogrammetry spurred rapid development The US-DOD GPS satellite constellation reached full configuration needed for widespread operations. High-accuracy inertial measurement units became available as certain military missile guidance systems were declassified By the mid-1990s, laser scanner manufacturers were delivering LiDAR sensors capable of 2,000 to 25,000 pulses per second. 550,000 pulses per second are now common By 2005, laser scanner manufacturers were delivering LiDAR sensors capable of 250,000 pulses per second. http://brianjohnsonaviation.weebly.com/brians-blog

Four (not so) basic needs… Sensor 2. GPS 3. Inertial Measurement Unit (IMU) 4. High precision Clocks

LiDAR Key Benefits… 1. Acquisition similar to aerial photography- The ground coverage of an airborne lidar sensor is very similar to that of a traditional aerial camera, so photogrammetric methods of flight planning could be directly applied to lidar. 2. Lidar is capable of "seeing" between trees in forested areas - Similar data processing, the development of end products from the dense lidar mass points is much like photogrammetric data processing. 3. Lidar presented fast, accurate, and direct (not inferential) data collection Generating 3-dimensional data point clouds. As the cost of instruments and services stabilized, it quickly became a very attractive mapping solution.

Light Detection & Ranging (LiDAR) LiDAR (Light Detection And Ranging) has become an established method for collecting very dense and accurate elevation values. This active remote sensing technique is similar to radar but uses light pulses instead of radio waves. The laser uses its own energy and surveys can be carried out at any time of day or night. Haze, cloud or foggy conditions are not ideal and lead to data anomalies.

ACTIVE vs PASSIVE Remote Sensing The laser transmitter emits a short pulse of coherent light in a very narrow (monochromatic) wavelength band that travels to the target and is reflected back. A very accurate clock is used to measure the time difference between the transmitted pulse and the return echo. Scanning the target by moving the laser records the three-dimensional surface of the target as a mass or cloud of individual points. Distance=(Speed of light × Time taken for light to reflect) / 2.

Two types of LiDAR used in forestry: Airborne LiDAR (ALS) Data acquired using fixed-wing, rotary aircraft or satellite. Point data is used to derive models. Terrestrial LiDAR (TLS) Ground-based system used for forest inventory work. Point data reflect direct measurements (basal area, tree height and stem density) on the ground.

LiDAR pulse Small-footprint vs. large-footprint system; Refers to the size (diameter) of the light beam. Small is usually 0.2m- 1.0m in diameter. Large can be up to 70m (ICEsat). “Continuous waveform” vs. “discrete return” systems;

Discrete Vs. Waveform The transmitted lidar pulse is a coherent waveform that could hit a solid object and be reflected back in one coherent return. – PULSE A. The waveform could also, be partially reflected by leaves and branches near the top of a tree, again be partially reflected by understory vegetation, and finally be reflected by the ground at the base of the tree – PULSE B. DISCRETE - Commercial mapping lidar systems are most often of the discrete-return type, recording up to five reflections per transmitted pulse. WAVEFORM – records entire waveform of the reflected pulse. Requires much more storage and more complex data processing, most often used in research applications to measure detailed structure of vegetation canopy.

LiDAR Point Cloud Interpretation Reporting minimum and maximum for all LAS point record entries ... X 136017726 136957032 Y 50580735 51236785 Z 97779 180239 intensity 0 255 return_number 1 5 number_of_returns 1 5 edge_of_flight_line 0 0 scan_direction_flag 0 1 classification 2 5 scan_angle_rank -28 33 number of first returns: 110,490,691 number of intermediate returns: 3,818,770 number of last returns: 110,449,287 number of single returns: 95,401,927 histogram of classification of points: 23,581,527 ground (2) 16,320,499 low vegetation (3) 4,453,570 medium vegetation (4) 85,001,225 high vegetation (5) Point Cloud-> A set of vertices in a three-dimensional coordinate system

Point densities…how high: 1,2…100? Tree 3 points per m2: Tree 8 points per m2: Point densities on DEM and CHM creation – need to see the ground! Point cloud degrading does not impact forest metrics down to 1pt/m2. Maximum canopy height is sensitive to reduced point clouds. Tree 12 points per m2:

Digital Elevation Model… ALS DEM TRIM DEM

Canopy Height Model… 1m CHM from 3pts/m2 2m CHM from 3pts/m2

LiDAR Enhanced Forest Inventory Provides a quantified description of 3-D canopy structure Used to improve canopy heights, density, vertical structure and for sampling applications Acquisition->Calibration->Modelling->Mapping

LiDAR Enhanced Forest Inventory Flow Chart Lastools Lastools/FUSION R ArcGIS

TFL18: Calibration data – observed (field) vs. predicted (ALS)

Note axis are reversed from previous slide

Forest Attribution…

LiDAR Vertical Plot Profiles Bimodal Top loaded These graphics show a comparison of ground plots and associated lidar point clouds. On the left are height/diameter distributions from tree measured in the field H y-axis, DBH on x-axis Next to those are vertical height distributions from the laser data. These distributions in some cases are: bimodal showing 2 unique height distributions or Top-loaded where most of the heights are in the upper canopy The bottom shows a normal distribution in an OG Hemlock plot with the majority of trees measured at heights around 25m mixed spruce Mixed hemlock OG hemlock

Stand Delineation & Species Multi-resolution inventory systems both at the stand level and sub-stand level are typical when using LiDAR outputs New paper in iEEE transactions on geoscience and remote sensing Wang et al. show 2pts/m2 is ok for dominant tree detection. 8 pts is better at penetrating the canopy to capture surpressed and intermediate trees. With higher point densities & different ALS wavelengths, software is available to identify tree species based on crown 3D architecture leading to individual tree inventories.

Inventory Enhancement Height 5m Height 10m Height 15m Canopy Height 1m We are currently developing standards around integrating ALS data into our current VRI framework. It is the hope for future projects that we will have the capacity to not only provide the generalized ALS data in VRI form but also preserve the finer resolution ALS data making it available to inventory users as well. Canopy Height 25m Inventory Enhancement: Consistent and measured high-resolution data Scalability and, Data Preservation.

LiDAR Specifications The following minimum specifications for forestry-related LiDAR acquisition have been assembled using information from various sources (White et al. 2013; Reutebuch and McGaughey 2008; Hopkinson 2007). Acquisition Parameter Specification for Coast Forests Specification for Interior Forests 1.Laser beam divergence Narrow beam divergence between 0.1-0.6 mrad. Influences the size of the laser footprint on the ground.   Narrow beam divergence between 0.1-0.6 mrad. Wider beam reduces peak pulse power. 2.Scan angle +/- 12 degrees from nadir. +/- 15 degrees from nadir– greater scan angles are permitted in open canopies. 3.Pulse Repetition Frequency Frequencies between 150kHz-250kHz. Higher frequency results in increases in noise and height range data. Frequencies between 50kHz-150kHz are satisfactory. 500khz are common in 2015. 4.Pulse Density A minimum of 8 pulses per square metre for stand-level canopy models. Greater densities provide an improved description of the forest canopy A minimum of 4 pulses per square metre for stand-level canopy models. Multiply X3 for individual tree analysis. 5.Returns per pulse Minimum four returns per pulse. Minimum two returns per pulse (first and last return). 6.Swath overlap Greater than 50% sidelap is required to ensure the area is covered with appropriate pulse densities and multiple look angles. Greater than 50% sidelap.

Fixed-wing Acquisition Specifications Merritt 2016 example… Operating altitude 1,200m AGL Target Groundspeed 160 knots Total Line Length 57,145km Field of View (FOV) 50 degrees Pulse Rate 444.4kHz Sidelap 50% Average Point Density (w/ overlap) ~8.0/m² Fundamental Vertical Accuracy (FVA) 15cm at 2 sigma Cost is $1.64 per hectare

LiDAR Summary ACTIVE remote sensing technology that uses a laser to measure distances to target points. Necessary requirements include; GPS, IMU and high-precision clocks. LiDAR is an established & direct method for collecting very dense and accurate 3-dimensional point cloud information.

Technological mashup…2016

2007-house of cards The making: https://www.youtube.com/watch?v=cyQoTGdQywY Video: https://www.youtube.com/watch?v=8nTFjVm9sTQ

References https://www.e-education.psu.edu/geog481/l1_p4.html http://www.lidar-uk.com/a-brief-history-of-lidar/ http://www.lasermap.com/laserM/en/doc02.htm http://brianjohnsonaviation.weebly.com/brians-blog