Wednesday, July 8, 2015

Notes from Chapter 1 of Manual of Airborne Topographic Lidar

My Summer GIS independent study through George Mason University, GGS 698: Urban Mapping with LIDAR involves two parts:
  1. A theoretical part where I will independently complete the OER course materials from Penn State's GEOG 481: Topographic Mapping with Lidar course.
  2. A practical part where I will continue development of the Photovoltaic Viability Map.
In a previous post I documented the first steps in setting up the Photovoltaic Viability Map web application. In this post I'll summarize the first chapter of  the Manual of Airborne Topographic Lidar -- part of the first reading assignment for the course.

The book is published by the ASPRS, which is mentioned several times in the forward and chapter 1, without description of the organization of expansion of the acronym. A bit of googling revealed that it is the American Society for Photogrammetry and Remote Sensing, founded in 1934.

Chapter 1: Introduction

In chapter 1 editor Michael S. Renslow describes lidar (LIght Detection And Ranging) as a mature mapping technology that provides 3D information for the earth's surface including terrain surface models, vegetation characteristics, and man-made features.  Lidar is an active remote sensing technology that generates pulses of light and then detects their reflections, much like radar and sonar do with radio and sound waves respectively.

Lidar is particularly useful for modern mapping and GIS integration due to its data being natively 3D and georeferenced.  The point cloud data generated by lidar systems can be processed for a wide variety of uses, including feature mapping, vegetation mapping, transportation mapping, transmission corridor mapping, 3D building models, natural hazard detection, and natural disaster evaluation.

Lidar is quite accurate, Renslow states, with between 5 and 15 cm vertical accuracy, and it can be processed rapidly to produce imagery for spatial referencing. It is a fairly recent technology, being developed originally by the defense industry in the early 1990s, and it has improved rapidly since then from a repetition rate of 5,000 to 10,000 pulses per second in the early systems to between 150,000 and 400,000 pulses per second returning up to 10 to 12 points per square meter in current systems.

Lidar technology progressed for over a decade without specific data standards, but standardization efforts now include the ASPRS LAS data standard for lidar data classification and the USGS's Base Lidar Specification for digital elevation model production.  Renslow concludes page 1 by mentioning two emerging lidar technologies that will be discussed in the book: FLASH Lidar and Gieger Mode Lidar.

The remainder of the first chapter is a list of commonly used lidar terms.

Commonly Used Lidar Terms

Airborne Laser Scanning (ALS)
a synonym for lidar, also referred to as laser altimetry. Comprised of a Direct Georeferencing System (often called a Position Orientation System, or POS) to accurately determing the position and orientation of the ALS platform, and the Laser Scanner System to emit and receive laser pulses.
Along Track Resolution
the spacing of pulses from the system in the platform's flight direction.
Detectable surface remnants of elevated features in a bare earth elevation model.
Attitude (pitch, roll, and yaw)
pitch: vertical rotation of the aircraft (nose up, nose down) roll: rotation around the flight vector (wing up, wing down) yaw: horizontal rotation (nose left, nose right)
Avalanche Photodiode (APD)
converts light signal to electronic voltage pulse.
electromagnetic energy reflected back toward its source by terrain or atmospheric particles.
Base Station
GPS or GNSS receiver at an accurately-known fixed location used to derive information for the onboard GNSS receiver. Base station receiver records raw GNSS data which are applied during processing of raw lidar data.
Bare Earth
digital elevation of the terrain free of vegetation, buildings, and other man-made structures.
Beam Divergence
increase in diameter of a beam with distance from the aperture. Typically measured in milliradians (mrad). Higher frequency beams generally have lower divergence.
calibration of sensor system equipped with IMU to determine attitude of active sensor pulses.
Canopy Height Model (CHM)
representation of difference between top of canopy surface and underlying ground topography derived by filtering and classifying lidar point clouds to separate ground from canopy hits.
light that does not disperse and thus has low beam divergence. Lidar laser beams are highly collimated. Collimated rays are described as parallel and focused at infinity.
Cross Track Resolution
spacing of pulses from lidar system in scanning direction (perpendicular to direction platform is moving).
Direct Georeferencing
direct measurement of position (x-y-z coordinates) and attitude (roll-pitch-yaw) of sensor to determine position and orientation.
each return from emitted laser pulse in a multiple-pulse-return laser scanning system (first, intermediate..., last).
Globalnaya navigasionnaya sputnikovaya sistema, a radio-based satellite navigation system operated by Russian Space Forces.
Global Navigation Satellite System, a satellite navigation system with global coverage.
Inertial Measurement Unit (IMU)
monitors angular accelerations (using accelerometers) and rotations (using gyroscopes) of aircraft (and sensor) with respect to attitude. Integrating these measurements with time enables determining precise orientation of sensor platform over time. Typically recorded at 50 Hz to 200 Hz.
Lever Arm Offsets
perpendicular distance from axis of rotation to line of action of force. Measured precisely between ALS system components to enable integration of information from all system subcomponents during postprocessing.
Near Infra-red (NIR)
wavelenghts between 800 nm and 2500 nm. Majority of ALS sensors use NIR wavelengths of 1000nm, 1047nm, 1064nm, and 1550nm due to availability of stable lasing materials, reflectivity of natural surfaces, low signal-to-noise ration in sunlight, and eye-safe nature of these wavelengths.
Nominal Point Spacing (NPS)
important depending on specific application because lidar senors are random sampling systems. Lidar technology is based on random distribution of mass points that result in point cloud of data. Flight planning process can design project to estimate NPS and density of resulting data set.
Point Cloud
cluster of points that comprise lidar data achieved from reflections of laser beams off various landscape features, thus representing feature height and 3D relationship between features.
Point Dropout
laser pulses for which no energy was returned to the sensor. Can occur if aircraft (sensor) is too high, surface material is absorbing, or ground level energy is reflected away from sensor.
Positioning Orientation System (POS)
accurately determines position and orientation of ALS platform in 3D space at time of pulse emission and reception. Comprised of Differential GPS System (DGPS) to determine geographic position of platform within 5 cm of true trajectory, and IMU to measure orientation changes through time.
Point Spacing
average ground distance between successive pulse returns.
Pulse Footprint
area of ground intersected by laser pulse. It is a function of range, angle of incidence, and beam divergence.
Pulse Footprint Smearing
elongation of pulse footprint caused when laser beam is reflected from sloped terrain (especially away from center of scan) causing increaded uncertainty of horizontal and vertical position.
Pulse Repetition Frequency (PRF)
frequency of transmitted laser pulses (i.e. points per second). High PRF enables dense point-spacing providing high-resolution representations of landscape.
Pulse Return
laser pulses reflected off surfaces below the sensor and received by the lidar sensor. First pulse returns measure range to first surface encountered (i.e. vegetation, canopy, building roofs); last pulse returns measure to last surface encountered (i.e. ground).
Pulse Return Intensity
reflective NIR intensity of pulse returns can be measured by most ALS systems and provide improved discrimination and classification of scanned features. Intensity data is a function of pulse range, pulse footprint size, angle of incidence at point of return, and spectral characteristics of encountered surface.
distance between laser aperture and detected object or surface.
Repetition Rate
pulses-per-second of laser denoted in KHz. A 200KHz system indicates lidar system will pulse 200,000 times per second, which means the receiver can (but usually does not) receive information from each of these 200,000 pulses.
Scan Angle
half the angle of full sweep of a scanning mirror scanner. Large scan angles are generally not chosen at high altitudes due to high dropout rates, increased error, and obstruction shadowing at edges of scan. Typically do not exceed 30 degrees.
Scan Rate
frequency of cross-track sweeps of a mirror scanner in Hz.
Swath Width
width of survey area covered by complete sweep of scanner. Related to flying height and scanner angle.
Time Interval Meter (TIM)
method used to time-stamp the pulse transmission and reception points to determine time difference between pulse transmission and reception and the resulting range and position of reflective surface.


Renslow, Michael S. Manual of Airborne Topographic Lidar. Bethesda, MD.: American Society for Photogrammetry Remote Sensing, 2012. Print.

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