ALS

• LiDAR (LIght Detection And Ranging) is a remote sensing technology that emits focused beams of light and measures the time it takes for the reflections to be detected by the sensor to compute ranges, or distances, to objects.

Computing 3D Coordinates

• The target objects' three-dimensional coordinates (x, y, z or latitude, longitude, and elevation) are computed as a cloud of points
• The computations are derived from three main data points:
  • The time difference between the emission and return of a laser pulse determines the distance (range) between the laser and the point on the ground
  • The angle at which the pulse was "fired"
  • The absolute location of the sensor on the Earth's surface

Preliminary Basic Consideration About Z Coordinates

• Maps use 2D (East, North) or 2.5D (E, N, Z) coordinate systems
• True 3D models represent the real world
• Maps usually display 2D coordinates, and may include height for some objects, creating a 2.5D representation
• 3D models can have multiple Z values for each North, East coordinate pair

ALS: Airborne Laser Scanning Systems

• A key application of ALS is to create precise 3D representations of terrain
• Digital elevation models (DEMs) that are widely available are often too coarse for small geomorphic features and processes
• LiDAR/ALS data makes DEMs possible at previously unattainable resolutions, even sub-meter resolutions
• Applicable to:
  • Geomorphology
  • Landslide & flood hazards
  • Forestry/Ecology
  • Civil Engineering
  • Urban planning
  • Volcanology
• Has become widely available in geosciences in the past 10-15 years

ALS System Components

• Laser scanner, transmitter, and receiver calculate the distance (range) between the emitter and the ground point hit by the laser beam
• GNSS (Global Navigation Satellite System) receiver determines the antenna's position after processing; which gives location of the laser sensor, requires a GNSS master station
• IMU/INS determines the attitude of the aircraft/laser system, reporting pitch, roll, and yaw angles
• RGB camera (optional)

Aerial LiDAR Platforms and Application Diversity

• Typical flight parameters:
  • 700-1200 m flight height
  • 30,000 points/second
• Ground samples can achieve multiple points per square meter
• Ground point position accuracy around ~15 cm
• Acquisition costs approximately $300 - $500 per sq. km
• Diverse applications include:
  • Wide-area mapping
  • Engineering-grade surveys
  • Corridor mapping

History of LiDAR

• 1970s: Development began at NASA
• 1980s: GPS was incorporated
• 1990s: Commercial LiDAR became available
• 2000s: included:
  • GPS positional accuracy increases (cm level accuracy)
  • INS orientation accuracy increases (Pitch/roll accuracy ≈ 0.005°)
  • Incorporation of LiDAR with other technologies (digital camera, signal intensity)
  • Multiple pulses emergence
  • High-end systems by Riegl, Leica, and Optech

The Accuracy of the Sensors

• LiDAR instrument components have different precisions
• Typical laser sensor accuracies:
  • 1-5 cm range accuracy
  • 2-5 cm GNSS accuracy
  • 0.01° scan angle accuracy
  • <0.005° INS accuracy for pitch/roll
  • <0.008° heading
  • 0.25 to 5 mrad beam divergence
• Data achieves vertical accuracies on the order of 5 to 15 cm and horizontal accuracies of 15-50 cm at one sigma

LASER: Light Amplification by Stimulated Emission of Radiation

• Active sensor, transforms energy in electromagnetic radiation for object detection; it uses its own radiation reflection
• Beam characteristics: monochromatic, coherent, small divergence
• Small divergence allows beam transport and energy concentration with optical systems

Safety and Laser Classification

• CLASS 1: Unable to produce damaging radiation under normal operation; e.g., laser printer, CD-Rom
• CLASS 2: Emits visible light; eye protection from defense reaction; not hazardous if viewed less than 0.25 second; maximum power is 1 mW; e.g., barcode scanner
• CLASS 3a: Probably not hazardous if viewed briefly; hazardous if viewed with collecting optics; maximum power is 5mW; some require DANGER labels; e.g., laser pointer
• CLASS 3b: Hazardous if viewed directly or by specular reflection; diffuse reflection not usually hazardous; upper limit is 0.5W.
• CLASS 5: Exceeds 0.5W; hazardous under all viewing conditions; not suitable for surveying

Techniques for Distance Measurement

• In range meausrements with laser, two major ranging principles are applied:
  • The pulsed ranging principle
  • Measuring phase differences between transmitted signals and backscattered signals for ranging
• Pulse ranging systems are the most common in the ALS market

Distance Measurement by Pulse Ranging (Time of Flight, t.o.f.)

• High precision clocks are critical; 1 mm accuracy requires measuring a time delay of 3.33 picoseconds
• Pulse Characteristics of Common Sensors:
  • Repetition rates: 4 - 83 Khz
  • Pulse durations: 6-12 nanoseconds
  • Pulse widths: 1.8-3.6 m
• Light travels ~30 cm in 1 ns

Footprint and Posting Density

• Illuminated Footprint: laser beam's diameter upon ground impact, diverges slightly
• Posting density depends on: laser pulse rate, flying height, flight speed, scan angle

Swath Width

• Swath width hinges on flight altitude and laser beam deflection angle, calculated as L = 2H tan γ

Systems Measuring Multiple Echoes

• Surveying forested areas uses the possibility generated by the amplitude to generate multiple reflections at different times

General Characteristics of LiDAR

• Signal wavelength in near infrared (or blue-green)
• Pulses at high frequency and power with narrow solid angles, which provides lower power dissipation and higher resolution
• Narrow spectral width (filters remove background radiation)
• Operates day or night, unaffected by shadows, but clouds must be above the airplane
• Distance measurement is range, derived from time, recording 50000 to 200000 measures per second
• Footprint size varies with system type and flight altitude, from 10 cm to 25 m, strip width from 50 m to 9 km
• Intensity is the amount of energy reflected, yielding a reflectance image co-registered to the cloud of X,Y,Z points

Reflectivity and Maximum Laser Range

• Target reflectivity and laser wavelength affect maximum range
• Maximum range specifications should detail target type, reflection (diffuse or specular), and reflectivity percentage
• Wavelength impacts a survey's scope
• 1535 nm lasers may be not suitable for glaciers due to ice/snow reflectance

General Characteristics of LiDAR (II)

• High resolution and especially in relative accuracy
• Quick survey for large areas
• Direct georeferencing of intensity data and XYZ coordinates
• Fast processing
• Output in ASCII XYZ, easily imported by GIS
• UTM-WGS84 result necessitates datum transformation for other systems
• Ellipsoidal WGS84 must have geoid undulation applied
• Complete system costs from $500,000 to $1.3 million

Data Filtering and Software

• Filtering from DSM to DTM:
  • Automated processing removes 90-95% of non-ground points, remaining 10% may consume most of the budget
• Geometric criteria:
  • Points can be viewed three dimensionally
  • Own point classes defined: ground, vegetation, buildings, or wire
  • 3D objects classified such as towers

Accuracy of ALS Empirical accuracy

• Accuracy declines when surface is poorly defined (rough) and laser point density decreases

Problems and Errors in ALS

• Errors in sensor positioning due to GPS, INS and GPS-INS integration.
• Angles of laser travel are problematic, instrument isn't aligned; differential shaking between laser scanner and INS
• Vector from GPS antenna to instrument in INS reference system required is a physical observation

Datasets Delivered

• Clouds of points: the interchange of 3D pt cloud data maintains specific LiDAR information; binary format; supports any 3D
• DEM (with elevations), ESRI grid format, filtered or not in ASCII
• Typically tile-organized, importance of cataloging and coding of the tiles
• Two main structures for the surface: TIN (Triangulated Irregular Network) or Grid

Advantages of LiDAR

• All data geo-referenced from inception
• High level of accuracy
• Ability to cover large areas quickly
• Quicker turnaround, less labor intensive, and lower costs than photogrammetric methods
• Collect data in steep terrain and shadows produces DEM and DSM

Disadvantages of LiDAR

• Large datasets are tricky to interpret
• No international protocols for data acquisition
• $200 - $300 / sq. km is costly

ALS Applications

• Includes the mapping for a variety of items: corridors, electrical transmission lines and towers, waterway landscapes

Further ALS applications

• DTM (Digital Terrain Model) generation, coastal areas, high-density applications
• Rapid mapping and damage assessment
• Measurements for numerous forms of areas. snow- and ice-covered, wetlands
• Derivation of other various parameters: vegetation, hydrographic surveys...

Lidar Applications

• Modeling areas and urban areas
• Census of buildings demolished or new buildings

Models

• Help to estimate stand volume Volume and biomass can be estimated by crowns which leads to fuel loading studies

LiDAR Applications for Archeology

• Can assist in field campaigns and can map features
• Used in creating high-resolution digital elevation models

Explore LiDAR technology, its evolution, and applications in creating 2.5D and true 3D models. Understand the key differences between these representations and the advantages of using true 3D models, focusing on component accuracy and real-world scenarios.