TLS

Terrestrial Laser Scanning (TLS) and 3D Scanning

• Terrestrial Laser Scanning (TLS) is a key method, among others, for 3D scanning.
• It has many applications in urban and regional analysis.

Applications of Terrestrial 3D Scanning

• This technique has uses in various fields like facilities management, architecture, and construction using BIM.
  • Also used for studying roads, tunnels, and mines.
• Archaeology relies on it for documenting heritage and restoration.
• Geology benefits from it in landslide and glaciology studies.
• Other uses include forensics, deformation monitoring, and creating content for movies, games, and virtual reality.

The Expanding Sector

• The Terrestrial Laser Scanning market is growing globally
• Expected to propel market growth is the increasing need for digitization in the infrastructure sector.

3D Terrestrial Scanning

• It generates a cloud of 3D points, as an active system.
• The coordinates are device-centered 3D cartesian.
• Final results come from combining several clouds.
• Georeferencing in an absolute system isn't always a must.
• Individual points' coordinates link to intensity values.
  • Highly reflective points appear as light grey, and highly absorbing points as dark grey
  • Absence of return is black
  • There's also optional color data (RGB values).

Types of Instruments

• Time-Of-Flight and Phase-based Terrestrial Laser Scanners
  • These are tripod mounted for large areas
• LASER (TLS)
  • Desktop versions suit smaller items
  • Handheld suits smaller, complex items
• Structured Light Projection (not laser)

Terrestrial Laser Scanners for Architectural Applications

• Terrestrial laser scanners for architecture are usually tripod-mounted.
• They measure slant range using a laser range finder
  • Along with two angles via angular encoders.
• Angular increments are user-defined. Step sizes typically match
  • Provides equal spatial sampling in a polar coordinate system.
• The instrument’s rotation axis need not be positioned vertically, unlike a total station.
• Another difference: the scan station need not be marked, which it often is.

Types of Scanners

• White-light scanners
  • Feature a projector and one or two cameras.
• Time-Of-Flight Laser Scanners
• Phase based Terrestrial Laser Scanners
  • For architectural applications

Range Determination

• Two main techniques exist for measuring range:
  • Time-Of-Flight
  • Phase measurement

Distance Measurement by Time-Of-Flight (t.o.f.)

• Pulses are emitted, and travel time to/from the object is measured.
  • This time, multiplied by light speed (c) and halved, yields the range.
• Beam deflection provides elevation/azimuth.
• Return pulse energy (intensity) + optional color is recorded.
  • Full waveform recording is available on newer TLS instruments.

Distance Measurement by Phase-Based Technique

• This avoids high-precision clocks by modulating laser beam power.
• Emitted light's amplitude is modulated + directed onto a surface.
• The scattered reflection is gathered, and the circuit measures the phase difference between transmitted/received waveforms to derive time delay

Scanner Reference

• Measurements use a local scanner space coordinate system.
• The scanner's origin is at the instrument's center.

Choosing a Ranging Scanner Instrument

• Precision and accuracy are important factors.
• Other factors include acquisition speed, maximum distance, and signal wavelength.
• Also consider resolution, divergence, field of view, ability to recognize targets automatically, and RGB acquisition.
• Data storage, download mode, portability, power type, and software are key.

3D Triangulators

• Triangulation involves creating a triangle using:
• Illumination angle aimed at a reflective surface.
• Observation angle with a known baseline.

Laser Triangulation for small objects

• Laser emitter and digital sensor (camera) are at a known, small distance
• The laser emits a point or line, usually visible in red.

Laser Triangulation vs Stereoscopic photogrammetry

• Triangulation is when an triangulation-based system has a light projector and a camera
• Stereo-based systems use two cameras
• Depth is inferred by triangulation in both cases.

Structured Light Projection Scanners

• The technique uses triangulation without a laser.
• White/blue light with imprinted pattern hits an object, deforming based on geometry which is decoded
• A video camera records frames per second to acquire image continuously

Structured Light Projection Scanning Characteristics

• Short distances + narrow field of action.
• High scanning speed, with submillimeter accuracy, and high density.

F6 Volumetric Structured Light Projection Scanner

• The Stonex F6 Smart scanner uses triangulation, and structured light
• Projects NIR (850 nm) light.
• An NIR sensor examines model’s geometry calculates distances.
• It works in infrared, independent of lighting.
• Range is 60 cm-4 m, acquires 8 fps, with accuracy 0.2–0.1% of distance (4 mm at 4 m).

The Laser Scanning Equipment

• Includes laser scanners, various range finders, laser measurement systems, and software.
• The laser and Risk Classes for Exposure to eyes.

Riegl VZ400

• Laser scanners are categorized by risk to the eye.
• Class 1 lasers are safe for occasional exposure over 7 meters.
• Class 3B lasers are safe for occasional exposure over 160 meters

T.o.f. vs Phase-based instruments

• Phase difference instruments are generally slightly less accurate
  • Their error is not much affected by distance
• They can be considered more advantageous
  • Better scanning speed and often lower cost

The Laser and the Risk Classes for Exposure to eyes

• Eye injury hazard descriptions are valid for for exposures relatively close to the laser
• Because the beam spreads, less light will enter the pupil at greater distances
• The hazard decreases the farther a person is from the laser, and the shorter the exposure time

Leica RTC 360

• A fast 3D laser scanner.
• Includes HDR spherical imaging + VIS (Visual Inertial System) for real-time registration in the field.
• High scanning speed creates colorized 3D point clouds in under 2 minutes.
• It uses visual Inertial System to track movement
  • An IMU ensures accuracy to 18".

Coordinate Systems in Scanning

• SOCS: Scanner Own Coordinate System: each scan position has its own system
• PRCS: PRoject Coordinate System: a local system for the whole project
• GLCS: GLobal Coordinate System: an absolute system (e.g., UTM, WGS84).

Workflow in 3D Scanning

• Includes design/planning of the survey
• Point cloud acquisition
• Editing/filtering
• Alignment/registration
• Fusion and mesh generation
• Editing, optimization, decimation, and generating final products
• Quality control is maintained throughout.

Design of the Survey

• Considers material/geometrical aspects and TLS features.
• Chooses acquisition distance (based on-site), for desired density.
• Overlap is 30-40%, helping reliable point cloud alignment.

On the field

• Before setting anything up, make sure to take a walk around the field site.
• Possible you might need more 4 targets dependin on the conditions.
• Can also draw a sketch the target arrangement.
• May need survey depending on the complexity
• It also depends when the Scan position is set

Problems Related to the Geometry of the Object

• Occlusion, shadows and orientation bias can affect scan data.
• Proper planning is crucial to minimize these issues.

Partitioning a Scan

• Partition scan by setting different step in order to maintain homogeneous on the entire object
• This is useful if the distance from the area constantly varies

Summarising

• The basic principle same no matter what scanner
• Each point stores xyz, reflectivity and RGB value
• Point cloud can have 1 million data per second
• Use multiple scans project need need to be taken from different positions to ensure a complete data set

Scan time

• Ranging from seconds to several minutes depends on user system and the number of scan
• High resolution means more time need to Scan

Divergence

• With a higher laser beam divergence, lost of content and data

Pointcloud visualisation

• Point cloud visualisation includes camera system (intensity vs RGB)
• It also includes the azimuth and zenith angles and 3D coordinates.

Data format for point cloud

• Various formats exist for point cloud data storage and each format has its advantages and disadvantages,

Editing and Filtering of the Pointcloud

• The step includes topographical survey to get quality 3D points
• Followed by the editing and filtering and also alignment

Point cloud cleaning

• Involves removing elements unrelated to the object's geometry
• Can be performed manually or with automatic filter procedure

Filtering

• Theory say that data should be unfiltered because the choice of instrument was inconsistent
• Robust filter might need to be implored anyway

Point decimation (data reduction)

• Point decimation reduces spatial density and/or make it uniform

Aligning/registering the point clouds

• Relies on two 3D-laser scans, with six degrees solved
• By doing so the parameters are applied so that the cloud's coordinates are expressed

Approaches for alignment / registration of point clouds

• Can be achieved by registration using target, or by using the ICP method
• Lastly by imploring other sensor (AI camera tech)

Aiming/registering the point clouds

• Relies on a common point to calculated the registration parameter
• Two clouds are acquired by different scan stations and translated towards another
• Another error it that affects how the linked scan is implored
• Errors occur as 6 degrees of freedom become available for the registration

Targets recognition and link between 2 scans

• Done by Scanning the Target with a High Scanning Step the center needs position's

Aligning of cloud points

• It helps join multiple scans together where there is unknown or know through a topographical survey
• It relies on new support since the pre-registration is done on the field

Cylindrical and Spherical targets

• Help during The cloud point alignments

Alignment of cloud points Approach

• Its easier with a semi automatic procedure that involves little time and no help from targets and surveyors

Minimising the scans

• This saves lot of scan time and means there is a lot lesser data to process

Steps after The Scanning is completed

• Data must be imported to make ready for visualization the program called RiScan Pro by Riegl

Aligning of cloud points Algorithm-

1. Select points of the same feature
2. Iterate until the all 3D Points are close

Summary the ICP operation

• Is related to Geometric complexity
• Original or final Unique model
• It's Easy to change at positioning.
• Has 6 Degrees Freedom in the scanner which have 3 Dimensions

Key Risks with ICP-based quality assurance

• Quality is a manual process can't be automated
• The more time spent more the quality
• Software that does the process
• Data can be very big

Leica RTC360 Information

• Uses IMU for Angular accuracy
• Visual cameras align scans
• A visual and LIDAR Based scanner

Final Cloud Fusion Process

• all 3d Point aligned and edited become one clean Data Structure and format

Meshing procedure

• Delaunay triangulation which gives a minimal amount of triangle created and can be visual

Mesh optimization and generation

• The subdivisions for the mesh creation and models should minimal

Edits of the data (in the data package)

• All the softwares does offer very cool data models for data imports exports manipulations and transformations that

Final product is a high quality mesh for export to final use

• Can be the imported and transformed to desired requirements
• High Quality final Resultant.