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Airborne and Terrestrial Laser Scanning.pdf
Front Matter
Preface
Table of Contents
1. Laser Scanning Technology
1.1 Basic Measurement Principles of Laser Scanners
1.1.1 Time-of-Flight Measurement
1.1.2 Phase Measurement Techniques
1.1.3 Triangulation-Based Measurements
1.2 Components of Laser Scanners
1.2.1 Light Sources
1.2.2 Laser Beam Propagation
1.2.3 Photodetection
1.2.4 Propagation Medium and Scene Effects
1.2.5 Scanning/Projection Mechanisms
1.3 Basics of Airborne Laser Scanning
1.3.1 Principle of Airborne Laser Scanning
1.3.2 Integration of On-Board Systems
1.3.3 Global Positioning System/Inertial Measurement Unit Combination
1.3.4 Laser Scanner Properties
1.3.5 Pulse Repetition Frequency and Point Density
1.3.6 Multiple Echoes and Full-Waveform Digitisation
1.3.7 Airborne Laser Scanner Error Budget
1.4 Operational Aspects of Airborne Laser Scanning
1.4.1 Flight Planning
1.4.2 Survey Flight
1.4.3 Data Processing
1.4.4 Airborne Laser Scanning and Cameras
1.4.5 Advantages and Limitations of Airborne Laser Scanning
1.5 Airborne Lidar Bathymetry
1.6 Terrestrial Laser Scanners
Acknowledgements
References
2. Visualisation and Structuring of Point Clouds
2.1 Visualisation
2.1.1 Conversion of Point Clouds to Images
2.1.1.1 Height Images
2.1.1.2 Shaded Images
2.1.1.3 Other Imagery
2.1.1.4 Visualisation for Quality Control
2.1.2 Point-Based Rendering
2.2 Data Structures
2.2.1 Delaunay Triangulation
2.2.2 Octrees
2.2.3 k-D Tree
2.3 Point Cloud Segmentation
2.3.1 3D Hough Transform
2.3.1.1 Detection of Lines in a 2D Space
2.3.1.2 Detection of Planes
2.3.1.3 Detection of Cylinders
2.3.2 The Random Sample Consensus Algorithm
2.3.3 Surface Growing
2.3.4 Scan Line Segmentation
2.4 Data Compression
References
3. Registration and Calibration
3.1 Geometric Models
3.1.1 Rotations
3.1.2 The Geometry of Terrestrial Laser Scanning
3.1.3 The Geometry of Airborne Laser Scanning
3.1.3.1 Sensor Frame - s
3.1.3.2 Earth-Centred, Earth-Fixed Frame - e
3.1.3.3 Local-Level Frame - l
3.1.3.4 Body Frame - b
3.1.3.5 Airborne Laser Scanner Observation Equation
3.1.3.6 Mapping Frame - m
3.1.3.7 Trajectory Determination by GPS/INS
3.2 Systematic Error Sources and Models
3.2.1 Systematic Errors and Models of Terrestrial Laser Scanning
3.2.1.1 Range
3.2.1.2 Horizontal Direction
3.2.1.3 Elevation Angle
3.2.1.4 Data Artefacts
3.2.2 Errors and Models for Airborne Laser Scanning
3.2.2.1 Trajectory Positioning Errors Delta X_b ^e
3.2.2.2 Trajectory Orientation Errors R_b ^b' Delta r Delta p Delta y
3.2.2.3 Scanner Component Errors Delta rho, Delta theta
3.2.2.4 Lever-Arm Error Delta X_b ^s
3.2.2.5 Bore-Sight Error R_s ^s' Delta omega Delta phi Delta kappa
3.2.2.6 Scanner Assembly Error Delta eta
3.2.2.7 Target Accuracy Error Budget
3.2.2.8 Scanning Geometry
3.3 Registration
3.3.1 Registration of Terrestrial Laser Scanning Data
3.3.1.1 Target-Based Registration
3.3.1.2 Iterative Closest Point Methods
3.3.1.3 Feature-Based Registration
3.3.2 Registration of Airborne Laser Scanning Data
3.3.2.1 Direct Registration in National Coordinates
3.3.2.2 Strip Adjustment
3.4 System Calibration
3.4.1 Calibration of Terrestrial Laser Scanners
3.4.2 Calibration of Airborne Laser Scanners
Summary
References
4. Extraction of Digital Terrain Models
4.1 Filtering of Point Clouds
4.1.1 Morphological Filtering
4.1.2 Progressive Densification
4.1.3 Surface-Based Filtering
4.1.4 Segment-Based Filtering
4.1.5 Filter Comparison
4.1.6 Potential of Full-Waveform Information for Advanced Filtering
4.2 Structure Line Determination
4.3 Digital Terrain Model Generation
4.3.1 Digital Terrain Model Determination from Terrestrial Laser Scanning Data
4.3.2 Digital Terrain Model Quality
4.3.3 Digital Terrain Model Data Reduction
References
5. Building Extraction
5.1 Building Detection
5.2 Outlining of Footprints
5.3 Building Reconstruction
5.3.1 Modelling and Alternatives
5.3.2 Geometric Modelling
5.3.3 Formal Grammars and Procedural Models
5.3.4 Building Reconstruction Systems
5.4 Issues in Building Reconstruction
5.4.1 Topological Correctness, Regularisation and Constraints
5.4.2 Constraint Equations and Their Solution
5.4.3 Constraints and Interactivity
5.4.4 Structured Introduction of Constraints and Weak Primitives
5.4.5 Reconstruction and Generalisation
5.4.6 Lessons Learnt
5.5 Data Exchange and File Formats for Building Models
References
6. Forestry Applications
6.1 Introduction
6.2 Forest Region Digital Terrain Models
6.3 Canopy Height Model and Biomass Determination
6.4 Single-Tree Detection and Modelling
6.5 Waveform Digitisation Techniques
6.6 Ecosystem Analysis Applications
6.7 Terrestrial Laser Scanning Applications
6.7.1 Literature Overview
6.7.2 Forest Inventory Applications
References
7. Engineering Applications
7.1 Reconstruction of Industrial Sites
7.1.1 Industrial Site Scanning and Modelling
7.1.2 Industrial Point Cloud Segmentation
7.1.3 Industrial Object Detection and Parameterisation
7.1.4 Integrated Object Parameterisation and Registration
7.2 Structural Monitoring and Change Detection
7.2.1 Change Detection
7.2.1.1 Direct DSM Comparison
7.2.1.2 3D: Octree-Based Change Detection
7.2.1.3 2.5D: Visibility Maps
7.2.2 Point-Wise Deformation Analysis
7.2.2.1 Repeated Virtual Target Monitoring
7.2.2.2 Point Cloud versus Reference Surface
7.2.2.3 Landslide Analysis by Local ICP
7.2.2.4 Observation Quality
7.2.2.5 Statistical Point-Wise Deformation Analysis
7.2.2.6 Testing for Stability of a Tunnel
7.2.2.7 Morphological Deformation
7.2.3 Object-Oriented Deformation Analysis
7.2.3.1 Deformations Relative to an Object
7.2.3.2 Local Object Movement
7.2.3.3 Moving Rocks
7.2.3.4 Changing Roof Orientations
7.2.3.5 Rock Face Geometry
7.2.4 Outlook
7.3 Corridor Mapping
7.3.1 Power Line Monitoring
7.3.1.1 Data Collection
7.3.1.2 Classification
7.3.1.3 Transmission Cable Modelling
7.3.2 Dike and Levee Inspection
Conclusions
References
8. Cultural Heritage Applications
8.1 Accurate Site Recording: 3D Reconstruction of the Treasury Al-Kasneh in Petra, Jordan
8.2 Archaeological Site: Scanning the Pyramids at Giza, Egypt
8.3 Archaeological Airborne Laser Scanning in Forested Areas
8.4 Archaeological Site: 3D Documentation of an Archaeological Excavation in Mauken, Austria
8.5 The Archaeological Project at the Abbey of Niedermunster, France
References
9. Mobile Mapping
9.1 Introduction
9.2 Mobile Mapping Observation Modes
9.2.1 Stop-and-Go Mode
9.2.2 On-the-Fly Mode
9.3 Mobile Mapping System Design
9.3.1 General System Design
9.3.2 Imaging and Referencing
9.3.3 Indoor Applications
9.3.4 Communication and Control
9.3.5 Processing Flow
9.4 Application Examples
9.4.1 Railroad-Track Based Systems
9.4.2 Road-Based Systems
9.5 Validation of Mobile Mapping Systems
Conclusions
References
List of Abbreviations
Index
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Airborne and Terrestrial
Laser Scanning
Edited by
George Vosselman
Hans-Gerd Maas
WHITTLES PUBLISHING
Whittles Publishing,
Caithness KW6 6EY,
Published by
Dunbeath,
Scotland, UK
www.whittlespublishing.com
Distributed in North America by
CRC Press LLC,
6000 Broken Sound Parkway NW, Suite 300,
Taylor and Francis Group,
Boca Raton, FL 33487, USA
© 2010 G Vosselman and H-G Maas
ISBN 978-1904445-87-6
USA ISBN 978-1-4398-2798-7
All rights reserved.
No part of this publication may be reproduced,
stored in a retrieval system, or transmitted,
in any form or by any means, electronic,
without prior permission of the publishers.
mechanical, recording or otherwise
The publisher and authors have used their best efforts in preparing this book, but assume
no responsibility for any injury and/or damage to persons or property from the use or
implementation of any methods, instructions, ideas or materials contained within this
book. All operations should be undertaken in accordance with existing legislation and
recognized trade practice. Whilst the information and advice in this book is believed to be
true and accurate at the time of going to press, the authors and publisher accept no legal
responsibility or liability for errors or omissions that may have been made.
Printed by
Preface
Within a time frame of only two decades airborne and terrestrial laser scanning have
become well established surveying techniques for the acquisition of (geo)spatial infor-
mation. A wide variety of instruments is commercially available, and a large number of
companies operationally use airborne and terrestrial scanners, accompanied by many
dedicated data acquisition, processing and visualisation software packages. The high
quality 3D point clouds produced by laser scanners are nowadays routinely used for
a diverse array of purposes including the production of digital terrain models and
3D city models, forest management and monitoring, corridor mapping, revamping of
industrial installations and documentation of cultural heritage.
However, the publicly accessible knowledge on laser scanning is distributed over a
very large number of scientific publications, web pages and tutorials. The objective of
this book is to give a comprehensive overview of the principles of airborne and terres-
trial laser scanning technology and the state of the art of processing 3D point clouds
acquired by laser scanners for a wide range of applications. It will serve as a textbook
for use in under- and post-graduate studies as well as a reference guide for practition-
ers providing laser scanning services or users processing laser scanner data.
Airborne and terrestrial laser scanning clearly differ in terms of data capture modes,
typical project sizes, scanning mechanisms, and obtainable accuracy and resolution.
Yet, they also share many features, especially those resulting from the laser ranging
technology. In particular, when it comes to the processing of point clouds, often the
same algorithms are applied to airborne as well as terrestrial laser scanning data. In this
book we therefore present, as far as possible, an integral treatment of airborne and ter-
restrial laser scanning technology and processing.
The book starts with an introduction to the technology of airborne and terrestrial
laser scanning covering topics such as range measurement principles, scanning mecha-
nisms, GPS/IMU integration, full-waveform digitisation, error budgets as well as
operational aspects of laser scanning surveys. The chapter focuses on principles rather
ix
x
Preface
than technical specifications of current laser scanners as the latter are rapidly outdated
with ongoing technological developments.
Common to all laser scanning projects are the needs to visualise and structure the
acquired 3D point clouds as well as to obtain a proper georeferencing of the data.
Chapter 2 discusses techniques to visualise both original point clouds and rasterised
data. Visualisation is shown to be an important tool for quality assessment. Information
extraction from point clouds often starts with the grouping of points into smooth or
planar surfaces of the recorded objects. The most common segmentation algorithms as
well as data structures suitable for point clouds are also presented in this chapter.
Chapter 3 deals with the registration of multiple datasets and the calibration of
airborne and terrestrial laser scanners. Models are elaborated that show the relation
between the observations made by laser scanners and the resulting coordinates of
the reflecting surface points. Based on these geometric models, the error sources of
typical instrument designs are discussed. Registration of point clouds is the process
of transforming a dataset to an externally defined coordinate system. For airborne
surveys this normally is the national mapping coordinate system. For terrestrial laser
scanning, point clouds acquired at different scan positions have to be registered with
respect to each other. This chapter elaborates upon the coordinate systems involved as
well as the registration methods. The chapter concludes with a discussion of calibra-
tion procedures that estimate and correct for potential systematic errors in the data
acquisition.
The applications oriented part of the book starts in Chapter 4 with the extraction
of digital terrain models (DTM) from airborne laser scanning data. The production
of high quality DTMs has been the major driving force behind the development of
airborne laser scanners. Compared to other surveying technologies, airborne laser
scanning enables DTM production with higher quality at lower costs. As a conse-
quence, in the few years since its introduction airborne laser scanning has become the
preferred technology for the acquisition of DTMs. Point clouds resulting from laser
scanning surveys contain points both on the terrain and also on vegetation, build-
ings and other objects. Whereas these points are a rich source of information for a
variety of applications, non-ground points need to be removed from the point clouds
for the purpose of DTM production. Chapter 4 reviews the most popular strategies
and algorithms that can be applied to this so-called filtering process. In addition it
also discusses the extraction of break lines, reduction of point densities and aspects of
DTM quality.
With increasing sensor resolution, point clouds acquired by airborne laser scan-
ners are nowadays dense enough to also retrieve detailed information on buildings
and trees. The detection of buildings and the extraction of 3D building models in
airborne laser data are the subject of Chapter 5. Detection of buildings typically makes
use of the separation in ground and non-ground points by filtering algorithms, but
needs to further classify the non-ground points. Following the detection of buildings,
Preface xi
algorithms for deriving their 2D outlines are reviewed. Building outlines may be used
to update traditional 2D building maps or serve as an intermediate step for a complete
3D building reconstruction procedure. Chapter 5 discusses several such procedures
as well as the ongoing research questions in this field with regard to regularisation,
constraints, interaction and generalisation. The chapter concludes with an overview on
data exchange formats for building models.
Chapter 6 is devoted to forestry applications, not only using airborne laser scan-
ning data, but also including analyses of terrestrially acquired data. The unique ability
of airborne laser scanning to obtain both points on vegetation as well as on the ground
led to the rapid acceptance of laser scanning for the purpose of forest inventory stud-
ies, forest management, carbon sink analysis, biodiversity characterisation and habitat
analysis. This chapter presents an overview of techniques and studies on stand-wise as
well as tree-wise monitoring of forests. Whereas stand-wise monitoring provides aver-
age values for forest inventory parameters such as tree height or timber volume for
larger areas, high resolution airborne laser scanning surveys can also be used to deline-
ate and model individual trees. Terrestrial laser scanning is exploited to obtain detailed
geometric models of selected plots.
The last three chapters present applications that mainly make use of terrestrial
laser scanners. A wide variety of engineering applications is presented in Chapter 7.
In industry 3D CAD models of installations are required for maintenance manage-
ment, safety analysis and revamping. Point clouds acquired by terrestrial laser scan-
ners directly capture the shapes of these installations and allow an efficient modelling
process. Change detection and deformation analysis play an important role in civil
and geotechnical engineering applications. Terrestrial laser scanning is shown to be of
considerable value for projects such as the monitoring of dams, tunnels and areas sus-
ceptible to land erosion or land slides, whereas airborne laser scanning depicts a very
efficient tool for monitoring power lines and water embankments.
Applications of laser scanning to the documentation of cultural heritage are dis-
cussed in some case studies in Chapter 8. Historical buildings and sculptures are often
difficult to model. The ability of terrestrial laser scanners to rapidly capture complex
surfaces makes laser scanning a preferred technology for documenting cultural her-
itage. Terrestrial as well as airborne laser scanning have also proven their value in
archaeological studies. Whereas terrestrial laser scanning is used to document exca-
vation sites, airborne laser scanning has been used successfully to uncover land use
patterns in forested areas.
Finally, Chapter 9 presents a review on vehicle-borne mobile mapping systems.
The chapter discusses various modes of observation (stop-and-go, on-the-fly), design
considerations and data processing flows. Present-day systems are shown together with
their application to corridor mapping of road and rail environments.
The obvious need for a textbook on laser scanning has been on our minds for sev-
eral years. For the realisation of this book we are extremely grateful that we could enlist
xii
Preface
renowned scientists to share their knowledge and invest their precious time in contrib-
uting dedicated chapters.
We would also like to express our gratitude to Keith Whittles, who gave us a push
to accept the challenge of editing this book. Sincere thanks also go to the staff of
Whittles Publishing for their professional handling of the manuscript and marketing
efforts.
We hope that this book may serve many students, researchers and practitioners for
their work in the exciting field of airborne and terrestrial laser scanning and 3D point
cloud processing.
George Vosselman
International Institute for Geo-Information Science
and Earth Observation (ITC)
Hans-Gerd Maas
Dresden University of Technology
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
The Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
List of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvi
Chapter 1 Laser Scanning Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Basic measurement principles of laser scanners . . . . . . . . . . . . . . . . . . . . . .2
1.1.1 Time-of-flight measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2 Phase measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.3 Triangulation-based measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2 Components of laser scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
1.2.1 Light sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.2 Laser beam propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.3 Photodetection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.4 Propagation medium and scene effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.5 Scanning/projection mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3 Basics of airborne laser scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
1.3.1 Principle of airborne laser scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.3.2
Integration of on-board systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.3.3 Global Positioning System/Inertial Measurement
Unit combination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.3.4 Laser scanner properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1.3.5 Pulse repetition frequency and point density . . . . . . . . . . . . . . . . . . . . . . . 26
1.3.6 Multiple echoes and full-waveform digitisation . . . . . . . . . . . . . . . . . . . . . 28
1.3.7 Airborne laser scanner error budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.4 Operational aspects of airborne laser scanning . . . . . . . . . . . . . . . . . . . . . .30
1.4.1 Flight planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
1.4.2 Survey flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
1.4.3 Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1.4.4 Airborne laser scanning and cameras . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
1.4.5 Advantages and limitations of airborne laser scanning . . . . . . . . . . . . . . . . 35
1.5 Airborne lidar bathymetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
v
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Contents
1.6 Terrestrial laser scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39
Chapter 2 Visualisation and Structuring of Point Clouds . . . . . . . . . . . . . 45
2.1 Visualisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46
2.1.1 Conversion of point clouds to images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.1.2 Point-based rendering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.2 Data structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58
2.2.1 Delaunay triangulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
2.2.2 Octrees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
2.2.3 k-D tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.3 Point cloud segmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63
2.3.1 3D Hough transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
2.3.2 The random sample consensus algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . 67
2.3.3 Surface growing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
2.3.4 Scan line segmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
2.4 Data compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79
Chapter 3 Registration and Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.1 Geometric models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83
3.1.1 Rotations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
3.1.2 The geometry of terrestrial laser scanning . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.1.3 The geometry of airborne laser scanning . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.2 Systematic error sources and models . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92
3.2.1 Systematic errors and models of terrestrial laser scanning . . . . . . . . . . . . . 92
3.2.2 Errors and models for airborne laser scanning . . . . . . . . . . . . . . . . . . . . . 100
3.3 Registration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109
3.3.1 Registration of terrestrial laser scanning data . . . . . . . . . . . . . . . . . . . . . . 111
3.3.2 Registration of airborne laser scanning data . . . . . . . . . . . . . . . . . . . . . . . 119
3.4 System calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .122
3.4.1 Calibration of terrestrial laser scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
3.4.2 Calibration of airborne laser scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .129
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .129
Chapter 4 Extraction of Digital Terrain Models . . . . . . . . . . . . . . . . . . . 135
4.1 Filtering of point clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137
4.1.1 Morphological filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
4.1.2 Progressive densification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
4.1.3 Surface-based filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
4.1.4 Segment-based filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
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