Special Issue "Laser Scanning"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Optics and Lasers".

Deadline for manuscript submissions: closed (15 January 2018).

Special Issue Editors

Prof. Francesco Pirotti
E-Mail Website
Guest Editor
University of Padova - Via dell'Università, 16, 35020 - Legnaro (PD), Italy
Interests: LiDAR; Geomatics; Natural Resources; Natural Hazard; Risk
Special Issues and Collections in MDPI journals
Prof. Dr. Qi Chen
E-Mail Website
Guest Editor
Department of Geography, University of Hawaiˈi at Mānoa, 2424 Maile Way, Honolulu, HI 96822, USA
Interests: LiDAR remote sensing of vegetation; statistical learning; mathematical models; geospatial analysis
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

The science of remote sensing has seen a disruptive change with laser scanning technology in the past years. Surveying objects in 3D space with lasers opened new perspectives and stimulated new research for innovative processing methods and applications. The ability of laser scanning to reach and measure surfaces behind partial occlusions (e.g., vegetation canopy) is one of many characteristics that make this technology unique. Today, laser sensors are present in a broad range of instruments, from orbiting to handheld platforms as well in unmanned aerial vehicles (UAVs) and mobile mapping systems.

Laser scanning technology is still undergoing important advances, such as increasing point densities from single-photon counting, multi-spectral laser scanners, full-waveform recording, and lighter sensors for UAVs. New processing methods, which allow sensors to do without GNSS positioning information, have opened new possibilities and applications for mapping where GNSS is not available (e.g., indoor, caves, under canopy). The proposed Special Issue welcomes reports describing innovative applications and latest technological advances related to laser scanning technology.

Prof. Francesco Pirotti
Dr. Xinlian Liang
Dr. Qi Chen
Guest Editors

Manuscript Submission Information

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Keywords

  • Orbiting sensors using laser-based technology

  • Terrestrial laser scanning

  • Mobile laser scanning

  • GNSS-less laser scanning sensors

  • Frontiers in laser-based sensor techniques, e.g., photon-counting scanners

  • UAV laser scanning

  • Processing point clouds: classification and modeling

  • Applications to the built environment

  • Agro-forestry applications

  • Indoor mapping

Published Papers (19 papers)

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Editorial

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Open AccessEditorial
Preface of Special Issue on Laser Scanning
Appl. Sci. 2019, 9(13), 2713; https://doi.org/10.3390/app9132713 - 04 Jul 2019
Abstract
A laser is a spatially coherent light that can travel through space with very little diffraction [...] Full article
(This article belongs to the Special Issue Laser Scanning)

Research

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Open AccessArticle
Improved Human Detection with a Fusion of Laser Scanner and Vision/Infrared Information for Mobile Applications
Appl. Sci. 2018, 8(10), 1967; https://doi.org/10.3390/app8101967 - 18 Oct 2018
Cited by 1
Abstract
This paper presents a method for human detection using a laser scanner with vision or infrared images. Mobile applications require reliable and efficient methods for human detection, especially as a part of driver assistance systems, including pedestrian collision systems. The authors propose an [...] Read more.
This paper presents a method for human detection using a laser scanner with vision or infrared images. Mobile applications require reliable and efficient methods for human detection, especially as a part of driver assistance systems, including pedestrian collision systems. The authors propose an efficient method for multimodal human detection based on a combination of the features and context information. Strictly, the human is detected in the vision/infrared images using a combination of local binary patterns and histogram of oriented gradients features with a neural network in a cascade manner. Next, using coordinates of detected humans from the vision system, the moving trajectory is predicted until the scanner working distance is reached by the individual human. Then the segmentation of data from the laser scanner is further carried out with respect to the predicted trajectory. Finally, human detection in the laser scanner working distance is performed based on modelling of the human legs. The modelling is based on the adaptive breakpoint detection algorithm and proposed improved polylines definition and fitting algorithm. The authors conducted a set of experiments in predefined scenarios, discussed the identified weakness and advantages of the proposed method, and outlined detailed future work, especially for night-time and low-light conditions. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Indoor Building Reconstruction from Occluded Point Clouds Using Graph-Cut and Ray-Tracing
Appl. Sci. 2018, 8(9), 1529; https://doi.org/10.3390/app8091529 - 01 Sep 2018
Cited by 10
Abstract
Despite the increasing demand of updated and detailed indoor models, indoor reconstruction from point clouds is still in an early stage in comparison with the reconstruction of outdoor scenes. Specific challenges are related to the complex building layouts and the high presence of [...] Read more.
Despite the increasing demand of updated and detailed indoor models, indoor reconstruction from point clouds is still in an early stage in comparison with the reconstruction of outdoor scenes. Specific challenges are related to the complex building layouts and the high presence of elements such as pieces of furniture causing clutter and occlusions. This work proposes an automatic method for modelling Manhattan-World indoors acquired with a mobile laser scanner in the presence of highly occluded walls. The core of the methodology is the transformation of indoor reconstruction into a labelling problem of structural cells in a 2D floor plan. Assuming the prevalence of orthogonal intersections between walls, indoor completion is formulated as an energy minimization problem using graph cuts. Doors and windows are detected from occlusions by implementing a ray-tracing algorithm. The methodology is tested in a real case study. Except for one window partially covered by a curtain, all building elements were successfully reconstructed. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Initial Results of Testing a Multilayer Laser Scanner in a Collision Avoidance System for Light Rail Vehicles
Appl. Sci. 2018, 8(4), 475; https://doi.org/10.3390/app8040475 - 21 Mar 2018
Cited by 3
Abstract
This paper presents an application to detect and track obstacles using a multilayer laser scanner. The goal of the detection system is to develop collision avoidance for the Light Rail Vehicle (LRV). The laser scanner, which is mounted in front of the tram, [...] Read more.
This paper presents an application to detect and track obstacles using a multilayer laser scanner. The goal of the detection system is to develop collision avoidance for the Light Rail Vehicle (LRV). The laser scanner, which is mounted in front of the tram, collects information in a four-scan plane. The object recognition and tracking module, which is composed of a three sub-modules segmentation, classification, and Kalman Filter tracking, was carried out on the raw data. Thus, data were provided for collision avoidance module. The proposed system was applied to a tram named “Silkworm” which is manufactured by Durmazlar Machine Inc. (Bursa, Turkey) and initial experimental tests have been conducted at the facilities of Durmazlar Machine Inc. in the city of Bursa, Turkey. This study aims to illustrate parts of the possible tests that can be carried out and to share with the scientific community an important application of multilayer laser scanners, although in the initial implementation phase, in urban rail transportation. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessFeature PaperArticle
Performance Evaluation of Two Indoor Mapping Systems: Low-Cost UWB-Aided Photogrammetry and Backpack Laser Scanning
Appl. Sci. 2018, 8(3), 416; https://doi.org/10.3390/app8030416 - 11 Mar 2018
Cited by 13
Abstract
During the past dozen years, several mobile mapping systems based on the use of imaging and positioning sensors mounted on terrestrial (and aerial) vehicles have been developed. Recently, systems characterized by an increased portability have been proposed in order to enable mobile mapping [...] Read more.
During the past dozen years, several mobile mapping systems based on the use of imaging and positioning sensors mounted on terrestrial (and aerial) vehicles have been developed. Recently, systems characterized by an increased portability have been proposed in order to enable mobile mapping in environments that are difficult to access for vehicles, in particular for indoor environments. In this work the performance of a low-cost mobile mapping system is compared with that of: (i) a state-of-the-art terrestrial laser scanning (TLS), considered as the control; (ii) a mobile mapping backpack system (Leica Pegasus), which can be considered as the state-of-the-art of commercial mobile mapping backpack systems. The aim of this paper is two-fold: first, assessing the reconstruction accuracy of the proposed low-cost mobile mapping system, based on photogrammetry and ultra-wide band (UWB) for relative positioning (and a GNSS receiver if georeferencing is needed), with respect to a TLS survey in an indoor environment, where the global navigation satellite system (GNSS) signal is not available; second, comparing such performance with that obtained with the Leica backpack. Both mobile mapping systems are designed to work without any control point, to enable an easy and quick survey (e.g., few minutes) and to be easily portable (relatively low weight and small size). The case study deals with the 3D reconstruction of a medieval bastion in Padua, Italy. Reconstruction using the Leica Pegasus backpack allowed obtaining a smaller absolute error with respect to the UWB-based photogrammetric system. In georeferenced coordinates, the root mean square (RMS) error was respectively 16.1 cm and 50.3 cm; relative error in local coordinates was more similar, respectively 8.2 cm and 6.1 cm. Given the much lower cost (approximately $6k), the proposed photogrammetric-based system can be an interesting alternative when decimetric reconstruction accuracy in georeferenced coordinates is sufficient. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Examination of Indoor Mobile Mapping Systems in a Diversified Internal/External Test Field
Appl. Sci. 2018, 8(3), 401; https://doi.org/10.3390/app8030401 - 09 Mar 2018
Cited by 22
Abstract
Nowadays, simultaneous localization and mapping (SLAM) algorithms support several commercial sensors which have recently been introduced to the market, and, like the more common mobile mapping systems (MMSs), are designed to acquire three-dimensional and high-resolution point clouds. The new systems are said to [...] Read more.
Nowadays, simultaneous localization and mapping (SLAM) algorithms support several commercial sensors which have recently been introduced to the market, and, like the more common mobile mapping systems (MMSs), are designed to acquire three-dimensional and high-resolution point clouds. The new systems are said to work both in external and internal environments, and completely avoid the use of targets and control points. The possibility of increasing productivity in three-dimensional digitization projects is fascinating, but data quality needs to be carefully evaluated to define appropriate fields of application. The paper presents the analytical measurement principle of these indoor mobile mapping systems (IMMSs) and the results of some tests performed on three commercial systems. A common test field was defined in order to acquire comparable data. By taking the already available terrestrial laser scan survey as the ground truth, the datasets under examination were compared with the reference and some assessments are presented which consider both quantitative and qualitative aspects. Geometric deformation in the final models was computed using the so-called Multiscale Model to Model Cloud Comparison (M3C2) algorithm. Cross sections and cloud to mesh (C2M) distances were also employed for a more detailed analysis. The real usability assessment is based on the features of recognizability, double surface evidence, and visualization effectiveness. For these evaluations, comparative images and tables are presented. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Field Navigation Using Fuzzy Elevation Maps Built with Local 3D Laser Scans
Appl. Sci. 2018, 8(3), 397; https://doi.org/10.3390/app8030397 - 08 Mar 2018
Cited by 5
Abstract
The paper describes the case study of the mobile robot Andabata navigating on natural terrain at low speeds with fuzzy elevation maps (FEMs). To this end, leveled three-dimensional (3D) point clouds of the surroundings are obtained by synchronizing ranges obtained from a 360 [...] Read more.
The paper describes the case study of the mobile robot Andabata navigating on natural terrain at low speeds with fuzzy elevation maps (FEMs). To this end, leveled three-dimensional (3D) point clouds of the surroundings are obtained by synchronizing ranges obtained from a 360 field of view 3D laser scanner with odometric and inertial measurements of the vehicle. Then, filtered point clouds are employed to produce FEMs and their corresponding fuzzy reliability masks (FRMs). Finally, each local FEM and its FRM are processed to choose the best motion direction to reach distant goal points through traversable areas. All these tasks have been implemented using ROS (Robot Operating System) nodes distributed among the cores of the onboard processor. Experimental results of Andabata during non-stop navigation on an urban park are presented. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Using Multispectral Airborne LiDAR Data for Land/Water Discrimination: A Case Study at Lake Ontario, Canada
Appl. Sci. 2018, 8(3), 349; https://doi.org/10.3390/app8030349 - 28 Feb 2018
Cited by 6
Abstract
Coastal areas are environmentally sensitive and are affected by nature events and human activities. Land/water interaction in coastal areas changes over time and, therefore, requires accurate detection and frequent monitoring. Multispectral Light Detection and Ranging (LiDAR) systems, which operate at different wavelengths, have [...] Read more.
Coastal areas are environmentally sensitive and are affected by nature events and human activities. Land/water interaction in coastal areas changes over time and, therefore, requires accurate detection and frequent monitoring. Multispectral Light Detection and Ranging (LiDAR) systems, which operate at different wavelengths, have become available. This new technology can provide an effective and accurate solution for the determination of the land/water interface. In this context, we aim to investigate a set of point features based on elevation, intensity, and geometry for this application, followed by a presentation of an unsupervised land/water discrimination method based on seeded region growing algorithm. The multispectral airborne LiDAR sensor, the Optech Titan, was used to acquire LiDAR data at three wavelengths (1550, 1064, and 532 nm) of a study area covering part of Lake Ontario in Scarborough, Canada for testing the discrimination methods. The elevation- and geometry-based features achieved an average overall accuracy of 75.1% and 74.2%, respectively, while the intensity-based features achieved 63.9% accuracy. The region growing method succeeded in discriminating water from land with more than 99% overall accuracy, and the land/water boundary was delineated with an average root mean square error of 0.51 m. The automation of this method is restricted by having double returns from water bodies at the 532 nm wavelength. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessFeature PaperArticle
Bias Impact Analysis and Calibration of UAV-Based Mobile LiDAR System with Spinning Multi-Beam Laser Scanner
Appl. Sci. 2018, 8(2), 297; https://doi.org/10.3390/app8020297 - 18 Feb 2018
Cited by 9
Abstract
Light Detection and Ranging (LiDAR) is a technology that uses laser beams to measure ranges and generates precise 3D information about the scanned area. It is rapidly gaining popularity due to its contribution to a variety of applications such as Digital Building Model [...] Read more.
Light Detection and Ranging (LiDAR) is a technology that uses laser beams to measure ranges and generates precise 3D information about the scanned area. It is rapidly gaining popularity due to its contribution to a variety of applications such as Digital Building Model (DBM) generation, telecommunications, infrastructure monitoring, transportation corridor asset management and crash/accident scene reconstruction. To derive point clouds with high positional accuracy, estimation of mounting parameters relating the laser scanners to the onboard Global Navigation Satellite System/Inertial Navigation System (GNSS/INS) unit, i.e., the lever-arm and boresight angles, is the foremost and necessary step. This paper proposes a LiDAR system calibration strategy for a Unmanned Aerial Vehicle (UAV)-based mobile mapping system that can directly estimate the mounting parameters for spinning multi-beam laser scanners through an outdoor calibration procedure. This approach is based on the use of conjugate planar/linear features in overlapping point clouds derived from different flight lines. Designing an optimal configuration for calibration is the first and foremost step in order to ensure the most accurate estimates of mounting parameters. This is achieved by conducting a rigorous theoretical analysis of the potential impact of bias in mounting parameters of a LiDAR unit on the resultant point cloud. The dependency of the impact on the orientation of target primitives and relative flight line configuration would help in deducing the configuration that would maximize as well as decouple the impact of bias in each mounting parameter so as to ensure their accurate estimation. Finally, the proposed analysis and calibration strategy are validated by calibrating a UAV-based LiDAR system using two different datasets—one acquired with flight lines at a single flying height and the other with flight lines at two different flying heights. The calibration performance is evaluated by analyzing correlation between the estimated system parameters, the a-posteriori variance factor of the Least Squares Adjustment (LSA) procedure and the quality of fit of the adjusted point cloud to planar/linear features before and after the calibration process. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
An Improved MbICP Algorithm for Mobile Robot Pose Estimation
Appl. Sci. 2018, 8(2), 272; https://doi.org/10.3390/app8020272 - 12 Feb 2018
Cited by 2
Abstract
This paper presents an improved version of the metric-based iterative closest point algorithm to estimate robot poses by matching 2D laser scans with different overlapping percentages. Because of the greatly varied density distribution of realistic point clouds, a resampling method is used to [...] Read more.
This paper presents an improved version of the metric-based iterative closest point algorithm to estimate robot poses by matching 2D laser scans with different overlapping percentages. Because of the greatly varied density distribution of realistic point clouds, a resampling method is used to accelerate the iteration process and protect the calculation of the rejection threshold from being distorted by reducing dense but unhelpful points. A new procedure that combines point-to-point and point-to-line metrics is used to determine the correct correspondence between partially overlapping scans, which maintains both efficiency and accuracy. In addition, a rejection threshold that is based on the MAD-from-median method is utilized to discard correspondences with large distances, which are likely to be incorrect. Experiments show that the improved algorithm is more accurate and robust than the standard algorithm with respect to the existence of non-overlapping areas, and testing demonstrates that it is valid in practice. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
HOVE-Wedge-Filtering of Geomorphologic Terrestrial Laser Scan Data
Appl. Sci. 2018, 8(2), 263; https://doi.org/10.3390/app8020263 - 09 Feb 2018
Cited by 2
Abstract
Terrestrial laser scanning has become an important surveying technique in many fields such as natural hazard assessment. To analyse earth surface processes, it is useful to generate a digital terrain model originated from laser scan point cloud data. To determine the terrain surface [...] Read more.
Terrestrial laser scanning has become an important surveying technique in many fields such as natural hazard assessment. To analyse earth surface processes, it is useful to generate a digital terrain model originated from laser scan point cloud data. To determine the terrain surface as precisely as possible, it is often necessary to filter out points that do not represent the terrain surface. Examples are vegetation, vehicles, and animals. In mountainous terrain with a small-structured topography, filtering is very difficult. Here, automatic filtering solutions usually designed for airborne laser scan data often lead to unsatisfactory results. In this work, we further develop an existing approach for automated filtering of terrestrial laser scan data, which is based on the assumption that no other surface point can be located in the area above a direct line of sight between scanner and another measured point. By taking into account several environmental variables and a repetitive calculation method, the modified method leads to significantly better results. The root-mean-square-error (RSME) for the same test measurement area could be reduced from 5.284 to 1.610. In addition, a new approach for filtering and interpolation of terrestrial laser scanning data is presented using a grid with horizontal and vertical angular data and the measurement length. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessFeature PaperArticle
Object Tracking with LiDAR: Monitoring Taxiing and Landing Aircraft
Appl. Sci. 2018, 8(2), 234; https://doi.org/10.3390/app8020234 - 03 Feb 2018
Cited by 2
Abstract
Mobile light detection and ranging (LiDAR) sensors used in car navigation and robotics, such as the Velodyne’s VLP-16 and HDL-32E, allow for sensing the surroundings of the platform with high temporal resolution to detect obstacles, tracking objects and support path planning. This study [...] Read more.
Mobile light detection and ranging (LiDAR) sensors used in car navigation and robotics, such as the Velodyne’s VLP-16 and HDL-32E, allow for sensing the surroundings of the platform with high temporal resolution to detect obstacles, tracking objects and support path planning. This study investigates the feasibility of using LiDAR sensors for tracking taxiing or landing aircraft close to the ground to improve airport safety. A prototype system was developed and installed at an airfield to capture point clouds to monitor aircraft operations. One of the challenges of accurate object tracking using the Velodyne sensors is the relatively small vertical field of view (30°, 41.3°) and angular resolution (1.33°, 2°), resulting in a small number of points of the tracked object. The point density decreases with the object–sensor distance, and is already sparse at a moderate range of 30–40 m. The paper introduces our model-based tracking algorithms, including volume minimization and cube trajectories, to address the optimal estimation of object motion and tracking based on sparse point clouds. Using a network of sensors, multiple tests were conducted at an airport to assess the performance of the demonstration system and the algorithms developed. The investigation was focused on monitoring small aircraft moving on runways and taxiways, and the results indicate less than 0.7 m/s and 17 cm velocity and positioning accuracy achieved, respectively. Overall, based on our findings, this technology is promising not only for aircraft monitoring but for airport applications. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Evaluation of Handheld Scanners for Automotive Applications
Appl. Sci. 2018, 8(2), 217; https://doi.org/10.3390/app8020217 - 31 Jan 2018
Cited by 3
Abstract
The process of generating a computerized geometric model for an existing part is known as Reverse Engineering (RE). It is a very useful technique in product development and plays a significant role in automotive, aerospace, and medical industries. In fact, it has been [...] Read more.
The process of generating a computerized geometric model for an existing part is known as Reverse Engineering (RE). It is a very useful technique in product development and plays a significant role in automotive, aerospace, and medical industries. In fact, it has been getting remarkable attention in manufacturing industries owing to its advanced data acquisition technologies. The process of RE is based on two primary steps: data acquisition (also known as scanning) and data processing. To facilitate point data acquisition, a variety of scanning systems is available with different capabilities and limitations. Although the optical control of 3D scanners is fully developed, still several factors can affect the quality of the scanned data. As a result, the proper selection of scanning parameters, such as resolution, laser power, shutter time, etc., becomes very crucial. This kind of investigation can be very helpful and provide its users with guidelines to identify the appropriate factors. Moreover, it is worth noting that no single system is ideal in all applications. Accordingly, this work has compared two portable (handheld) systems based on laser scanning and white light optical scanning for automotive applications. A car door containing a free-form surface has been used to achieve the above-mentioned goal. The design of experiments has been employed to determine the effects of different scanning parameters and optimize them. The capabilities and limitations have been identified by comparing the two scanners in terms of accuracy, scanning time, triangle numbers, ease of use, and portability. Then, the relationships between the system capabilities and the application requirements have been established. The results revealed that the laser scanner performed better than the white light scanner in terms of accuracy, while the white light scanner performed better in terms of acquisition speed and triangle numbers. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
From Point Clouds to Building Information Models: 3D Semi-Automatic Reconstruction of Indoors of Existing Buildings
Appl. Sci. 2017, 7(10), 1030; https://doi.org/10.3390/app7101030 - 12 Oct 2017
Cited by 55
Abstract
The creation of as-built Building Information Models requires the acquisition of the as-is state of existing buildings. Laser scanners are widely used to achieve this goal since they permit to collect information about object geometry in form of point clouds and provide a [...] Read more.
The creation of as-built Building Information Models requires the acquisition of the as-is state of existing buildings. Laser scanners are widely used to achieve this goal since they permit to collect information about object geometry in form of point clouds and provide a large amount of accurate data in a very fast way and with a high level of details. Unfortunately, the scan-to-BIM (Building Information Model) process remains currently largely a manual process which is time consuming and error-prone. In this paper, a semi-automatic approach is presented for the 3D reconstruction of indoors of existing buildings from point clouds. Several segmentations are performed so that point clouds corresponding to grounds, ceilings and walls are extracted. Based on these point clouds, walls and slabs of buildings are reconstructed and described in the IFC format in order to be integrated into BIM software. The assessment of the approach is proposed thanks to two datasets. The evaluation items are the degree of automation, the transferability of the approach and the geometric quality of results of the 3D reconstruction. Additionally, quality indexes are introduced to inspect the results in order to be able to detect potential errors of reconstruction. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
LiDARgrammetry: A New Method for Generating Synthetic Stereoscopic Products from Digital Elevation Models
Appl. Sci. 2017, 7(9), 906; https://doi.org/10.3390/app7090906 - 12 Sep 2017
Cited by 2
Abstract
There are currently several new technologies being used to generate digital elevation models that do not use photogrammetric techniques. For example, LiDAR (Laser Imaging Detection and Ranging) and RADAR (RAdio Detection And Ranging) can generate 3D points and reflectivity information of the surface [...] Read more.
There are currently several new technologies being used to generate digital elevation models that do not use photogrammetric techniques. For example, LiDAR (Laser Imaging Detection and Ranging) and RADAR (RAdio Detection And Ranging) can generate 3D points and reflectivity information of the surface without using a photogrammetric approach. In the case of LiDAR, the intensity level indicates the amount of energy that the object reflects after a laser pulse is transmitted. This energy mainly depends on the material and the wavelength used by LiDAR. This intensity level can be used to generate a synthetic image colored by this attribute (intensity level), which can be viewed as a RGB (red, green and blue) picture. This work presents the outline of an innovative method, designed by the authors, to generate synthetic pictures from point clouds to use in classical photogrammetric software (digital restitution or stereoscopic vision). This is conducted using available additional information (for example, the intensity level of LiDAR). This allows mapping operators to view the LiDAR as if it were stereo-imagery, so they can manually digitize points, 3D lines, break lines, polygons and so on. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Axial Fan Blade Vibration Assessment under Inlet Cross-Flow Conditions Using Laser Scanning Vibrometry
Appl. Sci. 2017, 7(8), 862; https://doi.org/10.3390/app7080862 - 22 Aug 2017
Cited by 3
Abstract
In thermal power plants equipped with air-cooled condensers (ACCs), axial cooling fans operate under the influence of ambient flow fields. Under inlet cross-flow conditions, the resultant asymmetric flow field is known to introduce additional harmonic forces to the fan blades. This effect has [...] Read more.
In thermal power plants equipped with air-cooled condensers (ACCs), axial cooling fans operate under the influence of ambient flow fields. Under inlet cross-flow conditions, the resultant asymmetric flow field is known to introduce additional harmonic forces to the fan blades. This effect has previously only been studied numerically or by using blade-mounted strain gauges. For this study, laser scanning vibrometry (LSV) was used to assess fan blade vibration under inlet cross-flow conditions in an adapted fan test rig inside a wind tunnel test section. Two co-rotating laser beams scanned a low-pressure axial fan, resulting in spectral, phase-resolved surface vibration patterns of the fan blades. Two distinct operating points with flow coefficients of 0.17 and 0.28 were examined, with and without inlet cross-flow influence. While almost identical fan vibration patterns were found for both reference operating points, the overall blade vibration increased by 100% at the low fan flow rate as a result of cross-flow, and by 20% at the high fan flow rate. While numerically predicted natural frequency modes could be confirmed from experimental data as minor peaks in the vibration amplitude spectrum, they were not excited significantly by cross-flow. Instead, primarily higher rotation-rate harmonics were amplified; that is, a synchronous blade-tip flapping was strongly excited at the blade-pass frequency. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Analysis on the Accuracy of Intraoral Scanners: The Effects of Mandibular Anterior Interdental Space
Appl. Sci. 2017, 7(7), 719; https://doi.org/10.3390/app7070719 - 13 Jul 2017
Cited by 4
Abstract
In this study, we evaluated the effects of mandibular anterior interdental space on the accuracy of intraoral scanners. Four models of mandibular arch with varying distances of anterior interdental space were analyzed; incisors were evenly spaced out between the two canines by 0 [...] Read more.
In this study, we evaluated the effects of mandibular anterior interdental space on the accuracy of intraoral scanners. Four models of mandibular arch with varying distances of anterior interdental space were analyzed; incisors were evenly spaced out between the two canines by 0 mm, 1 mm, 3 mm, and 5 mm. The full arch of each model was scanned 10 times with iTero® and Trios®. The images were superimposed with those from the reference scanner (Sensable S3) and compared using Geomagic Verify. Statistical analysis was conducted using a t-test, paired t-test, and one-way analysis of variance (ANOVA). Differences in the accuracy of images were statistically significant according to both iTero® and Trios®; a greater deviation was noted with increasing anterior interdental space (p < 0.05). Upon dividing the lower arch into five sections, larger deviation was observed with iTero® in the molar area, except in the model with 1 mm space. With Trios®, the largest deviation was observed in the right molar area in all models. The maximum deviations of intermolar width were 0.66 mm and 0.76 mm in iTero® and Trios®, respectively. This comparison suggests that Trios® demonstrates a higher accuracy than iTero® in all models and in all sections of the arch. However, the mean deviations indicate that both iTero® and Trios® are clinically acceptable. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Simulating the Effects of the Airborne Lidar Scanning Angle, Flying Altitude, and Pulse Density for Forest Foliage Profile Retrieval
Appl. Sci. 2017, 7(7), 712; https://doi.org/10.3390/app7070712 - 10 Jul 2017
Cited by 6
Abstract
Foliage profile is a key biophysical parameter for forests. Airborne Light Detection and Ranging is an effective tool for vegetation parameter retrieval. Data acquisition conditions influence the estimation of biophysical parameters. To acquire accurate foliage profiles at the lowest cost, we used simulations [...] Read more.
Foliage profile is a key biophysical parameter for forests. Airborne Light Detection and Ranging is an effective tool for vegetation parameter retrieval. Data acquisition conditions influence the estimation of biophysical parameters. To acquire accurate foliage profiles at the lowest cost, we used simulations to explore the effects of data acquisition conditions on forest foliage profile retrieval. First, a 3-D forest scene and the airborne small-footprint full-waveform LiDAR data were simulated by the DART model. Second, the foliage profile was estimated from LiDAR data based on a Geometric Optical and Radiative Transfer model. Lastly, the effects of the airborne LiDAR scanning angle, flying altitude, and pulse density on foliage profile retrieval were explored. The results indicated that the scanning angle was an important factor in the foliage profile retrieval, and the optimal scanning angle was 20°. The optimal scanning angle was independent of flying altitude and pulse density, and combinations of multiple scanning angles could improve the accuracy of the foliage profile estimation. The flying altitude and pulse density had little influence on foliage profile retrieval at plot level and could be ignored. In general, our study provides reliable information for selecting the optimal instrument operational parameters to acquire more accurate foliage profiles and minimize data acquisition costs. Full article
(This article belongs to the Special Issue Laser Scanning)
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Open AccessArticle
Analysis of Land Cover Classification Using Multi-Wavelength LiDAR System
Appl. Sci. 2017, 7(7), 663; https://doi.org/10.3390/app7070663 - 28 Jun 2017
Cited by 9
Abstract
The airborne multi-wavelength light detection and ranging (LiDAR) system measures different wavelengths simultaneously and usually includes two or more active channels in infrared and green to acquire both topographic and hydrographic information. The reflected multi-wavelength energy can also be used to identify different [...] Read more.
The airborne multi-wavelength light detection and ranging (LiDAR) system measures different wavelengths simultaneously and usually includes two or more active channels in infrared and green to acquire both topographic and hydrographic information. The reflected multi-wavelength energy can also be used to identify different land covers based on physical properties of materials. This study explored the benefits of multi-wavelength LiDAR in object-based land cover classification, focusing on three major issues: (1) the evaluation of single- and multi-wavelength LiDARs for land cover classification; (2) the performance of spectral and geometrical features extracted from multi-wavelength LiDAR; and (3) the comparison of the vegetation index derived from active multi-wavelength LiDAR and passive multispectral images. The three-wavelength test data were acquired by Optech Titan in green, near-infrared, and mid-infrared channels, and the reference data were acquired from Worldview-3 image. The experimental results show that the multi-wavelength LiDAR provided higher accuracy than single-wavelength LiDAR in land cover classification, with an overall accuracy improvement rate about 4–14 percentage points. The spectral features performed better compared to geometrical features for grass, road, and bare soil classes, and the overall accuracy improvement is about 29 percentage points. The results also demonstrated the vegetation indices from Worldview-3 and Optech Titan have similar characteristics, with correlations reaching 0.68 to 0.89. Overall, the multi-wavelength LiDAR system improves the accuracy of land cover classification because this system provides more spectral information than traditional single-wavelength LiDAR. Full article
(This article belongs to the Special Issue Laser Scanning)
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