Terrestrial Laser Scanning Instruments Research Articles

Deriving Vegetation Indices for 3D Canopy Chlorophyll Content Mapping Using Radiative Transfer Modelling

Leaf chlorophyll content is a major indicator of plant health and productivity. Optical remote sensing estimation of chlorophyll limits its retrievals to two-dimensional (2D) estimates, not allowing examination of its distribution within the canopy, although it exhibits large variation across the vertical profile. Multispectral and hyperspectral Terrestrial Laser Scanning (TLS) instruments can produce three-dimensional (3D) chlorophyll estimates but are not widely available. Thus, in this study, 14 chlorophyll vegetation indices were developed using six wavelengths employed in commercial TLS instruments (532 nm, 670 nm, 808 nm, 785 nm, 1064 nm, and 1550 nm). For this, 200 simulations were carried out using the novel bidirectional mode in the Discrete Anisotropic Radiative Transfer (DART) model and a realistic forest stand. The results showed that the Green Normalized Difference Vegetation Index (GNDVI) of the 532 nm and either the 808 nm or the 785 nm wavelengths were highly correlated to the chlorophyll content (R2 = 0.74). The Chlorophyll Index (CI) and Green Simple Ratio (GSR) of the same wavelengths also displayed good correlation (R2 = 0.73). This study was a step towards canopy 3D chlorophyll retrieval using commercial TLS instruments, but methods to couple the data from the different instruments still need to be developed.

Study on the LAI Single Tree Model Based on Terrestrial Laser Scanning

Leaf area index (LAI) is a key parameter for studying global terrestrial ecology and environment and has great ecological significance. How to accurately measure and calculate structural parameters of trees has become an urgent matter. This paper reports the use of terrestrial laser scanning (TLS) as a measurement tool to achieve accurate LAI estimation through point cloud preprocessing measures, the LeWos algorithm, and voxel methods. The accuracy and feasibility of this indirect measurement method were explored. It is found that the single wood structure parameters extracted from TLS have a good linear relationship with manual measurement, and the extraction errors meet the requirements of real-scene conversion. The study also found when the voxel size is consistent with the minimum distance of the point cloud set by TLS instrument, it has a strong correlation with the measured value of canopy analyser. These results lay the foundation for conveniently and quickly obtaining structural parameters of trees, tree growth state detection, and canopy ecological benefit assessment.

Extraction of discontinuity sets of rocky slopes using iPhone-12 derived 3DPC and comparison to TLS and SfM datasets

Characterisation of a rock mass requires data from the intact rock along with the discontinuities. Assuming that the discontinuities are planar, its characterisation requires its number and orientation. This leads to the analysis of the normal spacing, the persistence and the roughness, among others. The geometrical analysis of the surface enables the calculation of the parameters to characterise the discontinuities, and the use of digital datasets enhance them. Remote sensing techniques, such as the Terrestrial Laser Scanning (TLS) instruments of Structure from Motion (SfM) technique, provide 3D point clouds that enable the geometrical analysis. The scientific community has been testing both techniques since the 2000s, and companies are introducing their use in their workflows. However, the cost of the TLS instrument could still be a barrier to its use to most scholars. Because of this, the community shows a growing interest in Remotely Piloted Aircraft Systems (RPAS) equipped with digital cameras or in smartphones equipped with high-quality cameras to capture digital datasets of rocky slopes. The SfM workflow processes the captured images, reconstructing the rocky slope through a 3D point cloud and textured meshes. Although previous studies show that the SfM-derived point clouds present less quality than TLS-derived datasets in terms of accuracy, the use of SfM is still of interest because of its cost. In 2020 Apple launched the Iphone-12 device, which is equipped with a LiDAR sensor that is not used to capture the surface coordinates but to enhance the photo’s quality. Since then, the community has developed several applications to reconstruct 3D surfaces using this device. This leads to consider this device as an intermediate option between the TLS and SfM to characterise rocky slopes and their discontinuities. In this communication we explore the digitalisation of a rocky slope via TLS instruments, SfM technique and using the Iphone-12 device. It comprises a 26 meter high mechanically excavated rocky slope in Cretaceous marlstones and limestones. To capture the surface, we used three configurations, and we found that to scan ground surface the distance device-surface had to be less than 3 meters. The discontinuities are characterised using the three sources of information using the DSE software. The results show a promising match compared to the TLS or SfM. This evidences that these devices will soon be widely employed for evaluating rocky slopes.

Sensitivity of voxel-based estimations of leaf area density with terrestrial LiDAR to vegetation structure and sampling limitations: A simulation experiment

The need for fine scale description of vegetation structure is increasing as Leaf Area Density (LAD, m2/m3) becomes a critical parameter to understand ecosystem functioning and energy and mass fluxes in heterogeneous ecosystems. Terrestrial Laser Scanning (TLS) has shown great potential for retrieving the foliage area at stand, plant or voxel scales. Several sources of measurement errors have been identified and corrected over the past years. However, measurements remain sensitive to several factors, including, 1) voxel size and vegetation structure within voxels, 2) heterogeneity in sampling from TLS instrument (occlusion and shooting pattern), the consequences of which have been seldom analyzed at the scale of forest plots. In the present paper, we aimed at disentangling biases and errors in plot-scale measurements of LAD with TLS in a simulated vegetation scene. Two negative biases were formerly attributed to (i) the unsampled voxels and to (ii) the subgrid vegetation heterogeneity (i.e. clumping effect), and then quantified, thanks to a the simulation experiment providing known LAD references at voxel scale, vegetation manipulations and unbiased point estimators. We used confidence intervals to evaluate voxel-scale measurement accuracy.

Simulation of tree point cloud based on the ray-tracing algorithm and three-dimensional tree model

A terrestrial laser scanning (TLS) instrument can scan trees to get three-dimensional (3D) point clouds with detailed tree structures. However, some factors, such as surrounding environment, tree structure, the characteristics and scanning parameters of TLS instrument, can affect the scanning result. Simulation of tree point cloud is an efficient way to avoid and analyse the influence of the above factors. A simulation method was proposed to simulate tree point clouds by using the ray-tracing algorithm and 3D tree models. Thirteen tree models were tested with the proposed simulation method. Ten tree models are from the virtual model library, and other three tree models are constructed from real point cloud data. The accuracy of the simulated results was evaluated by using the absolute value of Hausdorff distances (AVOHD). Although the results showed that leaves and more branches will decrease simulation accuracy, the proposed method is effective and accurate on tree point cloud simulation.

THE IMPACT OF THE DISTRIBUTION OF OBSERVATIONS ON TERRESTRIAL LASER SCANNER SELF-CALIBRATION QUALITY

Abstract. Terrestrial laser scanning (TLS) instruments find routine use for a range of precision engineering measurement applications. Depending on the accuracy requirements for a specific project, the instrument may require self-calibration to determine systematic error model terms. One of the goals of first-order network design for self-calibration is to acquire observations throughout the full instrumental field-of-view. Experience calibrating TLS instruments has demonstrated that while this goal can be achieved for horizontal deflection angle observations, it is seldom realized for the vertical angle observations. This paper presents results from a preliminary investigation into the impact of the distribution of vertical angle observations on the estimation of two critical systematic error parameters in TLS instruments: the collimation axis error and the trunnion axis error. First, a model to characterize the empirical observation distributions is developed. The model is a function of a single shape parameter that quantifies observation dispersion. Then, a means to estimate the impact of the distribution on the parameter estimation is developed. Results from six real datasets show the distribution model characterizes the overall general trend of the observations. Simulated results show the relative independence of the collimation axis error and the strong dependence of the trunnion axis error on the shape parameter.

Comparing terrestrial laser scanners’ ability to measure tree height and diameter in a managed forest environment

ABSTRACT Terrestrial laser scanning (TLS) instruments like the Zebedee (including the Zeb-1 and its derivatives) and Dual-Wavelength Echidna LiDAR (DWEL) can usefully measure tree diameter, height and annual height growth for trees up to at least 10 m tall in a forest plantation setting. In a case study at the National Arboretum Canberra, these two types of scanner were compared with traditional methods (diameter tape and height stick) in the measurement of tree diameter, height and growth rate. The TLS instruments were accurate with height measurement correlations of not different to 1:1 and with r 2 = 0.99 and 0.98 (Zeb-1 and DWEL, respectively), whereas comparing measurements of height growth over a two-year period, the TLS instruments displayed an agreement of 83% and 93%, respectively. The Zebedee instruments also correlated significantly with diameter at breast height (DBH; r 2 = 0.97), although limited tests with the DWEL collected at a single point in inventory plots did not find a significant correlation. The additional detail provided by TLS instruments such as Zeb-1and DWEL has the potential to substantially improve data collected about the spatial distribution, canopy dimensions and trunk parameters of numerous trees in an operational inventory of a forest stand. However, there may be difficulties in developing and testing new reference definitions of these parameters, especially ‘diameter’, that would allow historical comparisons and improved future use for foresters and ecologists.

Realistic Forest Stand Reconstruction from Terrestrial LiDAR for Radiative Transfer Modelling

Forest biophysical variables derived from remote sensing observations are vital for climate research. The combination of structurally and radiometrically accurate 3D “virtual” forests with radiative transfer (RT) models creates a powerful tool to facilitate the calibration and validation of remote sensing data and derived biophysical products by helping us understand the assumptions made in data processing algorithms. We present a workflow that uses highly detailed 3D terrestrial laser scanning (TLS) data to generate virtual forests for RT model simulations. Our approach to forest stand reconstruction from a co-registered point cloud is unique as it models each tree individually. Our approach follows three steps: (1) tree segmentation; (2) tree structure modelling and (3) leaf addition. To demonstrate this approach, we present the measurement and construction of a one hectare model of the deciduous forest in Wytham Woods (Oxford, UK). The model contains 559 individual trees. We matched the TLS data with traditional census data to determine the species of each individual tree and allocate species-specific radiometric properties. Our modelling framework is generic, highly transferable and adjustable to data collected with other TLS instruments and different ecosystems. The Wytham Woods virtual forest is made publicly available through an online repository.

Estimation of vegetation water content at leaf and canopy level using dual-wavelength commercial terrestrial laser scanners.

Vegetation water content, quantified as the leaf equivalent water thickness (EWT), can serve as an indicator of vegetation stress. The intensity data recorded by terrestrial laser scanning (TLS) instruments, operating at shortwave infrared wavelengths, can be used to estimate the three-dimensional distribution of EWT, after a full and rigorous calibration for the range and incidence angle effects. However, TLS instruments do not record the incidence angles automatically, making calibration challenging. In this study, intensity data from two commercially available TLS instruments (Leica P40, 1550 nm shortwave infrared wavelength, and Leica P20, 808 nm near-infrared wavelength) were combined in a normalized difference index (NDI). The NDI was found to minimize the incidence angle effects with no need for further calibration. A dry-down experiment was conducted using deciduous and conifer canopies. The NDI was found to be highly correlated to EWT at leaf level (R2 of 0.91 and 0.74) and at canopy level (R2 of 0.89 and 0.74) for the deciduous and conifer canopies, respectively. Three-dimensional distributions of EWT at canopy level were generated, which revealed some vertical heterogeneity.

TLS-based profile model analysis of major composite structures with robust B-spline method

Abstract With the development of city constructions, tunnels are becoming important structures for underground transportation. Tunnels constitute layered composite structures with concrete, reinforcement, waterproof layers, etc. Deformation monitoring of this kind of wide-ranging composite structure is significant to assure their safety considering the development of their complexity. Terrestrial laser scanning (TLS) is one of the most accurate and fast measurement technologies for deformation analysis. It has been applied widely in survey fields with the advantages of non-contact and panoramic acquisition of information. In this situation, TLS instruments are being developed rapidly, which necessitates high requirements regarding software aspects, especially concerning high-accuracy model construction. Therefore, developing a reliable method for 3D modeling with complex and massive point clouds is urgent. In this paper, we propose an adaptive expectation maximization (EM) method based on the scaled t-distribution for B-spline estimation, where automation is achieved for the best approximation with the maximum probability density. The innovation of this paper lies in offering a robust, automatic and time-efficient solution to model practical tunnel structures with a complex point cloud.

Evaluation of the Range Accuracy and the Radiometric Calibration of Multiple Terrestrial Laser Scanning Instruments for Data Interoperability

Terrestrial laser scanning (TLS) data provide 3-D measurements of vegetation structure and have the potential to support the calibration and validation of satellite and airborne sensors. The increasing range of different commercial and scientific TLS instruments holds challenges for data and instrument interoperability. Using data from various TLS sources will be critical to upscale study areas or compare data. In this paper, we provide a general framework to compare the interoperability of TLS instruments. We compare three TLS instruments that are the same make and model, the RIEGL VZ-400. We compare the range accuracy and evaluate the manufacturer’s radiometric calibration for the uncalibrated return intensities. Our results show that the range accuracy between instruments is comparable and within the manufacturer’s specifications. This means that the spatial XYZ data of different instruments can be combined into a single data set. Our findings demonstrate that radiometric calibration is instrument specific and needs to be carried out for each instrument individually before including reflectance information in TLS analysis. We show that the residuals between the calibrated reflectance panels and the apparent reflectance measured by the instrument are greatest for highest reflectance panels (residuals ranging from 0.058 to 0.312).

Intensity data correction based on incidence angle and distance for terrestrial laser scanner

Terrestrial laser scanning (TLS) systems not only convey geometric information but also deliver backscattered intensity data of the scanned targets, which are generally acknowledged as significant observations that involve target reflectance information. Numerous studies have shown the potential of TLS intensity in many applications. However, directly using original intensity data is not recommended because of the existence of radiometric system variations, scan geometry discrepancies, environmental differences, and other factors. Performing radiometric correction is, therefore, indispensable to fully eliminate these effects and to compute target reflectance as accurately as possible. In this study, we investigate the effects of incidence angle and distance on intensity measurements and propose a practical method to eliminate the two effects for different TLS instruments. The results show that incidence angle and distance significantly influence TLS intensity data, and the proposed model with versatility can accurately correct the laser intensity value to eliminate the effects of incidence angle and distance.

Studying monogenetic volcanoes with a terrestrial laser scanner: case study at Croscat volcano (Garrotxa Volcanic Field, Spain)

Erosional processes (natural or anthropogenic) may partly destroy the relatively small-sized volcanic edifices characteristic of monogenetic volcanic zones, leaving their internal structure well exposed. Nevertheless, the study of these outcrops may be extremely challenging due to restricted accessibility or safety issues. Digital representations of the outcrop surface have been lately used to overcome such difficulties. Data acquired with terrestrial laser scanning instruments using Light Detection and Ranging technology enables the construction of such digital outcrops. The obtained high-precision 3-D terrain models are of greater coverage and accuracy than conventional methods and, when taken at different times, allow description of geological processes in time and space. Despite its intrinsic advantages and the proven satisfactory results, this technique has been little applied in volcanology-related studies. Here, we want to introduce it to the volcanological community together with a new and user-friendly digital outcrop analysis methodology for inexperienced users. This tool may be useful, not only for volcano monitoring purposes, but also to describe the internal structure of exposed volcanic edifices or to estimate outcrop erosion rates that may be helpful in terms of hazard assessment or preservation of volcanic landscapes. We apply it to the Croscat volcano, a monogenetic cone in the La Garrotxa Volcanic Field (Catalan Volcanic Zone, NE Spain), quarrying of which leads to a perfect view of its interior but restricts access to its uppermost parts. Croscat is additionally one of the most emblematic symbols of the La Garrotxa Volcanic Field Natural Park, and its preservation is a main target of the park administration.

Evaluation of the dynamic processes of a landslide with laser scanners and Bayesian methods

This paper deals with the study of the dynamics of a landslide from two different but complementary point of views. The landslide is situated within the Miozza basin, an area of approximately 10.7 km2 located in the Alpine region of Carnia (Italy). In the first part of the paper, the macro-scale analysis of volumetric changes occurred after the reactivation of landslide in 2004 is addressed by using a two-epoch laser scanning surveys from airborne (ALS) and terrestrial (TLS) platforms. airborne laser scanning (ALS) data were collected in 2003 (before reactivation of the phenomenon) with an ALTM 3033 OPTECH sensor while terrestrial laser scanning (TLS) measurements were acquired in 2008 with a Riegl LMS-Z620. The second part of the paper deals with the study of dynamic processes of the landslide at micro-scale. To this aim, a global navigation satellite system (GNSS)-based monitoring network is analysed using a statistical approach to discriminate between measurement noise and possible actual displacements. This task is accomplished using both “classical” statistical testing and a Bayesian approach. The second method has been employed to verify some apparent vertical displacements detected by the classical test.As regards the first topic of the paper, achieved results show that long-range TLS instruments can be profitably used in mountain areas to provide high-resolution digital terrain models (DTMs) with superior quality and detail with respect to aerial light detection and ranging data only, even in areas with very low accessibility. Moreover, ALS- and TLS-derived DTMs can be combined each other in order to fill gaps in ALS data, mainly due to the complexity of terrain morphology, and to perform quite accurate calculations of volume changes due to landslide phenomenon. Finally, the outcomes of the application of Bayesian inference demonstrate the effectiveness of this method to better detect statistically significant displacements of a GNSS monitoring network points. However, the application of this method in the geodetic field requires the identification of a preferring direction of displacements, what is not always feasible in advance.

Implications of sensor configuration and topography on vertical plant profiles derived from terrestrial LiDAR

The vertical distribution of plant constituents is a key parameter to describe vegetation structure and influences several processes, such as radiation interception, growth and habitat. Terrestrial laser scanning (TLS), also referred to as terrestrial LiDAR, has the potential to measure the canopy structure with high spatial detail and accuracy. Vertical plant profiles, which describe the plant area per unit volume (PAVD) as a function of height, are often used to quantify the vertical structure. However, most studies do not account for topography, use registered multiple TLS scans or use a detailed airborne LiDAR digital terrain model to account for this variation in ground height. Airborne LiDAR is often not available or expensive to acquire. Here, we present an approach that facilitates rapid, robust and automated assessment of the vertical structure of vegetation. We use single scans and local plane fitting to correct for topographic effects in vertical plant profiles and test our approach in five different Australian forest types with different topography and understorey. We validate our approach with topography-corrected vertical plant profiles with digital terrain models derived from airborne LiDAR. Our results demonstrate that not correcting for topography can lead to significant errors in the vertical distribution of plant constituents (CV(RMSE) up to 66.2%, typically ranging from 4.2% to 13.8%). This error decreases significantly when topography is accounted for with TLS plane fitting (CV(RMSE) up to 20.6%, typically ranging from 1.5% to 12.6%). We demonstrate that height metrics from vertical plant profiles that are not corrected for topography depart significantly from those that are inferred from the reference profile. The effect is most noticeable for canopy top height and the peak PAVD height. Correcting topography with a TLS plane fitting approach reduces the error in canopy top height by at least 77% and up to 100%, and reduces the error in peak PAVD height by 83.3% and up to 100%. We also show the advantage of a multiple return over a first return TLS instrument. The definition of the ground returns with a first return instrument might be problematic in environments with dense herbaceous understorey and there is an overall trend of lower height metrics compared to multiple return instruments. We present a data-driven approach that is based on single scan TLS data. The latter is of importance for large area sampling as it allows more sites to be sampled from existing resources and facilitate consistent processing of archived TLS data, which is often single scan data with no survey control.

Detecting surface moisture in aeolian environments using terrestrial laser scanning

Surface moisture plays a key role in determining erodibility of sandy and dusty surfaces in semi-arid and coastal environments. Where aeolian processes are active, sedimentation patterns may rapidly change the soil moisture on a thin veneer of the surface that determines sediment entrainment. Here we present terrestrial laser scanning (TLS) as a promising method for detecting moisture at high temporal and spatial resolution within the range where aeolian transport is possible and illustrate its applicability using playa and beach case studies. TLS instruments are active sensors that record the return intensity (or backscatter) of a laser pulse. This signal intensity is influenced by both distance and surface properties. Calibration relationships are outlined that correct for both distance and moisture and explore the influence of grain size and mineralogy. We also show that by normalising intensity using a dry surface, the resulting relative ratio infers changes in moisture patterns and is a useful alternative when sediment calibrations are not available.

In Situ Quantification of Experimental Ice Accretion on Tree Crowns Using Terrestrial Laser Scanning

In the eastern hardwood forests of North America ice storms are an important disturbance event. Ice storms strongly influence community dynamics as well as urban infrastructure via catastrophic branch failure; further, the severity and frequency of ice storms are likely to increase with climate change. However, despite a long-standing interest into the effects of freezing rain on forests, the process of ice accretion and thus ice loading on branches remains poorly understood. This is because a number of challenges have prevented in situ measurements of ice on branches, including: 1) accessing and measuring branches in tall canopies, 2) limitations to travel during and immediately after events, and 3) the unpredictability of ice storms. Here, utilizing a novel combination of outdoor experimental icing, manual measurements and terrestrial laser scanning (TLS), we perform the first in situ measurements of ice accretion on branches at differing heights in a tree crown and with increasing duration of exposure. We found that TLS can reproduce both branch and iced branch diameters with high fidelity, but some TLS instruments do not detect ice. Contrary to the expectations of ice accretion models, radial accretion varied sharply within tree crowns. Initially, radial ice accretion was similar throughout the crown, but after 6.5 hours of irrigation (second scanning) radial ice accretion was much greater on upper branches than on lower (∼factor of 3). The slope of the change in radial ice accretion along branches increased with duration of exposure and was significantly greater at the second scanning compared to the first. We conclude that outdoor icing experiments coupled with the use of TLS provide a robust basis for evaluation of models of ice accretion and breakage in tree crowns, facilitating estimation of the limiting breaking stress of branches by accurate measurements of ice loads.

Characterization and monitoring of the Åknes rockslide using terrestrial laser scanning

Abstract. Terrestrial laser scanning (TLS) provides high-resolution point clouds of the topography and new TLS instruments with ranges exceeding 300 m or even 1000 m are powerful tools for characterizing and monitoring slope movements. This study focuses on the 35 million m3 Åknes rockslide in Western Norway, which is one of the most investigated and monitored rockslides in the world. The TLS point clouds are used for the structural analysis of the steep, inaccessible main scarp of the rockslide, including an assessment of the discontinuity sets and fold axes. TLS acquisitions in 2006, 2007 and 2008 provide information on 3-D displacements for the entire scanned area and are not restricted like conventional survey instruments to single measurement points. The affine transformation matrix between two TLS acquisitions precisely describes the rockslide displacements and enables their separation into translational components, such as the displacement velocity and direction, and rotational components, like toppling. This study shows the ability of TLS to obtain reliable 3-D displacement information over a large, unstable area. Finally, a possible instability model for the upper part of Åknes rockslide explains the measured translational and rotational displacements by a combination of southward planar sliding along the gneiss foliation, gravitational vertical settlement along the complex, stepped basal sliding surface and northward toppling toward the opened graben structure.