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G. Mandlburger

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Karadeniz Technical University

Istanbul Technical University

Daniel Morleghem

Sapienza University of Roma

Historic Environment Scotland

Adam Mickiewicz University in Poznań

Kamila Reczynska

University of Wroclaw

Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic

German Research Center for Artificial Intelligence

SASTRA University

daniela gigante

University of Perugia, Italy

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Papers by G. Mandlburger

Ultra-High Precision Uav-Based Lidar and Dense Image Matching

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2018

This paper presents a study on the potential of ultra-high accurate UAV-based 3D data capture. It... more This paper presents a study on the potential of ultra-high accurate UAV-based 3D data capture. It is motivated by a project aiming at the deformation monitoring of a ship lock and its surrounding. This study is part of a research and development project initiated by the German Federal Institute of Hydrology (BfG) in Koblenz in partnership with the Office of Development of Neckar River Heidelberg (ANH). For this first official presentation of the project, data from the first flight campaign will be analysed and presented. Despite the fact that monitoring aspects cannot be discussed before data from additional flight campaigns will be available later this year, our results from the first campaign highlight the potential of high-end UAV-based image and LiDAR sensors and their data fusion. So far, only techniques from engineering geodesy could fulfil the aspired accuracy demands in the range of millimetres. To the knowledge of the authors, this paper for the first time addresses such ultra-high accuracy applications by combing high precision UAVbased LiDAR and dense image matching. As the paper is written at an early stage of processing only preliminary results can be given here.

Mapping of Water Surface Levels and Slopes with Single Photon Lidar – a Case Study at the River Rhine

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences

Precise knowledge of water surface level heights is crucial for safe ship navigation and as basis... more Precise knowledge of water surface level heights is crucial for safe ship navigation and as basis for calibration of hydrodynamicnumerical models. While Airborne Laser Scanning (ALS) is a well established technique for topographic mapping, ALS-based water surface mapping using conventional infrared lasers suffers from the high degree of specular reflection which leads to data voids for off-nadir angles beyond 5-7 degrees. The advent of single photon sensitive ALS systems using green laser sources presents the prospect of large-area, high-resolution water surface mapping due to the high receiver sensitivity and measurement rate of such systems. Building on previous studies on subject matters, we present the results of a pilot project initiated and conducted by the German Federal Institute of Hydrology (BfG, Koblenz) at the Rhine River. Three specific test sites with varying water surface and flow velocity properties were captured on October 30 th and 31 th , 2019 with the Leica SPL100 from flying altitudes of 3000 m, 2500 m, 1600 m, and 800 m, respectively. As anticipated, the water surface laser pulse density was high and exhibited 20-145 points/m 2 depending on flying altitude. After quality control, strip adjustment, and point cloud analysis, three water surface classification methods were implemented based on: (i) height quantiles, (ii) point cloud segmentation, and (iii) inverse DTM filtering. All approaches featured relative and absolute water level height accuracies better than 10 cm. We conclude that Single Photon LiDAR based high resolution mapping of water surface levels and tilts is feasible when employing application specific data acquisition parameters, i.e., off-nadir angle ≤10 • and flying altitude ≤3000 m.

Airborne Hydrographic LiDAR Mapping - Potential of a new technique for capturing shallow water bodies

Chan, F., Marinova, D. and Anderssen, R.S. (eds) MODSIM2011, 19th International Congress on Modelling and Simulation., 2011

In this paper, we present the general design of a hydrographic laser scanner (prototype instrumen... more In this paper, we present the general design of a hydrographic laser scanner (prototype instrument) manufactured by the company Riegl Laser Measurement Systems in cooperation with the University of Innsbruck, Unit of Hydraulic Engineering. The instrument utilizes a green laser source (λ=532 nm) capable of penetrating the water column. Very short laser pulses (1 ns) are emitted and the backscattered signal is digitized enabling sophisticated waveform processing online during the flight and in post processing. In combination with a traditional topographic airborne laser scanner (λ=1500 nm) mounted on the same platform a complete hydrographic and topographic survey of the riparian foreland, the water surface and river bed can be carried out in a single campaign. In contrast to existing bathymetric LiDAR systems, the presented system uses only medium pulse energy but a high pulse repetition rate of up to 250 kHz and, thus, focuses on a detailed description of shallow waters under clear water conditions.

Airborne laser bathymetry – detecting and recording submerged archaeological sites from the air

Journal of Archaeological Science, 2013

A new generation of airborne bathymetric laser scanners utilises short green laser pulses for hig... more A new generation of airborne bathymetric laser scanners utilises short green laser pulses for high resolution hydrographic surveying in very shallow waters. The paper investigates its use for the documentation of submerged archaeological structures, introducing the concept of airborne laser bathymetry and focussing on a number of challenges this novel technology still has to face. Using this method, an archaeological pilot study on the northern Adriatic coast of Croatia has revealed sunken structures of a Roman villa. The results demonstrate the potential of this novel technique to map submerged archaeological structures over large areas in high detail in 3D, for the first time providing the possibility for systematic, large-scale archaeological investigation of this environment. The resulting maps will provide unique means for underwater heritage management.

Airborne laser scanning derived terrain models for modelling overland flow

Measuring the Topography of Submerged Archaeological Sites from the Air

Archaeological Prospection, May 29, 2013

A Framework for Generic Spatial Search in 3D Point Clouds

ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2021

Modern data acquisition with active or passive photogrammetric imaging techniques generally resul... more Modern data acquisition with active or passive photogrammetric imaging techniques generally results in 3D point clouds. Depending on the acquisition or processing method, the spacing of the individual points is either uniform or irregular. In the latter case, the neighbourhood definition like for digital images (4-or 8-neighbourhood, etc.) cannot be applied. Instead, analysis requires a local point neighbourhood. The local point neighbourhood with conventional k-nearest neighbour or fixed distance searches often produce sub-optimal results suffering from the inhomogeneous point distribution. In this article, we generalize the neighbourhood definition and present a generic spatial search framework which explicitly deals with arbitrary point patterns and aims at optimizing local point selection for specific processing tasks like interpolation, surface normal estimation and point feature extraction, spatial segmentation, and such like. The framework provides atomic 2D and 3D search strategies, (i) k-nearest neighbour, (ii) region query, (iii) cell based selection, and (iv) quadrant/octant based selection. It allows to freely combine the individual strategies to form complex, conditional search queries as well as specifically tailored point sub-selection. The benefits of such a comprehensive neighbourhood search approach are showcased for feature extraction and surface interpolation of irregularly distributed points.

Water Surface Reconstruction in Airborne Laser Bathymetry from Redundant Bed Observations

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences, 2017

In airborne laser bathymetry knowledge of exact water level heights is a precondition for applyin... more In airborne laser bathymetry knowledge of exact water level heights is a precondition for applying run-time and refraction correction of the raw laser beam travel path in the medium water. However, due to specular reflection especially at very smooth water surfaces often no echoes from the water surface itself are recorded (drop outs). In this paper, we first discuss the feasibility of reconstructing the water surface from redundant observations of the water bottom in theory. Furthermore, we provide a first practical approach for solving this problem, suitable for static and locally planar water surfaces. It minimizes the bottom surface deviations of point clouds from individual flight strips after refraction correction. Both theoretical estimations and practical results confirm the potential of the presented method to reconstruct water level heights in dm precision. Achieving good results requires enough morphological details in the scene and that the water bottom topography is cap...

The Opals Data Manager – Efficient Data Management for Processing Large Airborne Laser Scanning Projects

ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2012

The fast measurement rate of today's Airborne Laser Scanners results in billions of points for si... more The fast measurement rate of today's Airborne Laser Scanners results in billions of points for single ALS projects. Efficient algorithms and data management methods are, therefore, a precondition for successful project handling. The software package OPALS (Orientation and Processing of Airborne Laser Scanning data) was especially designed to meet those criteria. Central core of the package is the OPALS Data Manager (ODM). It provides both, fast spatial access to huge point clouds, as well as a flexible attribute schema to store additional point related quantities. Concepts of the spatial data organization and implementation details about the attribute handling are presented. Additionally, design rationales of the ODM, its file format and the system performance are described.

State of the art in high accuracy high detail DTMs derived from ALS

Archaeological Ground Point Filtering of Airborne Laser Scan Derived Point-Clouds in a Difficult Mediterranean Environment

Journal of Computer Applications in Archaeology, 2020

Digital terrain models (DTM) based on airborne laser scanning (ALS) are an important source for i... more Digital terrain models (DTM) based on airborne laser scanning (ALS) are an important source for identifying and monitoring archaeological sites and landscapes. However, a DTM is only one of many representations of a given surface. Its accuracy and quality must conform to its purpose and are a result of several considerations and decisions along the processing chain. One of the most important factors of ALS-based DTM generation is ground point filtering, i.e., the classification of the acquired point-cloud into terrain and off-terrain points. Filtering is not straightforward. The resulting DTM is usually a compromise that might show the surface below very dense vegetation while losing detail in other areas. In this paper, we show that in very complex situations (e.g., strongly varying vegetation cover), an optimal compromise is difficult to achieve, and more than one filter with different settings adapted to the varying degree of vegetation cover is necessary. For practical reasons, the results need to be combined into a single DTM. This is demonstrated using the case study of a Mediterranean landscape in Croatia, which consists of open areas (agricultural and grassland), olive plantations, as well as extremely dense and evergreen macchia vegetation. The results are the first step toward an adaptive ground point filtering strategy that might be useful far beyond the field of archaeology.

Progress in LiDAR sensor technologychance and challenge for DTM generation and data administration

Proceedings of the 51st …, 2007

Airborne laser scanning (ALS, also referred to as LIDAR (light detection and ranging)) became a s... more Airborne laser scanning (ALS, also referred to as LIDAR (light detection and ranging)) became a standard method for the acquisition of topographic data. However, since the first commercial flights in 1994 a significant development in the sensor technology can be observed. These developments lead to a high increase in data volume due to a significant higher number of determined echoes as well as due to an increased number of information stored per echo. Next to a short presentation of the current sensor developments this paper focuses on the current challenges in management of ALS data (including the storage of additional information provided by full-waveform ALS sensors). Furthermore, advanced procedures for DTM generation integrating geometric criteria with these additional attributes per echo are discussed.

Vertical Vegetation Structure Analysis and Hydraulic Roughness Determination Using Dense Als Point Cloud Data–A Voxel Based Approach

In this contribution the complexity of the vertical vegetation structure, based on dense airborne... more In this contribution the complexity of the vertical vegetation structure, based on dense airborne laser scanning (ALS) point cloud data (25 echoes/m 2), is analyzed to calculate vegetation roughness for hydraulic applications. Using the original 3D ALS point cloud, three levels of abstractions are derived (cells, voxels and connections) to analyze ALS data based on a 1x1 m 2 raster over the whole data set. A voxel structure is used to count the echoes in predefined detrended height levels within each cell. In general, it is assumed that the number of voxels containing echoes is an indicator for elevated objects and consequently for increased roughness. Neighboring voxels containing at least one data point are merged together to connections. An additional height threshold is applied to connect vertical neighboring voxels with a certain distance in between. Thus, the connections indicate continuous vegetation structures. The height of the surface near or lowest connection is an indicator for hydrodynamic roughness coefficients. For cells, voxels and connections the laser echoes are counted within the structure and various statistical measures are calculated. Based on these derived statistical parameters a rule-based classification is developed by applying a decision tree to assess vegetation types. Roughness coefficient values such as Manning's n are estimated, which are used as input for 2D hydrodynamic-numerical modeling. The estimated Manning's values from the ALS point cloud are compared with a traditional Manning's map. Finally, the effect of these two different Manning's n maps as input on the 2D hydraulics are quantified by calculating a height difference model of the inundated depth maps. The results show the large potential of using the entire vertical vegetation structure for hydraulic roughness estimation.

A Comparison of Single Photon and Full Waveform Lidar

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

Single photon sensitive LiDAR sensors are currently competing with conventional multi-photon lase... more Single photon sensitive LiDAR sensors are currently competing with conventional multi-photon laser scanning systems. The advantage of the prior is the potentially higher area coverage performance, which comes at the price of an increased outlier rate and a lower ranging accuracy. In this contribution, the principles of both technologies are reviewed with special emphasis on their respective properties. In addition, a comparison of Single Photon LiDAR (SPL) and Full Waveform LiDAR data acquired in July and September 2018 in the City of Vienna are presented. From data analysis we concluded that (i) less flight strips are needed to cover the same area with comparable point density with SPL, (ii) the sharpness of the resulting 3D point cloud is higher for the waveform LiDAR dataset, (iii) SPL exhibits moderate vegetation penetration under leaf-on conditions, and (iv) the dispersion of the SPL point cloud assessed in smooth horizontal surface parts competes with waveform LiDAR but is higher by a factor of 2-3 for inclined and grassy surfaces, respectively. Still, SPL yielded satisfactory precision measures mostly below 10 cm.

Analyzing near water surface penetration in laser bathymetry-A case study at the River Pielach

Recent developments in sensor technology yielded a major progress in airborne laser bathymetry fo... more Recent developments in sensor technology yielded a major progress in airborne laser bathymetry for capturing shallow water bodies. Modern topo-bathymetric small foot print laser scanners do no longer use the primary near infrared (NIR) signal (λ=1064 nm) but only emit and receive the frequency doubled green signal (λ=532 nm). For calculating correct water depths accurate knowledge of the water surface (air-water-interface) is mandatory for obtaining accurate spot positions and water depths. Due to the ability of the green signal to penetrate water the first reflections do not exactly represent the water surface but, depending on environmental parameters like turbidity, a certain penetration into the water column can be observed. This raises the question if it is even feasible to determine correct water level heights from the green laser echoes only. In this article, therefore, the near water surface penetration properties of the green laser signal are analyzed based on a test flight of the River Pielach (Austria) carried out with Riegl's VQ-820-G (532 nm) and VQ-580 (1064 nm) scanners mounted on the same airborne platform. It is shown that within the study area the mean penetration into the water column is in the range of 10-25 cm compared to the NIR signal as reference. However, as the upper hull of the green water surface echoes coincides with the NIR signal with cm-precision, it is still possible to derive water surface models from the green laser echoes only via statistical analysis of aggregated neighboring echoes and robustly keep the underestimation of the water level below 6 cm. This especially holds for still and stationary flowing water bodies.

Efficient Loading and Visualization of Massive Feature-Rich Point Clouds Without Hierarchical Acceleration Structures

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences

Nowadays, point clouds are the standard product when capturing reality independent of scale and m... more Nowadays, point clouds are the standard product when capturing reality independent of scale and measurement technique. Especially, Dense Image Matching (DIM) and Laser Scanning (LS) are state of the art capturing methods for a great variety of applications producing detailed point clouds up to billions of points. In-depth analysis of such huge point clouds typically requires sophisticated spatial indexing structures to support potentially long-lasting automated non-interactive processing tasks like feature extraction, semantic labelling, surface generation, and the like. Nevertheless, a visual inspection of the point data is often necessary to obtain an impression of the scene, roughly check for completeness, quality, and outlier rates of the captured data in advance. Also intermediate processing results, containing additional per-point computed attributes, may require visual analyses to draw conclusions or to parameterize further processing. Over the last decades a variety of commercial, free, and open source viewers have been developed that can visualise huge point clouds and colorize them based on available attributes. However, they have either a poor loading and navigation performance, visualize only a subset of the points, or require the creation of spatial indexing structures in advance. In this paper, we evaluate a progressive method that is capable of rendering any point cloud that fits in GPU memory in real time without the need of time consuming hierarchical acceleration structure generation. In combination with our multi-threaded LAS and LAZ loaders, we achieve load performance of up to 20 million points per second, display points already while loading, support flexible switching between different attributes, and rendering up to one billion points with visually appealing navigation behaviour. Furthermore, loading times of different data sets for different open source and commercial software packages are analysed.

Comparison of Subaquatic Digital Elevation Models from Airborne Laser Scanning and Imagery

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

The paper describes and compares the workflows and results of generating digital elevation models... more The paper describes and compares the workflows and results of generating digital elevation models (DEMs) of underwater areas from airborne laser scanning and aerial stereo images. Based on a combined laser scanning/image data set of an artificial lake, both methods are described and pros/cons are highlighted. The authors focus on the final results, especially on accuracy, completeness and spatial resolution of the underwater DEM's. Further, practical aspects of processing and complexity of both methods are highlighted too.

Hybrid Orientation of Airborne Lidar Point Clouds and Aerial Images

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

Airborne LiDAR (Light Detection And Ranging) and airborne photogrammetry are both proven and wide... more Airborne LiDAR (Light Detection And Ranging) and airborne photogrammetry are both proven and widely used techniques for the 3D topographic mapping of extended areas. Although both techniques are based on different reconstruction principles (polar measurement vs. ray triangulation), they ultimately serve the same purpose, the 3D reconstruction of the Earth's surface, natural objects or infrastructure. It is therefore obvious for many applications to integrate the data from both techniques to generate more accurate and complete results. Many works have been published on this topic of data fusion. However, no rigorous integrated solution exists for the first two steps that need to be carried out after data acquisition, namely (a) the lidar strip adjustment and (b) the aerial triangulation. A consequence of solving these two optimization problems independently can be large discrepancies (of up to several decimeters) between the lidar block and the image block. This is especially the case in challenging situations, e.g. corridor mapping with one strip only or in case few or no ground control data. To avoid this problem and thereby profit from many other advantages, a first rigorous integration of these two tasks, the hybrid orientation of lidar point clouds and aerial images, is presented in this work.

Investigating the Use of Coastal Blue Imagery for Bathymetric Mapping of Inland Water Bodies

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences

In this contribution, we report on an experimental airborne data acquisition with two medium form... more In this contribution, we report on an experimental airborne data acquisition with two medium format cameras (Coastal Blue, RGB) and a topo-bathymetric laser scanner for capturing the bathymetry of a dozen of groundwater supplied lakes located near Augsburg, Germany. The specific research question was to investigate whether the use of high-resolution Coastal Blue imagery (λ=400-460 nm) provides added value for mapping bathymetry and characterization of water bottom features. While data processing is still in progress, preliminary results indicate that the blue (λ=420-500 nm) and green (λ=490-570 nm) color channels of the RGB camera are better suited for estimating bathymetry, but the Coastal Blue channel adds an additional water penetrating band increasing the number of useful band combinations with a positive effect on the water bottom classification capabilities. Whereas Coastal Blue channels are rather used from satellite platforms (Landsat 8, WorldView-2) with spatial resolutions in the meter range, our experiment aims at using higher resolution Coastal Blue imagery with a ground sampling distance of around 5 cm enabling not only spectrally based shallow water depth mapping but also the application of multi-media photogrammetry in high spatial resolution. To the best of our knowledge the use of high-resolution Coastal Blue captured from airborne platforms is novel in the context of mapping shallow water bathymetry.

Feasibility Investigation on Single Photon Lidar Based Water Surface Mapping

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

The recent advent of single photon sensitive airborne LiDAR (Light Detection And Ranging) sensors... more The recent advent of single photon sensitive airborne LiDAR (Light Detection And Ranging) sensors has enabled higher areal coverage performance at the price of an increased outlier rate and a lower ranging accuracy compared to conventional Multi-Photon LiDAR. Single Photon LiDAR, in particular, uses green laser light capable of penetrating clear shallow water. Although primarily designed for large area topographic mapping, the technique can also be used for mapping the water surface and shallow water bathymetry. In this contribution we investigate the capability of Single Photon LiDAR for large area mapping of water surface heights. While interface returns from conventional green-only bathymetric sensors generally suffer from water level underestimation due to the water penetration capabilities of green laser radiation, the specific questions are, if Single Photon LiDAR (i) is less affected by this well known effect due to the high receiver sensitivity and (ii) consequently delivers a higher number of water surface echoes. The topic is addressed empirically in a case study by comparing the water surface responses of Single Photon LiDAR (Navarra, Spain) and Multi-Photon Topo-Bathymetric LiDAR (Neubacher Au, Austria) for selected water bodies with a horizontal water surface (reservoirs, ponds). Although flown at different altitudes, both datasets are well comparable as they exhibit the same strip point density of ca. 14 points/m 2. The expected superiority of Single Photon LiDAR over conventional green-only bathymetric LiDAR for mapping water surfaces could not be verified in this investigation. While both datasets show good agreement compared to a reference water level when aggregating points into cells of 10 x10 m 2 (mean deviations <5 cm), higher resolution Single Photon LiDAR based water surface models (grid size 1-5 m) show a systematic water level underestimation of 5-20 cm. However, independently measured ground truth observations and simultaneous data acquisition of the same area with both techniques are necessary to verify the results.

Ultra-High Precision Uav-Based Lidar and Dense Image Matching

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2018

This paper presents a study on the potential of ultra-high accurate UAV-based 3D data capture. It... more This paper presents a study on the potential of ultra-high accurate UAV-based 3D data capture. It is motivated by a project aiming at the deformation monitoring of a ship lock and its surrounding. This study is part of a research and development project initiated by the German Federal Institute of Hydrology (BfG) in Koblenz in partnership with the Office of Development of Neckar River Heidelberg (ANH). For this first official presentation of the project, data from the first flight campaign will be analysed and presented. Despite the fact that monitoring aspects cannot be discussed before data from additional flight campaigns will be available later this year, our results from the first campaign highlight the potential of high-end UAV-based image and LiDAR sensors and their data fusion. So far, only techniques from engineering geodesy could fulfil the aspired accuracy demands in the range of millimetres. To the knowledge of the authors, this paper for the first time addresses such ultra-high accuracy applications by combing high precision UAVbased LiDAR and dense image matching. As the paper is written at an early stage of processing only preliminary results can be given here.

Mapping of Water Surface Levels and Slopes with Single Photon Lidar – a Case Study at the River Rhine

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences

Precise knowledge of water surface level heights is crucial for safe ship navigation and as basis... more Precise knowledge of water surface level heights is crucial for safe ship navigation and as basis for calibration of hydrodynamicnumerical models. While Airborne Laser Scanning (ALS) is a well established technique for topographic mapping, ALS-based water surface mapping using conventional infrared lasers suffers from the high degree of specular reflection which leads to data voids for off-nadir angles beyond 5-7 degrees. The advent of single photon sensitive ALS systems using green laser sources presents the prospect of large-area, high-resolution water surface mapping due to the high receiver sensitivity and measurement rate of such systems. Building on previous studies on subject matters, we present the results of a pilot project initiated and conducted by the German Federal Institute of Hydrology (BfG, Koblenz) at the Rhine River. Three specific test sites with varying water surface and flow velocity properties were captured on October 30 th and 31 th , 2019 with the Leica SPL100 from flying altitudes of 3000 m, 2500 m, 1600 m, and 800 m, respectively. As anticipated, the water surface laser pulse density was high and exhibited 20-145 points/m 2 depending on flying altitude. After quality control, strip adjustment, and point cloud analysis, three water surface classification methods were implemented based on: (i) height quantiles, (ii) point cloud segmentation, and (iii) inverse DTM filtering. All approaches featured relative and absolute water level height accuracies better than 10 cm. We conclude that Single Photon LiDAR based high resolution mapping of water surface levels and tilts is feasible when employing application specific data acquisition parameters, i.e., off-nadir angle ≤10 • and flying altitude ≤3000 m.

Airborne Hydrographic LiDAR Mapping - Potential of a new technique for capturing shallow water bodies

Chan, F., Marinova, D. and Anderssen, R.S. (eds) MODSIM2011, 19th International Congress on Modelling and Simulation., 2011

In this paper, we present the general design of a hydrographic laser scanner (prototype instrumen... more In this paper, we present the general design of a hydrographic laser scanner (prototype instrument) manufactured by the company Riegl Laser Measurement Systems in cooperation with the University of Innsbruck, Unit of Hydraulic Engineering. The instrument utilizes a green laser source (λ=532 nm) capable of penetrating the water column. Very short laser pulses (1 ns) are emitted and the backscattered signal is digitized enabling sophisticated waveform processing online during the flight and in post processing. In combination with a traditional topographic airborne laser scanner (λ=1500 nm) mounted on the same platform a complete hydrographic and topographic survey of the riparian foreland, the water surface and river bed can be carried out in a single campaign. In contrast to existing bathymetric LiDAR systems, the presented system uses only medium pulse energy but a high pulse repetition rate of up to 250 kHz and, thus, focuses on a detailed description of shallow waters under clear water conditions.

Airborne laser bathymetry – detecting and recording submerged archaeological sites from the air

Journal of Archaeological Science, 2013

A new generation of airborne bathymetric laser scanners utilises short green laser pulses for hig... more A new generation of airborne bathymetric laser scanners utilises short green laser pulses for high resolution hydrographic surveying in very shallow waters. The paper investigates its use for the documentation of submerged archaeological structures, introducing the concept of airborne laser bathymetry and focussing on a number of challenges this novel technology still has to face. Using this method, an archaeological pilot study on the northern Adriatic coast of Croatia has revealed sunken structures of a Roman villa. The results demonstrate the potential of this novel technique to map submerged archaeological structures over large areas in high detail in 3D, for the first time providing the possibility for systematic, large-scale archaeological investigation of this environment. The resulting maps will provide unique means for underwater heritage management.

Airborne laser scanning derived terrain models for modelling overland flow

Measuring the Topography of Submerged Archaeological Sites from the Air

Archaeological Prospection, May 29, 2013

A Framework for Generic Spatial Search in 3D Point Clouds

ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2021

Modern data acquisition with active or passive photogrammetric imaging techniques generally resul... more Modern data acquisition with active or passive photogrammetric imaging techniques generally results in 3D point clouds. Depending on the acquisition or processing method, the spacing of the individual points is either uniform or irregular. In the latter case, the neighbourhood definition like for digital images (4-or 8-neighbourhood, etc.) cannot be applied. Instead, analysis requires a local point neighbourhood. The local point neighbourhood with conventional k-nearest neighbour or fixed distance searches often produce sub-optimal results suffering from the inhomogeneous point distribution. In this article, we generalize the neighbourhood definition and present a generic spatial search framework which explicitly deals with arbitrary point patterns and aims at optimizing local point selection for specific processing tasks like interpolation, surface normal estimation and point feature extraction, spatial segmentation, and such like. The framework provides atomic 2D and 3D search strategies, (i) k-nearest neighbour, (ii) region query, (iii) cell based selection, and (iv) quadrant/octant based selection. It allows to freely combine the individual strategies to form complex, conditional search queries as well as specifically tailored point sub-selection. The benefits of such a comprehensive neighbourhood search approach are showcased for feature extraction and surface interpolation of irregularly distributed points.

Water Surface Reconstruction in Airborne Laser Bathymetry from Redundant Bed Observations

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences, 2017

In airborne laser bathymetry knowledge of exact water level heights is a precondition for applyin... more In airborne laser bathymetry knowledge of exact water level heights is a precondition for applying run-time and refraction correction of the raw laser beam travel path in the medium water. However, due to specular reflection especially at very smooth water surfaces often no echoes from the water surface itself are recorded (drop outs). In this paper, we first discuss the feasibility of reconstructing the water surface from redundant observations of the water bottom in theory. Furthermore, we provide a first practical approach for solving this problem, suitable for static and locally planar water surfaces. It minimizes the bottom surface deviations of point clouds from individual flight strips after refraction correction. Both theoretical estimations and practical results confirm the potential of the presented method to reconstruct water level heights in dm precision. Achieving good results requires enough morphological details in the scene and that the water bottom topography is cap...

The Opals Data Manager – Efficient Data Management for Processing Large Airborne Laser Scanning Projects

ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2012

The fast measurement rate of today's Airborne Laser Scanners results in billions of points for si... more The fast measurement rate of today's Airborne Laser Scanners results in billions of points for single ALS projects. Efficient algorithms and data management methods are, therefore, a precondition for successful project handling. The software package OPALS (Orientation and Processing of Airborne Laser Scanning data) was especially designed to meet those criteria. Central core of the package is the OPALS Data Manager (ODM). It provides both, fast spatial access to huge point clouds, as well as a flexible attribute schema to store additional point related quantities. Concepts of the spatial data organization and implementation details about the attribute handling are presented. Additionally, design rationales of the ODM, its file format and the system performance are described.

State of the art in high accuracy high detail DTMs derived from ALS

Archaeological Ground Point Filtering of Airborne Laser Scan Derived Point-Clouds in a Difficult Mediterranean Environment

Journal of Computer Applications in Archaeology, 2020

Digital terrain models (DTM) based on airborne laser scanning (ALS) are an important source for i... more Digital terrain models (DTM) based on airborne laser scanning (ALS) are an important source for identifying and monitoring archaeological sites and landscapes. However, a DTM is only one of many representations of a given surface. Its accuracy and quality must conform to its purpose and are a result of several considerations and decisions along the processing chain. One of the most important factors of ALS-based DTM generation is ground point filtering, i.e., the classification of the acquired point-cloud into terrain and off-terrain points. Filtering is not straightforward. The resulting DTM is usually a compromise that might show the surface below very dense vegetation while losing detail in other areas. In this paper, we show that in very complex situations (e.g., strongly varying vegetation cover), an optimal compromise is difficult to achieve, and more than one filter with different settings adapted to the varying degree of vegetation cover is necessary. For practical reasons, the results need to be combined into a single DTM. This is demonstrated using the case study of a Mediterranean landscape in Croatia, which consists of open areas (agricultural and grassland), olive plantations, as well as extremely dense and evergreen macchia vegetation. The results are the first step toward an adaptive ground point filtering strategy that might be useful far beyond the field of archaeology.

Progress in LiDAR sensor technologychance and challenge for DTM generation and data administration

Proceedings of the 51st …, 2007

Airborne laser scanning (ALS, also referred to as LIDAR (light detection and ranging)) became a s... more Airborne laser scanning (ALS, also referred to as LIDAR (light detection and ranging)) became a standard method for the acquisition of topographic data. However, since the first commercial flights in 1994 a significant development in the sensor technology can be observed. These developments lead to a high increase in data volume due to a significant higher number of determined echoes as well as due to an increased number of information stored per echo. Next to a short presentation of the current sensor developments this paper focuses on the current challenges in management of ALS data (including the storage of additional information provided by full-waveform ALS sensors). Furthermore, advanced procedures for DTM generation integrating geometric criteria with these additional attributes per echo are discussed.

Vertical Vegetation Structure Analysis and Hydraulic Roughness Determination Using Dense Als Point Cloud Data–A Voxel Based Approach

In this contribution the complexity of the vertical vegetation structure, based on dense airborne... more In this contribution the complexity of the vertical vegetation structure, based on dense airborne laser scanning (ALS) point cloud data (25 echoes/m 2), is analyzed to calculate vegetation roughness for hydraulic applications. Using the original 3D ALS point cloud, three levels of abstractions are derived (cells, voxels and connections) to analyze ALS data based on a 1x1 m 2 raster over the whole data set. A voxel structure is used to count the echoes in predefined detrended height levels within each cell. In general, it is assumed that the number of voxels containing echoes is an indicator for elevated objects and consequently for increased roughness. Neighboring voxels containing at least one data point are merged together to connections. An additional height threshold is applied to connect vertical neighboring voxels with a certain distance in between. Thus, the connections indicate continuous vegetation structures. The height of the surface near or lowest connection is an indicator for hydrodynamic roughness coefficients. For cells, voxels and connections the laser echoes are counted within the structure and various statistical measures are calculated. Based on these derived statistical parameters a rule-based classification is developed by applying a decision tree to assess vegetation types. Roughness coefficient values such as Manning's n are estimated, which are used as input for 2D hydrodynamic-numerical modeling. The estimated Manning's values from the ALS point cloud are compared with a traditional Manning's map. Finally, the effect of these two different Manning's n maps as input on the 2D hydraulics are quantified by calculating a height difference model of the inundated depth maps. The results show the large potential of using the entire vertical vegetation structure for hydraulic roughness estimation.

A Comparison of Single Photon and Full Waveform Lidar

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

Single photon sensitive LiDAR sensors are currently competing with conventional multi-photon lase... more Single photon sensitive LiDAR sensors are currently competing with conventional multi-photon laser scanning systems. The advantage of the prior is the potentially higher area coverage performance, which comes at the price of an increased outlier rate and a lower ranging accuracy. In this contribution, the principles of both technologies are reviewed with special emphasis on their respective properties. In addition, a comparison of Single Photon LiDAR (SPL) and Full Waveform LiDAR data acquired in July and September 2018 in the City of Vienna are presented. From data analysis we concluded that (i) less flight strips are needed to cover the same area with comparable point density with SPL, (ii) the sharpness of the resulting 3D point cloud is higher for the waveform LiDAR dataset, (iii) SPL exhibits moderate vegetation penetration under leaf-on conditions, and (iv) the dispersion of the SPL point cloud assessed in smooth horizontal surface parts competes with waveform LiDAR but is higher by a factor of 2-3 for inclined and grassy surfaces, respectively. Still, SPL yielded satisfactory precision measures mostly below 10 cm.

Analyzing near water surface penetration in laser bathymetry-A case study at the River Pielach

Recent developments in sensor technology yielded a major progress in airborne laser bathymetry fo... more Recent developments in sensor technology yielded a major progress in airborne laser bathymetry for capturing shallow water bodies. Modern topo-bathymetric small foot print laser scanners do no longer use the primary near infrared (NIR) signal (λ=1064 nm) but only emit and receive the frequency doubled green signal (λ=532 nm). For calculating correct water depths accurate knowledge of the water surface (air-water-interface) is mandatory for obtaining accurate spot positions and water depths. Due to the ability of the green signal to penetrate water the first reflections do not exactly represent the water surface but, depending on environmental parameters like turbidity, a certain penetration into the water column can be observed. This raises the question if it is even feasible to determine correct water level heights from the green laser echoes only. In this article, therefore, the near water surface penetration properties of the green laser signal are analyzed based on a test flight of the River Pielach (Austria) carried out with Riegl's VQ-820-G (532 nm) and VQ-580 (1064 nm) scanners mounted on the same airborne platform. It is shown that within the study area the mean penetration into the water column is in the range of 10-25 cm compared to the NIR signal as reference. However, as the upper hull of the green water surface echoes coincides with the NIR signal with cm-precision, it is still possible to derive water surface models from the green laser echoes only via statistical analysis of aggregated neighboring echoes and robustly keep the underestimation of the water level below 6 cm. This especially holds for still and stationary flowing water bodies.

Efficient Loading and Visualization of Massive Feature-Rich Point Clouds Without Hierarchical Acceleration Structures

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences

Nowadays, point clouds are the standard product when capturing reality independent of scale and m... more Nowadays, point clouds are the standard product when capturing reality independent of scale and measurement technique. Especially, Dense Image Matching (DIM) and Laser Scanning (LS) are state of the art capturing methods for a great variety of applications producing detailed point clouds up to billions of points. In-depth analysis of such huge point clouds typically requires sophisticated spatial indexing structures to support potentially long-lasting automated non-interactive processing tasks like feature extraction, semantic labelling, surface generation, and the like. Nevertheless, a visual inspection of the point data is often necessary to obtain an impression of the scene, roughly check for completeness, quality, and outlier rates of the captured data in advance. Also intermediate processing results, containing additional per-point computed attributes, may require visual analyses to draw conclusions or to parameterize further processing. Over the last decades a variety of commercial, free, and open source viewers have been developed that can visualise huge point clouds and colorize them based on available attributes. However, they have either a poor loading and navigation performance, visualize only a subset of the points, or require the creation of spatial indexing structures in advance. In this paper, we evaluate a progressive method that is capable of rendering any point cloud that fits in GPU memory in real time without the need of time consuming hierarchical acceleration structure generation. In combination with our multi-threaded LAS and LAZ loaders, we achieve load performance of up to 20 million points per second, display points already while loading, support flexible switching between different attributes, and rendering up to one billion points with visually appealing navigation behaviour. Furthermore, loading times of different data sets for different open source and commercial software packages are analysed.

Comparison of Subaquatic Digital Elevation Models from Airborne Laser Scanning and Imagery

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

The paper describes and compares the workflows and results of generating digital elevation models... more The paper describes and compares the workflows and results of generating digital elevation models (DEMs) of underwater areas from airborne laser scanning and aerial stereo images. Based on a combined laser scanning/image data set of an artificial lake, both methods are described and pros/cons are highlighted. The authors focus on the final results, especially on accuracy, completeness and spatial resolution of the underwater DEM's. Further, practical aspects of processing and complexity of both methods are highlighted too.

Hybrid Orientation of Airborne Lidar Point Clouds and Aerial Images

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

Airborne LiDAR (Light Detection And Ranging) and airborne photogrammetry are both proven and wide... more Airborne LiDAR (Light Detection And Ranging) and airborne photogrammetry are both proven and widely used techniques for the 3D topographic mapping of extended areas. Although both techniques are based on different reconstruction principles (polar measurement vs. ray triangulation), they ultimately serve the same purpose, the 3D reconstruction of the Earth's surface, natural objects or infrastructure. It is therefore obvious for many applications to integrate the data from both techniques to generate more accurate and complete results. Many works have been published on this topic of data fusion. However, no rigorous integrated solution exists for the first two steps that need to be carried out after data acquisition, namely (a) the lidar strip adjustment and (b) the aerial triangulation. A consequence of solving these two optimization problems independently can be large discrepancies (of up to several decimeters) between the lidar block and the image block. This is especially the case in challenging situations, e.g. corridor mapping with one strip only or in case few or no ground control data. To avoid this problem and thereby profit from many other advantages, a first rigorous integration of these two tasks, the hybrid orientation of lidar point clouds and aerial images, is presented in this work.

Investigating the Use of Coastal Blue Imagery for Bathymetric Mapping of Inland Water Bodies

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences

In this contribution, we report on an experimental airborne data acquisition with two medium form... more In this contribution, we report on an experimental airborne data acquisition with two medium format cameras (Coastal Blue, RGB) and a topo-bathymetric laser scanner for capturing the bathymetry of a dozen of groundwater supplied lakes located near Augsburg, Germany. The specific research question was to investigate whether the use of high-resolution Coastal Blue imagery (λ=400-460 nm) provides added value for mapping bathymetry and characterization of water bottom features. While data processing is still in progress, preliminary results indicate that the blue (λ=420-500 nm) and green (λ=490-570 nm) color channels of the RGB camera are better suited for estimating bathymetry, but the Coastal Blue channel adds an additional water penetrating band increasing the number of useful band combinations with a positive effect on the water bottom classification capabilities. Whereas Coastal Blue channels are rather used from satellite platforms (Landsat 8, WorldView-2) with spatial resolutions in the meter range, our experiment aims at using higher resolution Coastal Blue imagery with a ground sampling distance of around 5 cm enabling not only spectrally based shallow water depth mapping but also the application of multi-media photogrammetry in high spatial resolution. To the best of our knowledge the use of high-resolution Coastal Blue captured from airborne platforms is novel in the context of mapping shallow water bathymetry.

Feasibility Investigation on Single Photon Lidar Based Water Surface Mapping

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences

The recent advent of single photon sensitive airborne LiDAR (Light Detection And Ranging) sensors... more The recent advent of single photon sensitive airborne LiDAR (Light Detection And Ranging) sensors has enabled higher areal coverage performance at the price of an increased outlier rate and a lower ranging accuracy compared to conventional Multi-Photon LiDAR. Single Photon LiDAR, in particular, uses green laser light capable of penetrating clear shallow water. Although primarily designed for large area topographic mapping, the technique can also be used for mapping the water surface and shallow water bathymetry. In this contribution we investigate the capability of Single Photon LiDAR for large area mapping of water surface heights. While interface returns from conventional green-only bathymetric sensors generally suffer from water level underestimation due to the water penetration capabilities of green laser radiation, the specific questions are, if Single Photon LiDAR (i) is less affected by this well known effect due to the high receiver sensitivity and (ii) consequently delivers a higher number of water surface echoes. The topic is addressed empirically in a case study by comparing the water surface responses of Single Photon LiDAR (Navarra, Spain) and Multi-Photon Topo-Bathymetric LiDAR (Neubacher Au, Austria) for selected water bodies with a horizontal water surface (reservoirs, ponds). Although flown at different altitudes, both datasets are well comparable as they exhibit the same strip point density of ca. 14 points/m 2. The expected superiority of Single Photon LiDAR over conventional green-only bathymetric LiDAR for mapping water surfaces could not be verified in this investigation. While both datasets show good agreement compared to a reference water level when aggregating points into cells of 10 x10 m 2 (mean deviations <5 cm), higher resolution Single Photon LiDAR based water surface models (grid size 1-5 m) show a systematic water level underestimation of 5-20 cm. However, independently measured ground truth observations and simultaneous data acquisition of the same area with both techniques are necessary to verify the results.