Cornelis Stal

Airborne photogrammetry and lidar for DSM extraction and 3D change detection over an urban area – a comparative study

A digital surface model (DSM) extracted from stereoscopic aerial images, acquired in March 2000, is compared with a DSM derived from airborne light detection and ranging (lidar) data collected in July 2009. Three densely built-up study areas in the city centre of Ghent, Belgium, are selected, each covering approximately 0.4 km2. The surface models, generated from the two different 3D acquisition methods, are compared qualitatively and quantitatively as to what extent they are suitable in modelling an urban environment, in particular for the 3D reconstruction of buildings. Then the data sets, which are acquired at two different epochs t 1 and t 2, are investigated as to what extent 3D (building) changes can be detected and modelled over the time interval. A difference model, generated by pixel-wise subtracting of both DSMs, indicates changes in elevation. Filters are proposed to differentiate ‘real’ building changes from false alarms provoked by model noise, outliers, vegetation, etc. A final 3D building change model maps all destructed and newly constructed buildings within the time interval t 2 – t 1. Based on the change model, the surface and volume of the building changes can be quantified.

Using image-based modelling (SfM–MVS) to produce a 1935 ortho-mosaic of the Ethiopian highlands

Approximately 34,000 aerial photographs covering large parts of Ethiopia and dating back to 1935–1941 have been recently recovered. These allow investigating environmental dynamics for a past period that until now is only accessible from terrestrial photographs or narratives. As the archive consists of both oblique and vertical aerial photographs that cover rather small areas, methods of image-based modelling were used to orthorectify the images. In this study, 9 vertical and 18 low oblique aerial photographs were processed as an ortho-mosaic, covering an area of 25 km2, west of Wukro town in northern Ethiopia. Using 15 control points (derived from Google Earth), a Root Means Square Error of 28.5 m in X 35.4 m in Y were achieved. These values can be viewed as optimal, given the relatively low resolution and poor quality of the imagery, the lack of metadata, the geometric quality of the Google Earth imagery and the recording characteristics. Land use remained largely similar since 1936, with large parts of the land being used as cropland or extensive grazing areas. Most remarkable changes are the strong expansion of the settlements as well as land management improvements. In a larger effort, ortho-mosaics covering large parts of Ethiopia in 1935–1941 will be produced.

Digital Representation of Historical Globes: Methods to Make 3D and Pseudo-3D Models of Sixteenth Century Mercator Globes

Abstract In this paper, the construction of digital representations of a terrestrial and celestial globe will be discussed. Virtual digital (3D) models play an important role in recent research and publications on cultural heritage. The globes discussed in this paper were made by Gerardus Mercator (1512–1594) in 1541 and 1551. Four techniques for the digital representation are discussed and analysed, all using high-resolution photographs of the globes. These photographs were taken under studio conditions in order to get equal lighting and to avoid unwanted light spots. These lighting conditions are important, since the globes have a highly reflective varnish covering. Processing these images using structure from motion, georeferencing of separate scenes and the combination of the photographs with terrestrial laser scanning data results in true 3D representations of the globes. Besides, pseudo-3D models of these globes were generated using dynamic imaging, which is an extensively used technique for visualisations over the Internet. The four techniques and the consequent results are compared on geometric and radiometric quality, with a special focus on their usefulness for distribution and visualisation during an exhibition in honour of the five hundredth birthday of Gerardus Mercator.

Accurate and Cost-Efficient 3D Modelling Using Motorized Hexacopter, Helium Balloons and Photo Modelling: A Case Study

The destructive nature of archaeological excavations and the spatial character of archaeological finds make 3D models valuable contributions to the documentation of archaeological information. Laser scanning allows highly accurate 3D reconstructions, but involves considerable costs and expert knowledge. Therefore, photo modelling could be considered as a useful alternative. In this paper, we will demonstrate on the one hand the cost-efficiency and the consequent time-efficiency of the technique and on the other hand its (sub-decimeter) accuracy. Furthermore, the possibilities and advantages of motorized unmanned aerial vehicles (UAV) and helium balloons as airborne platform for image acquisition are shown. For this purpose, a case study is performed at the Mayan archaeological site of Edzna (Mexico). Using the Structure from Motion (SfM) and Multi-View Stereo (MVS) algorithm, terrestrial and aerial photographic recordings are processed into the final 3D models. For the quality assessment the photographic recordings are supplemented with topographic measurements.

Classification of airborne laser scanning point clouds based on binomial logistic regression analysis

This article presents a newly developed procedure for the classification of airborne laser scanning (ALS) point clouds, based on binomial logistic regression analysis. By using a feature space containing a large number of adaptable geometrical parameters, this new procedure can be applied to point clouds covering different types of topography and variable point densities. Besides, the procedure can be adapted to different user requirements. A binomial logistic model is estimated for all a priori defined classes, using a training set of manually classified points. For each point, a value is calculated defining the probability that this point belongs to a certain class. The class with the highest probability will be used for the final point classification. Besides, the use of statistical methods enables a thorough model evaluation by the implementation of well-founded inference criteria. If necessary, the interpretation of these inference analyses also enables the possible definition of more sub-classes. The use of a large number of geometrical parameters is an important advantage of this procedure in comparison with current classification algorithms. It allows more user modifications for the large variety of types of ALS point clouds, while still achieving comparable classification results. It is indeed possible to evaluate parameters as degrees of freedom and remove or add parameters as a function of the type of study area. The performance of this procedure is successfully demonstrated by classifying two different ALS point sets from an urban and a rural area. Moreover, the potential of the proposed classification procedure is explored for terrestrial data.

Statistical comparison of urban 3D models from photo modeling and airborne laser scanning

Since a few years, structure from motion has been commercially applied and interesting 3D models are discussed in recent literature. Structure from motion is known to be a relatively fast and low-cost technique for the generation of photo-realistic 3D models. This technique uses a series of digital images taken from different positions. In this image series, each part of the object is recorded on at least three images. Combining these images with information from a metadata file allows to determine the different camera positions and to calculate a textured mesh or colored point set in 3D, by solving a system of geometric matrices. In this article, the geometric quality of urban airborne digital surface models is assessed. These digital surface models are generated by structure from motion. A statistical comparison is performed for an urban area between models generated by structure from motion and models generated by a high density reference point set, acquired by airborne laser scanning. The ease of processing the images in professional software allows to consider structure from motion as an alternative for conventional digital photogrammetry. Although the results for the visual representations of the objects are good, a research on the geometrical accuracy of the final products is discussed in this paper. The statistical analysis of the structure from motion results and the digital surface models generated by other techniques, enables a thorough comparison of the different techniques and the influence of terrain dependent and terrain independent control points.

The use of SfM-photogrammetry to quantify and understand gully degradation at the temporal scale of rainfall events: an example from the Ethiopian drylands

Abstract With the recent technological advances offered by SfM-photogrammetry, we now have the possibility to study gully erosion at very high spatial and temporal scales from multi-temporal DEMs, and thus to enhance our understanding of both gully erosion processes and controls. Here, we examine gully degradation and aggradation at a gully headcut and at four re-incisions along a gully reach in Northern Ethiopia. Environmental controls recorded are topography rainfall, runoff, land use and cover, land management, and soil characteristics. The overall vulnerability of the catchment to erosion is low as calculated from the RUSLE (average 11.83 t ha−1 y−1). This reflects the successful land management of the past years. The runoff coefficient was on average 7.3% (maximum 18.2%). Runoff events caused most geomorphic change in the gully, but slumping of the gully bank also occurred on dry days. Most geomorphic change was caused by one major rainfall event of 54.8 mm d−1, and smaller runoff events caused both degradation and aggradation, often asynchronous between studied sites. Although most research focuses on gully heads alone, re-incisions at lower locations can still cause important gully degradation, which ultimately will reach the gully head and cause instability.

International cooperation in education: the VASSIVIERE Erasmus intensive training program (2011-2013) on hydrography and geomatics

In the framework of the European Union Erasmus Intensive Program which aims at sponsoring intentive courses jointly proposed by european universities, ENSTA Bretagne (France), Ghent University (Belgium) and HCU University (Germany), and the CIDCO (Interdisciplinary Centre for the Development of Ocean Mapping) (Canada), as an associate partner, organised a hydrographic and topographic surveying camp. This project took place in Lake of Vassiviere, one of France’s largest artificial lakes exploited by EDF, France’s global energy company. Five teams of students were tasked to achieve the survey instructions given by EDF like in a real professional context. The main objective was to survey this 10 km² lake to focus on points of interest like dam infrastructures, ports, bridges and to open navigation channels for a given level of the lake. A survey boat from the ENSTA Bretagne and land survey equipment from all universities have been mobilised for this purpose. The project received sponsorship from the industry (Boskalis, Netherland) which is interested in the concept of educating students 7at an international level. Both land and bathymetric surveys has been conducted using a large variety of equipments. This paper will analyze the educational benefit of international cooperation in the field of hydrographic and land survey training.

Laser Scanning for Precise Ovalization Measurements: Standard Deviations and Smoothing Levels

AbstractMonitoring of measurements needs to be supported by a thorough knowledge of the achievable accuracy of the measurement equipment and the processing methodology. This study on ovalization monitoring with laser scanning in tunnels under construction focused on fast-performing laser scanners and their achievable accuracy. This research also investigated the optimum smoothing level of the laser scanning data to achieve the best estimates of ovalization. The experimentally based standard deviations indicate that the used phase-based laser scanner delivers highly accurate results with standard deviations between 0.34 and 0.58 mm, depending on the applied smoothing level. In this research, this laser scanner was used, together with a [x – 0.008 rad; x + 0.008 rad] smoothing interval on the calculated radius values to meet the accuracy requirement requested by the client (0.5 mm), even when using multiple setups. Another used type of laser scanner (pulse-based) also delivers standard deviations around thi...

Deformation monitoring of a bridge with tram rail infrastructure

This paper describes the monitoring of a bridge with tram rail infrastructure using a Robotic total station, a Terrestrial Laser Scanner (TLS) and an Inertial Navigation System (INS). The presented methodology is elaborated using a case study performed in the city of Ghent, Belgium. Several modalities of dense and heavy traffic cross the historical bridge with a high frequency. In particular, when trams cross the bridge, the tram rails locally move in a vertical way which is visual to the naked eye. This raises concerns that the bridge in itself is also moving and may become unstable, making the infrastructure very vulnerable. If this would be the case, an urgent renovation of the bridge needs to be planned. To investigate whether these costly and imminent renovation works will be necessary, a thorough measuring campaign was set up. Together with the surveying team of the city of Ghent, Ghent University used a total station and a terrestrial laser scanner to monitor the bridge as a whole and the tram rails in particular. An additional experiment with an Inertial Navigation System (INS) was also implemented to monitor vibrations of the construction. A measurement strategy was set up to compare the movement of the rails and the top and bottom of the bridge in rest, as well as during crossing of a tram. The workflow of the data capture and the data processing is described in this paper. Combining all measurements, it is confirmed that the bridge is secure and it can be concluded that only the tram rails have become unstable.