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Cifuentes R.,Catholic University of Leuven | van der Zande D.,Management Unit of the North Sea Mathematical Models | Salas C.,University of the Frontier | Farifteh J.,Catholic University of Leuven | Coppin P.,Catholic University of Leuven
Forests | Year: 2014

Terrestrial laser scanning (TLS) data makes possible to directly characterize the three-dimensional (3D) distribution of canopy foliage elements. The scanned edges of these elements may result in incorrectly point measurements (i.e., "ghost points") impacting the quality of point cloud data. Therefore, estimation of the ghost points' spatial visibilities, measurement of their characteristics and their removal are essential. In order to quantify the improvements on data quality, a method is developed in this study to efficiently correct for ghost points. Since the occurrence of ghost points is governed by a number of factors, (e.g., scanning resolution and distance, object properties, incident angle); the developed method is based on the analysis of the effects of these factors under controlled conditions where canopy-like objects (i.e., leaves, branches and layers of leaves) were scanned using a continuous-wave TLS system that employs phase-shift technology. Manual extraction of ghost points was done in order to calculate the relative amount of ghost points per scan, or ghost points ratio (gpr). The gpr values were computed in order to: (i) analyze their relationships with variables representing the above factors; and (ii) be used as a reference to evaluate the performance of filters used for extraction of ghost points. The ghost points' occurrence was modeled by fitting regression models using different predictor variables that represent the variables under study. The obtained results indicated that reduced models with three predictors were suitable for gpr estimation in artificial leaves and in artificial branches, with a relative root mean squared error (RMSE) of 4.7% and 3.7%, respectively; while the full model with four predictors was appropriate for artificial layers of leaves, with relative RMSE of 4.5%. According to the statistical analysis, scanning distance was identified as the most important variable for modeling ghost points occurrence. Results indicated that optimized distance-based filters relative to the scanning distance have improved the outcomes in ghost points detection, in comparison to standard filtering criteria. These results suggest that more accurate characterization of forest canopy 3D structures can be achieved by removing ghost points using the new developed method. © 2014 by the authors. Source


Cifuentes R.,Catholic University of Leuven | Van der Zande D.,Management Unit of the North Sea Mathematical Models | Farifteh J.,Catholic University of Leuven | Salas C.,University of the Frontier | Coppin P.,Catholic University of Leuven
Agricultural and Forest Meteorology | Year: 2014

Assessments of forest canopy structure remain a challenge and are most often conducted using indirect techniques limited to a two-dimensional perspective. Using terrestrial laser scanner (TLS) technology, a three-dimensional (3D) approach to study canopy structure was conducted by modeling forest scenes from three broadleaved forest stands with different canopy features. Field TLS data were collected from each stand using a phase based FARO® LS880 laser scanner on four sampling setups. The capability of TLS-derived data to represent canopy structure was evaluated by comparing gap fraction estimates from the 3D models with gap fraction values from digital hemispherical photographs (DHP). Firstly, the collected 3D point clouds were processed to obtain fully representative voxel-based models of the forest canopy. Secondly, ray tracing algorithms were applied on these models to simulate hemispherical views and estimate gap fraction. Finally, a sensitivity analysis was done using different voxel sizes and the four sampling setups on the simulations, in order to assess their impact on the gap fraction estimates derived from TLS data. Results of TLS-derived gap fraction showed that combining nine scans produced better results in all forest stand. Similarly, the dimension of voxels have a marked influence on these results. Voxel sizes of 1cm, 2cm and 4cm were found to have less error when compared with real gap fraction values derived from DHP for young, intermediate and mature forest stands, respectively (RMSE ranging from 9% to 16%). However, substantial differences in gap fraction were observed and described at different zenith angles. These results suggest that specific TLS sampling setup and processing are required depending upon the forest type under analysis. Overall, this research indicates that phase based TLS data can be used for objective calculation of gap fraction. © 2014 Elsevier B.V. Source


Prins T.C.,Deltares | Desmit X.,Deltares | Desmit X.,Management Unit of the North Sea Mathematical Models | Baretta-Bekker J.G.,BarettaBekker Mariene Ecologie
Journal of Sea Research | Year: 2012

The Southern Bight of the North Sea is a shallow shelf sea, strongly influenced by river-borne nutrient loads. Eutrophication symptoms manifest themselves as high levels of chlorophyll-a and long-lasting, extensive blooms of Phaeocystis globosa, especially in the waters along the continental coast. As a consequence of measures to reduce eutrophication, riverine phosphorus loads have decreased more than 50% in the last two decades, and nitrogen loads show a decrease of ca 30%. While decreases in riverine N and P loads are observed, an increase in summer river-borne loads of silica occurred. Since 1990, The Netherlands has carried out a routine monitoring program in the North Sea, including analysis of phytoplankton composition and carbon biomass. An analysis of these data for the period 1990-2007 shows a trend in phytoplankton composition, toward an increase in diatom biomass, increased bloom frequency and maximum bloom cell numbers of several diatom species, in particular Chaeotoceros socialis, in the coastal waters. These changes coincide with increases in riverine Si loadings and increased Si concentrations in coastal waters as a consequence of changed river loads. © 2012 Elsevier B.V. Source


Wille K.,Ghent University | Claessens M.,Ghent University | Rappe K.,Ghent University | Monteyne E.,Management Unit of the North Sea Mathematical Models | And 3 more authors.
Journal of Chromatography A | Year: 2011

The presence of both pharmaceuticals and pesticides in the aquatic environment has become a well-known environmental issue during the last decade. An increasing demand however still exists for sensitive and reliable monitoring tools for these rather polar contaminants in the marine environment. In recent years, the great potential of passive samplers or equilibrium based sampling techniques for evaluation of the fate of these contaminants has been shown in literature. Therefore, we developed a new analytical method for the quantification of a high number of pharmaceuticals and pesticides in passive sampling devices. The analytical procedure consisted of extraction using 1:1 methanol/acetonitrile followed by detection with ultra-high performance liquid chromatography coupled to high resolution and high mass accuracy Orbitrap mass spectrometry. Validation of the analytical method resulted in limits of quantification and recoveries ranging between 0.2 and 20. ng per sampler sheet and between 87.9 and 105.2%, respectively. Determination of the sampler-water partition coefficients of all compounds demonstrated that several pharmaceuticals and most pesticides exert a high affinity for the polydimethylsiloxane passive samplers. Finally, the developed analytical methods were used to measure the time-weighted average (TWA) concentrations of the targeted pollutants in passive samplers, deployed at eight stations in the Belgian coastal zone. Propranolol, carbamazepine and seven pesticides were found to be very abundant in the passive samplers. These obtained long-term and large-scale TWA concentrations will contribute in assessing the environmental and human health risk of these emerging pollutants. © 2011 Elsevier B.V. Source


Ghekiere A.,A.R.C.H.E | Verdonck F.,A.R.C.H.E | Claessens M.,Ghent University | Monteyne E.,Management Unit of the North Sea Mathematical Models | And 5 more authors.
Marine Pollution Bulletin | Year: 2013

The environmental risks of 33 micropollutants occurring in Belgian coastal zone were assessed as single-substances and as mixtures. Water and sediment samples were taken in harbors, coastal waters and the Scheldt estuary during 2007-2009. Measured environmental concentrations were compared to quality standards such as Predicted No Effect Concentrations (PNECs), Environmental Quality Standards (EQSs), and Ecotoxicological Assessment Criteria (EAC). Out of a total of 2547 samples analyzed, 232 and 126 samples exceeded the EQS and EAC, respectively. Highest risks were observed for TBT, PBDEs, PCBs and the PAHs anthracene, indeno(1,2,3-cd)pyrene, benzo(g,h,i)perylene, benzo(k)fluoranthene, and benzo(b)fluoranthene in the water compartment and for TBT and PCBs in the sediment compartment. Samples taken at all stations during the April 2008 campaign indicate a potential risk of the contaminant mixtures to the aquatic environment (except W06 station). This study argues the need to revise quality standards when appropriate and hence the overall regulatory implication of these standards. © 2013 Elsevier Ltd. Source

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