Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Bezoek landelijke commissie Tomaat bij Wageningen UR Bleiswijk
    Varekamp, J. ; Wageningen UR Glastuinbouw, - \ 2012
    LTO Groeiservice
    Stochastic modelling of migration from polyolefins
    Helmroth, I.E. ; Varekamp, C. ; Dekker, M. - \ 2005
    Journal of the Science of Food and Agriculture 85 (2005)6. - ISSN 0022-5142 - p. 909 - 916.
    polymeric packaging material - additive migration - general validity - diffusion - foods - proportionality - component - quantity - proofs
    A method is presented to predict diffusion coefficients in polyolefins using stochastic modelling. A large number of experimental diffusion coefficients, published in the literature as one dataset, was used to derive probability distributions of diffusion coefficients in the polymers low-density polyethylene and linear low-density polyethylene, medium- and high-density polyethylene, and polypropylene. An equation is proposed to describe the diffusion coefficient as a function of the molar mass of the migrant. Model parameters and standard deviations are predicted by minimizing the sum of squared errors and the residuals are used to check the assumed types of probability distribution. The experimental data can be described by a log-normal distribution. It is shown how the derived probability distributions can be used as input for migration predictions. The method presented provides information about the most likely migration results for a given packaging¿food simulant combination. This is important for prediction of the probability that a given migration limit may be exceeded
    High-Resolution InSAR Image Simulation for Forest Canopies
    Varekamp, C. ; Hoekman, D.H. - \ 2002
    IEEE Transactions on Geoscience and Remote Sensing 40 (2002)7. - ISSN 0196-2892 - p. 1648 - 1655.
    High-resolution interferometric airborne synthetic aperture radar (SAR) images of Indonesian tropical rain forests have been acquired during the European Space Agency (ESA) Indonesian Radar Experiment (INDREX) 1996 campaign. Research efforts are directed toward development of automated canopy reconstruction algorithms. In this paper, interferometric synthetic aperture radar (InSAR) image simulation is discussed as one of the tools to support development of such inversion algorithms. First, the relevant physics, observation geometry, and radar characteristics are described. It is assumed that a forest can be modeled as a cloud of uniformly distributed isotropically scattering elements located within crown volumes. These volumes were measured during a field experiment for a 7.2 ha plot. Simulated data comprise intensity, phase, as well as coherence images. These are compared, in a statistical sense, with real data acquired in C- and X-bands. The canopy attenuation was simulated over a range of values. The normalized second intensity moment, the mean coherence magnitude, the coherence histogram, and the autocorrelation function of coherence were taken as measures for comparison. It can be concluded that simulated and real C-band images compare well for an extinction coefficient in the range of 0.15-0.3 m/sup -1/. For X-band, the selected measures of agreement lead to contradictions, indicating that the physical assumptions made may be less valid than for C-band.
    Observation of Tropical Rain Forest Trees by Airborne High-Resolution Interferometric Radar
    Hoekman, D.H. ; Varekamp, C. - \ 2001
    IEEE Transactions on Geoscience and Remote Sensing 39 (2001)3. - ISSN 0196-2892 - p. 584 - 594.
    tropische regenbossen - remote sensing - radar - tropical rain forests - remote sensing - radar
    The Indonesian Radar Experiment (INDREX) Campaign was executed in Indonesia to study the potential of high-resolution interferometric airborne radar in support of sustainable tropical forest management. Severe cloud cover limits the use of aerial photography, which is currently applied on a routine basis to extract information at the tree level. Interferometric radar images may be a viable alternative once radar imaging at the tree level is sufficiently understood. It is shown that interferometric height images can contain large height and displacement errors for individual trees but that this problem can be solved to a large extent using models for the vertical distribution of backscatter intensity and an extension of the Van Cittert-Zernike theorem. The predicted loss of coherence in lay-over regions of emergent trees is shown to be in good agreement with the loss of coherence as observed in the high resolution radar data (Pierson correlation coefficient=0.94). Several correction methods for height and displacement errors are proposed. It is shown that a simple approach already gives a good correction. Semi-empirical correction models, which can be calibrated for forest structure, perform even better
    Canopy reconstruction from interferometric SAR
    Varekamp, C. - \ 2001
    Wageningen University. Promotor(en): R.A. Feddes; D.H. Hoekman. - S.l. : S.n. - ISBN 9789058083531 - 88
    remote sensing - tropische bossen - kroondak - radar - remote sensing - radar - tropical forests - canopy
    Interferometric Synthetic Aperture Radar (InSAR) is investigated as a method for 3D tree mapping. When operational, the method may be important for monitoring forests with a persistent cloud cover such as tropical rain forests. The problem of crown displacement due to lay-over in a vegetation with a large vertical variation in scatterer height is studied. It is shown that crown displacements can be corrected for, by using coherence. An analytical expression for the coherence is derived which can be used for crown position corrections. The model is validated and shown to be consistent with observations. The water cloud model has been extended with interferometric phase, coherence and crown geometry. This model is used to simulate realistic InSAR images of a 3D forest canopy using measurements of 1064 trees. It is validated by comparing observed with simulated image statistics. The normalised second intensity moment, coherence histogram and coherence autocorrelation in range direction are used for this purpose.
    Results indicate a good agreement at C-band for an extinction coefficient of 0.3 per meter. Two image transforms are derived based on Gaussian scattering assumptions. These transforms may be used for automated canopy reconstruction. Finally, the simulation method and the intensity image transform are used to study the effect of InSAR system parameters on the position accuracy.
    Segmentation of high-resolution InSar data of tropical forest using Fourier parameterised deformable models
    Varekamp, C. ; Hoekman, D.H. - \ 2001
    International Journal of Remote Sensing 22 (2001)12. - ISSN 0143-1161 - p. 2339 - 2350.
    Currently, tree maps are produced from field measurements that are time consuming and expensive. Application of existing techniques based on aerial photography is often hindered by cloud cover. This has initiated research into the segmentation of high resolution airborne interferometric Synthetic Aperture Radar (SAR) data for deriving tree maps. A robust algorithm is constructed to optimally position closed boundaries. The boundary of a tree crown will be best approximated when at all points on the boundary, the z-coordinate image gradient is maximum, and directed inwards orthogonal to the boundary. This property can be expressed as the result of a line integral along the boundary. Boundaries with a large value for the line integral are likely to be tree crowns. This paper focuses on the search procedure and on illustrating how smoothing can be used to prevent the search from becoming trapped in a local optimum. The final crown detection stage is not described in this paper but could be based on the gradient and implemented using the above described value for the line integral. Results of this paper indicate that a Fourier parametrization with only three harmonics (nine parameters) can describe the shape variation in the 2D crown projection in sufficient detail. Current ground datasets are not suitable for obtaining detection statistics such as the percentage of tree crowns detected and the number of false alarms. Better ground datasets will be needed to evaluate algorithm performance for real tree mapping situations.
    Combining inSAR data from two orthogonal tracks for the retrieval of three-dimensional tree crown maps
    Varekamp, C. ; Hoekman, D.H. - \ 2000
    In: PIERS 2000, Boston, 5-14 July 2000 - p. 198 - 198.
    Comparison of simulated and measured high-resolution inSAR data of a tropical forest scene
    Varekamp, C. ; Hoekman, D.H. - \ 2000
    In: 3-rd European Conference on Synthetic Aperture Radar, Munich, 23-25 May 2000 - p. 47 - 50.
    Segmentation of High-resolution InSAR Data of Tropical Forest using Fourier Parameterised Deformable Models
    Varekamp, C. ; Hoekman, D.H. - \ 2000
    In: INDREX, Noordwijk, 1999 en Jakarta, 1999 / M. Wooding. - Noordwijk : ESA Publications Division, 2000. ( ; SP-489) - p. 57 - 62.
    Observation of tropical rain forest trees by airborne high-resolution interferometric radar
    Hoekman, D.H. ; Varekamp, C. - \ 2000
    In: INDREX, Noordwijk, 1999 en Jakarta, 1999 / M. Wooding. - Noordwijk : ESA Publications Division, 2000. ( ; SP-489) - p. 47 - 56.
    SIRAMHUTAN Sistem Informasi untuk Manajemen Hutan - A demonstration project for forestry in Indonesia
    Grim, R.J.A. ; Seifert, F.M. ; Hoekman, D.H. ; Varekamp, C. ; Hussin, Y.A. ; Sharifi, M.A. ; Weir, M. - \ 2000
    Delft : Netherlands Remote Sensing Board (BCRS), Programme Bureau, Rijkswaterstaat Survey Department - ISBN 9789054113140 - 83
    bosbedrijfsvoering - duurzaamheid (sustainability) - remote sensing - geografische informatiesystemen - indonesië - forest management - sustainability - geographical information systems - indonesia
    Interferometric phase difference segmentation using Fourier parameterised deformable models
    Varekamp, C. ; Hoekman, D.H. - \ 1999
    In: ASCI'99 : proceedings of the Fifth Annual Conference of the Advanced School for Computing and Imaging, Heijen, The Netherlands, 15-17 June 1999 / M. Boasson, J.A. Kaandorp, J.F.M. Tonino and M.G. Vosselman (eds). - [S.l.] : [s.n.], 1999 - p. 315 - 320.
    Remote sensing monitoring system for sustainable forest management and land cover change in Indonesia
    Hoekman, D.H. ; Varekamp, C. ; Jong, J.J. de; Valkengoed, E.H. van; Vissers, M.A.M. ; Wooding, M.G. - \ 1999
    Delft : Netherlands Remote Sensing Board (BCRS) - ISBN 9789054112730 - 77
    tropische regenbossen - landgebruik - monitoring - remote sensing - indonesië - tropical rain forests - land use - indonesia
    Sociaal-economische betekenis van de glastuinbouw
    Groot, N.S.P. de; Zachariasse, L.C. - \ 1998
    In: De glastuinbouw in het derde millennium / Alleblas, J.T.W., Varekamp, M.J.,
    Energieaspecten nu en in de toekomst
    Velden, N.J.A. van der; Duffhues, W.F.S. - \ 1998
    In: De glastuinbouw in het derde millennium / Alleblas, J.T.W., Varekamp, M.J.,
    De glastuinbouw in het derde millennium; wendingen en kansen
    Alleblas, J.T.W. ; Varekamp, M.J. - \ 1998
    Naaldwijk : Stichting Gemeente Naaldwijk 800 jaar in bloei - ISBN 9789090119755 - 214
    tuinbouw - beleid - economische situatie - nederland - toekomst - glastuinbouw - horticulture - policy - economic situation - netherlands - future - greenhouse horticulture
    Naar een nieuwe structuur voor de glastuinbouw
    Alleblas, J.T.W. - \ 1998
    In: De glastuinbouw in het derde millennium / Alleblas, J.T.W., Varekamp, M.J.,
    Energie en middelenverbruik
    Verhaegh, A.P. - \ 1998
    In: De glastuinbouw in het derde millennium / Alleblas, J.T.W., Varekamp, M.J.,
    Aspecten van arbeid en motivatie
    Oosten, H.J. van; Hendrix, A.T.M. - \ 1998
    In: De glastuinbouw in het derde millennium / Alleblas, J.T.W., Varekamp, M.J., - p. 166 - 172.
    An inversion algorithm fot automatic retrieval of tree crown characteristics from high-resolution interferometric SAR data.
    Varekamp, C. ; Hoekman, D.H. - \ 1998
    In: Proceedings of the 2nd International Workshop on Retrieval of Bio- and Geophysical parameters from SAR data for land applications, Noordwijk, The Netherlands. M. Borgeaud and T. Guyenne (eds.) - p. 241 - 244.
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