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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    Fast and nondestructive method for leaf level chlorophyll estimation using hyperspectral LiDAR
    Nevalainen, O. ; Hakala, T. ; Suomalainen, J.M. ; Mäkipää, R. ; Peltoniemi, M. ; Krooks, A. ; Kaasalainen, S. - \ 2014
    Agricultural and Forest Meteorology 198-199 (2014). - ISSN 0168-1923 - p. 250 - 258.
    supercontinuum laser source - vegetation indexes - reflectance spectra - precision agriculture - canopy reflectance - red edge - airborne - model - spectroscopy - validation
    We propose an empirical method for nondestructive estimation of chlorophyll in tree canopies. The first prototype of a full waveform hyperspectral LiDAR instrument has been developed by the Finnish Geodetic Institute (FGI). The instrument efficiently combines the benefits of passive and active remote sensing sensors. It is able to produce 3D point clouds with spectral information included for every point, which offers great potential in the field of environmental remote sensing. The investigation was conducted by using chlorophyll sensitive vegetation indices applied to hyperspectral LiDAR data and testing their performance in chlorophyll estimation. The amount of chlorophyll in vegetation is an important indicator of photosynthetic capacity and stress, and thus important for monitoring of forest condition and carbon sequestration on Earth. Performance of chlorophyll estimation was evaluated for 27 published vegetation indices applied to waveform LiDAR collected from ten Scots pine shoots. Reference data were collected by laboratory chlorophyll concentration analysis. The performance of the indices in chlorophyll estimation was determined by linear regression and leave-one-out cross-validation. The chlorophyll estimates derived from hyperspectral LiDAR linearly correlate with the laboratory analyzed chlorophyll concentrations, and they are able to represent a range of chlorophyll concentrations in Scots pine shoots (R2 = 0.88, RMSE = 0.10 mg/g). Furthermore, they are insensitive to measurement scale as nearly the same values of vegetation indices were measured in natural setting while scanning the whole canopy and from clipped shoots re-measured with hyperspectral LiDAR in laboratory. The results indicate that the hyperspectral LiDAR instrument has the potential to estimate vegetation biochemical parameters such as the chlorophyll concentration. The instrument holds much potential in various environmental applications and provides a significant improvement over single wavelength LiDAR or passive optical systems for environmental remote sensing.
    Implications of sensor configuration and topography on vertical plant profiles derived from terrestrial LiDAR
    Calders, K. ; Armston, J. ; Newnham, G. ; Herold, M. ; Goodwin, N. - \ 2014
    Agricultural and Forest Meteorology 194 (2014). - ISSN 0168-1923 - p. 104 - 117.
    ground-based lidar - pulsed-laser systems - wave-form lidar - tropical forests - deciduous forest - leaf-area - canopy - airborne - heterogeneity - environments
    The vertical distribution of plant constituents is a key parameter to describe vegetation structure and influences several processes, such as radiation interception, growth and habitat. Terrestrial laser scanning (TLS), also referred to as terrestrial LiDAR, has the potential to measure the canopy structure with high spatial detail and accuracy. Vertical plant profiles, which describe the plant area per unit volume (PAVD) as a function of height, are often used to quantify the vertical structure. However, most studies do not account for topography, use registered multiple TLS scans or use a detailed airborne LiDAR digital terrain model to account for this variation in ground height. Airborne LiDAR is often not available or expensive to acquire. Here, we present an approach that facilitates rapid, robust and automated assessment of the vertical structure of vegetation. We use single scans and local plane fitting to correct for topographic effects in vertical plant profiles and test our approach in five different Australian forest types with different topography and understorey. We validate our approach with topography-corrected vertical plant profiles with digital terrain models derived from airborne LiDAR. Our results demonstrate that not correcting for topography can lead to significant errors in the vertical distribution of plant constituents (CV(RMSE) up to 66.2%, typically ranging from 4.2% to 13.8%). This error decreases significantly when topography is accounted for with TLS plane fitting (CV(RMSE) up to 20.6%, typically ranging from 1.5% to 12.6%). We demonstrate that height metrics from vertical plant profiles that are not corrected for topography depart significantly from those that are inferred from the reference profile. The effect is most noticeable for canopy top height and the peak PAVD height. Correcting topography with a TLS plane fitting approach reduces the error in canopy top height by at least 77% and up to 100%, and reduces the error in peak PAVD height by 83.3% and up to 100%. We also show the advantage of a multiple return over a first return TLS instrument. The definition of the ground returns with a first return instrument might be problematic in environments with dense herbaceous understorey and there is an overall trend of lower height metrics compared to multiple return instruments. We present a data-driven approach that is based on single scan TLS data. The latter is of importance for large area sampling as it allows more sites to be sampled from existing resources and facilitate consistent processing of archived TLS data, which is often single scan data with no survey control.
    Calibration and Quality Assurance of Flux Observations from a Small Research Aircraft
    Vellinga, O.S. ; Dobosy, R.J. ; Dumas, E.J. ; Beniamino, G. ; Elbers, J.A. ; Hutjes, R.W.A. - \ 2013
    Journal of Atmospheric and Oceanic Technology 30 (2013). - ISSN 0739-0572 - p. 161 - 181.
    boundary-layer development - lift-induced upwash - carbon-dioxide - wind-tunnel - co2 fluxes - airborne - turbulence - surface - variability - landscape
    Small environmental research aircraft (ERA) are becoming more common for detailed studies of air–surface interactions. The Sky Arrow 650 ERA, used by multiple groups, is designed to minimize the complexity of high-precision airborne turbulent wind measurement. Its relative wind probe, of a nine-port design, is furthermore used with several other airplanes. This paper gives an overview of 1) calibration of the model that converts the probe’s raw measurements to meteorological quantities; 2) quality control and assurance (QC–QA) in postprocessing of these quantities to compute fluxes; and 3) sensitivity of fluxes to errors in calibration parameters. The model, an adapted version of standard models of potential flow and aerodynamic upwash, is calibrated using an integrated method to derive a globally optimum set of parameters from in-flight maneuvers. Methods of QC–QA from the tower flux community are adopted for use with airborne flux data to provide more objective selection criteria for large datasets. Last, measurements taken from a standard operational flight are used to show fluxes to be most sensitive to calibration parameters that directly affect the vertical wind component. In another test with the same data, varying all calibration parameters simultaneously by ±10% of their optimum values, the model computes a response in the fluxes smaller than 10%, though a larger response may occur if only a subset of parameters is perturbed. A MATLAB toolbox has been developed that facilitates the procedures presented here
    Detection of hydrocarbons in clay soils: A laboratory experiment using spectroscopy in the mid- and thermal infrared
    Meijde, M. van der; Knox, N. ; Cundill, S.L. ; Noomen, M.F. ; Werff, H.M.A. van der; Hecker, C. - \ 2013
    International Journal of applied Earth Observation and Geoinformation 23 (2013). - ISSN 0303-2434 - p. 384 - 388.
    pipeline leakage - natural-gas - reflectance - airborne - spectrometer - regression
    Remote sensing has been used for direct and indirect detection of hydrocarbons. Most studies so far focused on indirect detection in vegetated areas. We investigated in this research the possibility of detecting hydrocarbons in bare soil through spectral analysis of laboratory samples in the short wave and thermal infrared regions. Soil/oil mixtures were spectrally measured in the laboratory. Analysis of spectra showed development of hydrocarbon absorption features as soils became progressively more contaminated. The future application of these results airborne seems to be a challenge as present and future sensors only cover the diagnostic regions to a limited extent
    Performance of spectral fitting methods for vegetation fluorescence quantification
    Meroni, M. ; Busetto, D. ; Colombo, R. ; Guanter, L. ; Moreno, J. ; Verhoef, W. - \ 2010
    Remote Sensing of Environment 114 (2010)2. - ISSN 0034-4257 - p. 363 - 374.
    induced chlorophyll fluorescence - in-vivo - reflectance - model - photosynthesis - spectroradiometer - luminescence - scattering - airborne - canopy
    The Fraunhofer Line Discriminator (FLD) principle has long been considered as the reference method to quantify solar-induced chlorophyll fluorescence (F) from passive remote sensing measurements. Recently, alternative retrieval algorithms based on the spectral fitting of hyperspectral radiance observations, Spectral Fitting Methods (SFMs), have been proposed. The aim of this manuscript is to investigate the performance of such algorithms and to provide relevant information regarding their use. FLD and SFMs were used to estimate F starting from Top Of Canopy (TOC) fluxes at very high spectral resolution (0.12 nm) and sampling interval (0.1 nm), exploiting the O2-B (687.0 nm) and O2-A (760.6 nm) atmospheric oxygen absorption bands overlapping the fluorescence emissions at the red and far-red spectral window. Specific parameters affecting FLD and SFM performances are investigated and the accuracy of F estimation of the two methods is compared. The problem related to the lack of independent measurements of F at canopy level, which prevents the direct assessment of F estimation accuracy with actual measurements, is overcome in this study by using a modeled database of TOC reflectance spectra. In order to compute accuracy figures valid for operative applications the simulated spectra were perturbed by the addition of radiometric noise. An investigation was conducted to determine the best FLD channel configuration; it showed that violation of FLD assumptions results in a positive bias in F estimation at both oxygen absorption bands that cannot be avoided even at the high spectral resolution considered. SFMs were shown to be more accurate than FLD under any noise configuration considered
    Sensible and latent heat flux from radiometric surface temperatures at the regional scale: methodology and validation
    Miglietta, F. ; Gioli, B. ; Brunet, Y. ; Hutjes, R.W.A. ; Matese, A. ; Sarrat, C. ; Zaldei, A. - \ 2009
    Biogeosciences 6 (2009). - ISSN 1726-4170 - p. 1975 - 1986.
    klimaatverandering - kooldioxide - temperatuur - modellen - climatic change - carbon dioxide - temperature - models - convective boundary-layer - evapotranspiration rates - eddy covariance - carbon-dioxide - water-vapor - aircraft - model - airborne - field - emissivity
    The CarboEurope Regional Experiment Strategy (CERES) was designed to develop and test a range of methodologies to assess regional surface energy and mass exchange of a large study area in the south-western part of France. This paper describes a methodology to estimate sensible and latent heat fluxes on the basis of net radiation, surface radiometric temperature measurements and information obtained from available products derived from the Meteosat Second Generation (MSG) geostationary meteorological satellite, weather stations and ground-based eddy covariance towers. It is based on a simplified bulk formulation of sensible heat flux that considers the degree of coupling between the vegetation and the atmosphere and estimates latent heat as the residual term of net radiation. Estimates of regional energy fluxes obtained in this way are validated at the regional scale by means of a comparison with direct flux measurements made by airborne eddy-covariance. The results show an overall good matching between airborne fluxes and estimates of sensible and latent heat flux obtained from radiometric surface temperatures that holds for different weather conditions and different land use types. The overall applicability of the proposed methodology to regional studies is discussed
    Rainfall rate retrieval in presence of path attenuation using C-band polarimetric weather radars
    Vulpiani, G. ; Marzano, F.S. ; Chandrasekar, V. ; Berne, A.D. ; Uijlenhoet, R. - \ 2006
    Natural Hazards and Earth System Sciences 6 (2006). - ISSN 1561-8633 - p. 439 - 450.
    drop size distributions - differential phase - reflectivity - algorithm - frequencies - validation - airborne - profile
    Weather radar systems are very suitable tools for the monitoring of extreme rainfall events providing measurements with high spatial and temporal resolution over a wide geographical area. Nevertheless, radar rainfall retrieval at C-band is prone to several error sources, such as rain path attenuation which affects the accuracy of inversion algorithms. In this paper, the so-called rain profiling techniques (namely the surface reference method FV and the polarimetric method ZPHI) are applied to correct rain path attenuation and a new neural network algorithm is proposed to estimate the rain rate from the corrected measurements of reflectivity and differential reflectivity. A stochastic model, based on disdrometer measurements, is used to generate realistic range profiles of raindrop size distribution parameters while a T-matrix solution technique is adopted to compute the corresponding polarimetric variables. A sensitivity analysis is performed in order to evaluate the expected errors of these methods. It has been found that the ZPHI method is more reliable than FV, being less sensitive to calibration errors. Moreover, the proposed neural network algorithm has shown more accurate rain rate estimates than the corresponding parametric algorithm, especially in presence of calibration errors
    Tropical mangrove species discrimination using hyperspectral data: A laboratory study
    Vaiphasa, C. ; Ongsomwang, S. ; Vaiphasa, T. ; Skidmore, A.K. - \ 2005
    Estuarine Coastal and Shelf Science 65 (2005)1-2. - ISSN 0272-7714 - p. 371 - 379.
    feature-selection - reflectance variability - chlorophyll - vegetation - canopy - classification - ecosystems - airborne - wetland - leaves
    The aim of this study is to test whether spectra of crown canopy leaves of various tropical mangrove species measured under laboratory conditions contain sufficient spectral information for discriminating mangroves at the species level. This laboratory-level study is one of the most important prerequisites to the future use of airborne and satellite hyperspectral sensors for mangrove studies. First, spectral responses of 16 Thai tropical mangrove species (2151 spectral bands between 350 nm and 2500 nm) were recorded from the leaves, using a spectrometer under laboratory conditions. Next, the mangrove spectra were statistically tested using one-way ANOVA to see whether they significantly differ at every spectral location. Finally, the spectral separability between each pair of mangrove species was quantified using the Jeffries¿Matusita (J¿M) distance measure. It turned out that the 16 mangrove species under study were statistically different at most spectral locations, with a 95% confidence level (p <0.05). The total number of spectral bands that had p-values less than 0.05 was 1941, of which 477 bands had a 99% confidence level (p <0.01). Moreover, the J¿M distance indices calculated for all pairs of the mangrove species illustrated that the mangroves were spectrally separable except the pairs that comprised the members of Rhizophoraceae. Although the difficulties of discriminating the members of Rhizophoraceae are expected, the overall result encourages further investigations into the use of on-board hyperspectral sensors to see whether mangrove species can be separated when the difficulties of the field conditions are taken into account.
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