Dr. Aleixandre Verger Profile

Dr. Aleixandre Verger

PhD Student at Univ de València

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Publications (2)

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Proceedings Article | 26 October 2004 Paper

Testing parametric BRDF models with CHRIS/PROBA acquisitions over agricultural crops

Aleixandre Verger, Fernando Camacho-de Coca, Joaquin Meli

Proceedings Volume 5568, (2004) https://doi.org/10.1117/12.565551

KEYWORDS: Bidirectional reflectance transmission function, Data modeling, Reflectivity, Unmanned aerial vehicles, Solar radiation models, Vegetation, Scattering, Atmospheric modeling, Agriculture, Anisotropy

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A proper determination of the BRDF is of interest for land surface studies in different topics such as albedo estimation, correction of anisotropy effects, and retrieval of vegetation parameters by defining optimal geometries. In this paper, we evaluate a set of parametric models widely-used for BRDF characterisation (Roujean model, Ambrals combinations, non-linear RPV and the empirical Walthall's model). These models are inverted and tested against atmospherically-corrected BRF measurements acquired with the CHRIS (Compact High Resolution Imaging Spectrometer) instrument on board the PROBA (Project for On-Board Autonomy) satellite over an agricultural test site located in Barrax (Spain) during the SPARC (SPectra bARrax Campaign) 2003 campaign. The study area presents different land crops with high variability in LAI values from 0 to 6. The objectives of the present study are to determine how well the different parametric BRDF models are able to fit CHRIS/PROBA's observed multiangular reflectances in order to determine the nadir-zenith reflectance, which is the optimal geometry to retrieve the fractional vegetation coverage (FVC), and to describe the anisotropy of vegetation canopies, which can be useful to estimate accurately the leaf area index (LAI). To do so, performance indicators are obtained for the different models. The results of this study show that all the tested models are fairly accurate in the entire spectral range (RMS<0.016 at 674 nm and RMS<0.025 at 803 nm) and thus are suitable for normalisation purposes. However, most of them are not able to describe BRDF features such as the hot spot, which hampers the use of these models for exploiting the directional information. There are no significant differences, for the experimental conditions, among those evaluated although the best models appear to be the linear Ross-Li model (low RMS) and the non-linear RPV model (more realistic BRDF).

Proceedings Article | 26 October 2004 Paper

Validation of MSG vegetation products: part I. Field retrieval of LAI and FVC from hemispherical photographs

Beatriz Martinez, Frederic Baret, Fernando Camacho-de Coca, F. Javier Garcia-Haro, Aleixandre Verger, Joaquin Melia

Proceedings Volume 5568, (2004) https://doi.org/10.1117/12.565491

KEYWORDS: Photography, Vegetation, Cameras, Satellites, Biological research, Optical components, Digital cameras, Spatial resolution, Data acquisition, Image resolution

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The Satellite Application Facilities on Land Surface Analysis (LSA SAF) is aimed to produce and disseminate geophysical products using data from EUMETSAT satellites such as the geostationary MSG1 and the polar orbiting METOP. One of the main scientific objectives for LSA SAF validation activities is to provide the User Community with measures of uncertainty for all derived products. In this context, this document is the first of a two-part set which proposes a consistent methodology for the validation of the LSA SAF vegetation products (LAI/FVC/fAPAR) derived from SEVIRI /MSG . The methodology includes (1) an appropriate field data sampling strategy over different test sites, (2) derivation of high-resolution biophysical variable maps over a larger area (approximately the same size as the SPOT4-HRVIR2 multispectral image) along with an associated uncertainty, and (3) up-scaling to medium and coarse (MSG) resolution scales. This paper aims at developing the stage (1) of the methodology at the specific test site of Barrax, an agricultural area in Central Spain (39°3'N, 2°12'W), whereas the part (2) is addressed in a second document (this issue) and the part (3) will be addressed for future tasks. This work includes a detailed description along with an exhaustive analysis of the vegetation product estimates by the hemispherical camera during the SPARC'03 field campaign, which took place in July 2003 at Barrax test site. The hemispherical photographs have proved to provide accurate estimates of biophysical parameters in crop canopies with significant advantages such as the possibility to evaluate the gap fraction in all viewing direction. On the other hand, a test analysis of the (CAN-EYE) software package used for the hemispherical photographs processing was undertaken. This paper also includes the intercomparison with another ground data set collected by the optical instrument LI-COR LAI2000 during the same campaign.

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