The performance of different atmospheric correction algorithms for the Ocean and Land Colour Instrument (OLCI) on board of Sentinel-3 (S3) is evaluated for retrieval of water-leaving radiance reflectance, and derived parameters chlorophyll-a concentration and turbidity in turbid coastal waters in the Belgian Coastal Zone (BCZ). This is performed using in situ measurements from an autonomous pan-and-tilt hyperspectral radiometer system (PANTHYR). The PANTHYR provides validation data for any satellite band between 400 and 900 nm, with the deployment in the BCZ of particular interest due to the wide range of observed Near-InfraRed (NIR) reflectance. The Dark Spectrum Fitting (DSF) atmospheric correction algorithm is adapted for S3/OLCI processing in ACOLITE, and its performance and that of 5 other processing algorithms (L2-WFR, POLYMER, C2RCC, SeaDAS, and SeaDAS-ALT) is compared to the in situ measured reflectances. Water turbidities across the matchups in the Belgian Coastal Zone are about 20–100 FNU, and the overall performance is best for ACOLITE and L2-WFR, with the former providing lowest relative (Mean Absolute Relative Difference, MARD 7–27\%) and absolute errors (Mean Average Difference, MAD -0.002, Root Mean Squared Difference, RMSD 0.01–0.016) in the bands between 442 and 681 nm. L2-WFR provides the lowest errors at longer NIR wavelengths (754–885 nm). The algorithms that assume a water reflectance model, i.e. POLYMER and C2RCC, are at present not very suitable for processing imagery over the turbid Belgian coastal waters, with especially the latter introducing problems in the 665 and 709 nm bands, and hence the chlorophyll-a and turbidity retrievals. This may be caused by their internal model and/or training dataset not being well adapted to the waters encountered in the BCZ. The 1020 nm band is used most frequently by ACOLITE/DSF for the estimation of the atmospheric path reflectance (67\% of matchups), indicating its usefulness for turbid water atmospheric correction. Turbidity retrieval using a single band algorithm showed good performance for L2-WFR and ACOLITE compared to PANTHYR for e.g. the 709 nm band (MARD 15 and 17\%), where their reflectances were also very close to the in situ observations (MARD 11\%). For the retrieval of chlorophyll-a, all methods except C2RCC gave similar performance, due to the RedEdge band-ratio algorithm being robust to typical spectrally flat atmospheric correction errors. C2RCC does not retain the spectral relationship in the Red and RedEdge bands, and hence its chlorophyll-a concentration retrieval is not at all reliable in Belgian coastal waters. L2-WFR and ACOLITE show similar performance compared to in situ radiometry, but due to the assumption of spatially consistent aerosols, ACOLITE provides less noisy products. With the superior performance of ACOLITE in the 490–681 nm wavelength range, and smoother output products, it can be recommended for processing of S3/OLCI data in turbid waters similar to those encountered in the BCZ. The ACOLITE processor for OLCI and the in situ matchup dataset used here are made available under an open source license.
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RBINS Staff Publications 2021
Roof coverings are crucial elements in the architectural design of any building, from the most rudimentary to the most elaborate, because of the protection against weathering they offer to the buildings. Despite its important role, it remains many gaps or grey areas in our historical and technical knowledge of this stony materials used to make them. In north-western Europe, although roof tiles seem to have attracted the attention of researchers in France, Germany, the Netherlands and Great Britain, it must be said that slate did not benefit from the same interest. In Belgium as well roofing using slate has not been the subject of particular attention for periods r anging from the late medieval period to the 18th century. Situated at the crossroads of archaeology, archaeometry and history, this study aims to take stock of the use of this material in the southern part of the former Duchy of Brabant and more particularly in the Brussels Region. The available bibliography, archival sources and data from the geological analysis of samples from archaeological excavations will be mobilised to address several key questions: the goeographical origin of the roofing slates and, therefore, their transport – the Brussels Region is devoid of slate deposits and is dependent on import trade routes for its supply; the particular uses of slate (social groups, types of building); the question of the organisation of the slate roofers’ trade; the question of costs in relation to other raw material, tiles in particular; and finally, special attention will be paid to the various implementations observed in situ. This last approach will include a reflection on the diverted uses of this stony material in other types of structural work. In short, this contribution will seek to characterise the roofing slate in the history of construction in Brabant and Brussels and will resonate with a major ongoing research project dedicated to the evolution of the Brussels roof frames.
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RBINS Staff Publications 2023
