Δημοσιεύσεις
2018 |
Karageorgis, A.; Drakopoulos, P. G.; Iona, S.; Kapsimalis, V.; Psarra, S.; Sofianos, S.; Spyridakis, N.; Topouzelis, K.; Zervakis, V. Advancing ocean colour retrieval for the Hellenic Seas Conference Joint efforts of Research Institutes and Academia, Book of Abstracts, Hellenic Geological Society 1st meeting on Remote Sensing and Space Applications Committee of the Geological Society of Greece 2018. BibTeX | Ετικέτες: Hellenic Seas, ocean colour, retrieval @conference{Karageorgis2018, |
2017 |
Tsapanou, A.; Drakopoulos, P. G.; Oikonomou, E.; Poulos, S. 3rd International Ocean Colour Science meeting Lisbon Portugal, 2017, (15-19 Μay). Abstract | Links | BibTeX | Ετικέτες: Evros, ocean colour, river, satellite, water @conference{Tsapanou2017,This study investigates the potential of remotely sensed data to map surface suspended particulate matter (SPM) in the Continental shelf area of the North-East Aegean in Greece, by using Landsat-8 satellite sensor. Initially, simultaneous above water ocean color measurements were validated against SPM field data and compared to a concurrent Landsat-8 image, obtained on low river discharge period (June 2016). The satellite imagery was atmospherically corrected and Landsat-8 Surface Reflectance data were generated from: the Landsat Surface Reflectance Code (LaSRC) provided by US Geological Survey and the FLAASH algorithm within the ENVI software. In both cases, the computed remote sensing reflectances (Rrs) from Landsat-8 are converted into SPM by adopting an algorithm based on single band analysis. After validation, the optical in-situ reflectances are correlated to Sentinel-2A corresponding spectral responses, thus leading to a single value for each band. A comparison between Landsat-8 estimations of SPM and concurrent optical measurements at 11 stations indicates that satellite-derived concentrations tend to overestimate sea-truth. Image processing through ENVI & USGS shows 83% correlation, whereas the best linear correlation (90%) is observed between optical and USGS-derived values. The Sentinel-2A optical responses are quite similar to those of Landsat-8. We further discuss the improvements required in coupled studies of SPM retrieval within the coastal zone, the relationship between the examined remote sensors, as well as the necessity to evaluate the Sentinel-2A imagery performance, not only at surface but within the water column using in-situ measurements. |
Papadopoulou, A.; Drakopoulos, P. G.; Psarra, S.; Karageorgis, A. P.; Lagaria, A.; Spyridakis, N.; Zervakis, Vassilis 3rd International Ocean Colour Science meeting Lisbon Portugal, 2017, (15-19 Μay ). Abstract | Links | BibTeX | Ετικέτες: NE Aegean Sea, ocean colour, ocean optics, remote sensing @conference{Papadopoulou2017,In the Aegean Sea and Eastern Mediterranean there are large discrepancies between in situ and satellite ocean colour derived chlorophyll concentrations. The quantity that is monitored by ocean colour satellites and that can be used in the estimation of chlorophyll concentration is the remote sensing reflectance, defined as the ratio of the water leaving spectral radiance to the downwelling spectral irradiance. It can be determined in the field, with either above or in-water radiance and irradiance measurements. The complex optical properties of the North-East Aegean Sea, including radiance and irradiance, were studied during the AegeanMarTech project. Chlorophyll concentration estimates were derived from simultaneous above and in-water radiometric measurements. These were validated against chlorophyll concentration field data and compared against concurrent MODIS data from which chlorophyll was derived using two simple empirical algorithms. It was found that the MedOC3 algorithm outperforms the operational OC3M-547 algorithm and produces the least bias when compared against HPLC derived in situ chlorophyll. It is concluded that the greatest uncertainty in the inversion arises due to CDOM absorption below the 488 nm band. The reflectance ratios indicated that there is always an excess of yellow matter present in the study area and the water type could not be characterized optically as "typical open ocean" Case 1. |
2015 |
Drakopoulos, P. G.; Gialelis, Th.; Pateras, E.; Poulos, S.; Stefanou, E. Predicting Underwater Visibility in Tourist Beaches Conference Proceedings of the Twelfth International Conference on the Mediterranean Coastal Environment MEDCOAST 2015. Abstract | Links | BibTeX | Ετικέτες: beaches, colour, ocean colour, underwater @conference{Drakopoulos2015d,Aiming towards supporting recreational snorkelling in tourist beaches, we have investigated the relationship between underwater visibility range and backscattering coefficient. A series of field experiments involving simultaneous monitoring of beam transmission, backscattering and underwater horizontal black disk sighting have been conducted in various locations. The first results indicate that monitoring backscattering, even in the red part of the spectrum, can provide a useful proxy for visibility determination with an estimated accuracy of20%. |
Karageorgis, A. P.; Psarra, S.; Drakopoulos, P. G.; Assimakopoulou, G.; Banks, A. C.; Kabouri, G.; Kanellopoulos, Th.; Krasakopoulou, E.; Lagaria, A.; Pagou, K.; Papadopoulos, V.; Papadopoulou, A.; Parinos, K.; Renieris, P.; Spyridakis, N.; Velaoras, D. Final Scientific Conference of PERSEUS "Integrated Marine Research in the Mediterranean and the Black Sea" Brussels, 2015, (7-9 December ). Links | BibTeX | Ετικέτες: chlorophyll a, ocean colour, optics @conference{Karageorgis2015b, |
Papadopoulou, A.; Drakopoulos, P.; Psarra, S.; Banks, A. Clive; Karageorgis, A.; Lagaria, A.; Spyridakis, N.; V. Zervakis, Deriving euphotic zone depth in the North Aegean Sea from field and MODIS ocean colour measurements Conference Fifth International CEMEPE and SECOTOX Conference Mykonos Island, Greece , 2015. Abstract | Links | BibTeX | Ετικέτες: chlorophyll, euphotic zone, MODIS, ocean colour, remote sensing @conference{Papadopoulou2015,Euphotic zone depth (Zeu) is defined as the depth at which the photosynthetically active radiation (PAR) value is 1% of the surface value. In this work, three approaches were used to estimate Zeu from field data (in the North Aegean Sea): (a) the Chl-approach, where surface chlorophyll-a values were determined via the high performance liquid chromatography (HPLC) and the fluorometric technique, (b) the PAR-approach, where photosynthetically active radiation was measured with a hyperspectral irradiance meter and (c) the Reflectance-approach, in which surface reflectance values were produced from the in situ downwelling spectral irradiance and upwelling spectral radiance field measurements. Using the Seawifs Data Analysis System (SeaDAS), MODIS daily Level-2 Zeu datasets were produced with two different bio-optical algorithms, which were based on chlorophyll-a concentration (satellite Chl-approach). The comparison of field and satellite Zeu values showed that MODIS tended to underestimate the Zeu for all different approaches. In more detail, the two bio-optical algorithms were found to underestimate Zeu values by ~20-31% and ~18-21% when compared to field measured Zeu data via Chl-approach and PAR-approach, respectively. The percentage range of the underestimation of the Zeu satellite values compared with the one obtained from the field Zeu values via the Reflectance-approach, was found particularly small (~1-5%), a fact that explains the differences in the products as delivered by space agencies. Furthermore, the high uncertainties between field and satellite Zeu measurements require new approaches for ocean colour algorithms in the North Aegean Sea. |