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Uncertainty measurements and validation of ocean colour data in optically complex waters
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. (Marine ecology)ORCID iD: 0000-0003-3777-6788
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis addresses validation activities associated with the use of in situ and satellite-based radiometers to assess water quality parameters, such as chlorophyll-a (Chl-a), total suspended matter (TSM) and coloured dissolved organic matter (CDOM- also known as yellow substances, YEL) in CDOM-dominated waters. In paper I, an inter-comparison of in situ radiometers is presented. Here, a new hand-held radiometer, the Water Insight Spectrometer (WISP-3) was tested and evaluated for routine water monitoring against other common radiometers used for validation. The WISP-3 measures the reflectance at the surface, and thus also works in shallow depths. The WISP-3 is designed for validation in places where other radiometers are difficult to deploy. As it is hyperspectral it can be used to develop in-water algorithms for the retrieval of water-quality information. In paper II, satellite data from the 3rd reprocessing archives of the MEdium Resolution Imaging Spectrometer (MERIS) sensor on board of ENVISAT is evaluated. MERIS level 2 reflectance and water products are assessed against in situ data. This assessment is required in order to ensure the reliability of  monitoring systems based on MERIS data such as the Swedish coastal and lake monitoring system www.vattenkvalitet.se. The evaluation of reflectance data is based on a pair-wise comparison of the standard MEGS processor and three coastal processors that are provided as source free plug-ins for the VISAT BEAM software; all pairs are compared to data measured in situ. The derived water products are evaluated both on a pair-wise comparison of processors as well as on an individual comparison of some processors to sea-truthing data. The studied processors improved the retrievals of MERIS reflectance when used the latest MERIS FR 3rd reprocessing, equalized and “smile” corrected and a land adjacency effects were corrected using the improved contrast between ocean and land (ICOL). The blue spectral bands remain problematic for all processors. Chlorophyll was retrieved best using FUB with an overestimation between 18% - 26.5% (MNB) dependent on the compared pairs. At low chlorophyll < 2.5 mg m-3, random errors dominates the retrievals of MEGS. MEGS showed lower bias and random errors when deriving suspended particulate matter (SPM) with an overestimation in the range 8-16% (MNB). All processors failed to retrieve CDOM correctly, but FUB could at least resolve variations in CDOM, however with a systematic underestimation that may be corrected for by using a local correction factor . MEGS has shown already potential to be used as operational processor in the Himmerfjärden bay and adjacent areas, but it requires further improvement of the atmospheric correction for the blue bands and better definition at relatively low chlorophyll concentrations in presence of CDOM.

Place, publisher, year, edition, pages
Stockholm, Sweden: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2013. , 22 p.
Keyword [en]
Ocean colour validation, MERIS, in situ radiometry, water quality, CDOM
National Category
Oceanography, Hydrology, Water Resources
Research subject
Marine and Brackish Water Ecology
Identifiers
URN: urn:nbn:se:su:diva-90128OAI: oai:DiVA.org:su-90128DiVA: diva2:622953
Presentation
2013-06-10, 105, Frescati Backe, Svante Arrhenius väg 21-A, Stockholm, 17:28
Opponent
Supervisors
Funder
EU, FP7, Seventh Framework Programme, 251527
Note

Main research funders:

Swedish National Space Board (Dnr. 99/09)

European Space Agency (contract no. 21524/08/I-OL)

Available from: 2013-06-04 Created: 2013-05-23 Last updated: 2015-12-07Bibliographically approved
List of papers
1. Intercomparison in the field between the new WISP-3 and other radiometers (TriOS Ramses, ASD FieldSpec, and TACCS)
Open this publication in new window or tab >>Intercomparison in the field between the new WISP-3 and other radiometers (TriOS Ramses, ASD FieldSpec, and TACCS)
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2012 (English)In: Journal of Applied Remote Sensing, ISSN 1931-3195, E-ISSN 1931-3195, Vol. 6, 063615Article in journal (Refereed) Published
Abstract [en]

Optical close-range instruments can be applied to derive water quality parameters for monitoring purposes and for validation of optical satellite data. In situ radiometers are often difficult to deploy, especially from a small boat or a remote location. The water insight spectrometer (WISP-3) is a new hand-held radiometer for monitoring water quality, which automatically performs measurements with three radiometers (L-sky, L-u, E-d) and does not need to be connected with cables and electrical power during measurements. The instrument is described and its performance is assessed by an intercomparison to well-known radiometers, under real fieldwork conditions using a small boat and with sometimes windy and cloudy weather. Root mean squared percentage errors relative to those of the TriOS system were generally between 20% and 30% for remote sensing reflection, which was comparable to those of the other instruments included in this study. From this assessment, it can be stated that for the tested conditions, the WISP-3 can be used to obtain reflection spectra with accuracies in the same range as well-known instruments. When tuned with suitable regional algorithms, it can be used for quick scans for water quality monitoring of Chl, SPM, and aCDOM.

Keyword
WISP-3, radiometer, intercomparison, reflectance
National Category
Environmental Sciences Remote Sensing
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-88750 (URN)10.1117/1.JRS.6.063615 (DOI)000315260200001 ()
Funder
EU, FP7, Seventh Framework Programme
Note

AuthorCount:11;

Available from: 2013-03-26 Created: 2013-03-26 Last updated: 2017-12-06Bibliographically approved
2. Evaluation of MERIS products from Baltic Sea coastal waters rich in CDOM
Open this publication in new window or tab >>Evaluation of MERIS products from Baltic Sea coastal waters rich in CDOM
2014 (English)In: Ocean Science, ISSN 1812-0784, E-ISSN 1812-0792, Vol. 10, no 3, 377-396 p.Article in journal (Refereed) Published
Abstract [en]

In this study, retrievals of the medium resolution imaging spectrometer (MERIS) reflectances and water quality products using four different coastal processing algorithms freely available are assessed by comparison against sea-truthing data. The study is based on a pair-wise comparison using processor-dependent quality flags for the retrieval of valid common macro-pixels. This assessment is required in order to ensure the reliability of monitoring systems based on MERIS data, such as the Swedish coastal and lake monitoring system (http://vattenkvalitet.se). The results show that the pre-processing with the Improved Contrast between Ocean and Land (ICOL) processor, correcting for adjacency effects, improves the retrieval of spectral reflectance for all processors. Therefore, it is recommended that the ICOL processor should be applied when Baltic coastal waters are investigated. Chlorophyll was retrieved best using the FUB (Free University of Berlin) processing algorithm, although overestimations in the range 18-26.5 %, dependent on the compared pairs, were obtained. At low chlorophyll concentrations (< 2.5 mg m(-3)), data dispersion dominated in the retrievals with the MEGS (MERIS ground segment processor) processor. The lowest bias and data dispersion were obtained with MEGS for suspended particulate matter, for which overestimations in the range of 8-16% were found. Only the FUB retrieved CDOM (coloured dissolved organic matter) correlate with in situ values. However, a large systematic underestimation appears in the estimates that nevertheless may be corrected for by using a local correction factor. The MEGS has the potential to be used as an operational processing algorithm for the Himmerfjarden bay and adjacent areas, but it requires further improvement of the atmospheric correction for the blue bands and better definition at relatively low chlorophyll concentrations in the presence of high CDOM attenuation.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-106536 (URN)10.5194/os-10-377-2014 (DOI)000338651700006 ()
Note

AuthorCount:3;

Available from: 2014-08-11 Created: 2014-08-11 Last updated: 2017-12-05Bibliographically approved

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