Aimed optical active components

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Aimed optical active components

As analyzed in 2.1 there are several optical active water quality variables, able to be retrieved by satellites datasets. In the current study, which is based on a short-term satellite dataset, we aimed mainly to observe general trends of phytoplankton density (represented by Chl-a concentrations), which is generally influenced by higher content of lakes’ main nutrients (phosphates and nitrates). The choice of aimed optical active components was basically limited by the low availability of in-situ data due to the recent date of analyzed satellite images.

Validation with in situ measurements

Validation of satellite images treatment results was made using an independent unpublished in situ dataset provided by Moisset (Moisset , 2016). It is important here to notice that since this is a rather qualitative approach study than quantitative, available data will be used for simple comparisons and not extensive and precise correlation.
Provided in situ dataset, consisted of two different sampling points in the lake, SHL2 and GE3 (Figure 12). It included measurements of Chl-a, phosphates (PO4) and nitrates (NO 3) at several lake depths and in two different dates in August 2015, on the 3rd and the 14th (full table of data is given in Appendix A1). As analyzed in 2.1, light penetration gets attenuated after the first meters of water depth, hence any validation was made taking into account mean values of measurements included in the first 10 m of depth.
Figure 12: In situ sampling sites in Lake Geneva (CIPEL, 2016).

Satellite datasets selection

Our satellite dataset consisted of two different satellite missions’ images, Sentinel-2A and Landsat 7. According to the availability of in situ data (in August 2015) and the simultaneous availability of both Sentinel and Landsat captures, we concluded to a common image selection on the 29th of August 2015. To be more precise, available captures of Sentinel-2 in August 2015 were for the 12th, 19th, 22nd and the 29th of August. However, images of the 12th and 22nd were unsuitable due to the fact that they did not include the lake in total and capture on the 19th of August was covered by clouds (relevant information illustrations in Appendix A2). Available Landsat captures in August 2015 were, for Landsat 7 on the 13th and the 29th of August and for Landsat 8 on the 5th and 21st of August (Appendix A3). Thus, available common captures for both satellite missions was that of the 29th of August for Sentinel-2A and Landsat 7 satellites. In situ data, as explained in 3.3, dated on the 3rd and 14th of August, which has a time discrepancy with our satellite dataset but is still valid to draw conclusions.
Data acquisition for Sentinel-2A scenes is possible in the Scientific Data Hub platform, which provides complete, free and open access to all Sentinel products (ESA, Sentinels Scientific Data Hub, 2016). Sentinel data policy aims for maximum availability of data and indeed this is a free access platform with friendly interface. Data format after downloading is in SENTINEL-SAFE format, which has been designed as a common archiving and conveying data within ESA. It basically wraps a folder containing image data in binary data format and a metadata file in XML. Figure 13 shows Sentinel-2 product format. The folder contains a manifest.safe file which holds the information in XML, a preview image in JPEG2000 format, subfolders for measuring datasets (granules/tiles) in GML-JPEG2000 and datastrip level information, auxiliary data subfolders and HTML previews. Selected Sentinel-2A scene’s details can be found in Appendix A4.
Figure 13: Sentinel-2 products folder format (ESA, European Space Agency , 2016).
USGS decided an open and free availability of its archive since 2008. Landsat data are available for immediate download, mainly in EarthExplorer platform (USGS, EarthExplorer, 2016). Landsat scenes products folders contain several files, including band and metadata files. Landsat 7 folders include Gap Mask files for each band. An overview of Landsat scenes folders format is shown in Figure 14. Basic details of Landsat 7 ETM+ scene selected can be browsed in Appendix A5.
Figure 14: Available file formats of Landsat 7 and 8 scenes (USGS, 2016).

Image preparations

Image preparation and treatments were done using exclusively ArcGIS software (version 10.3) (ESRI ArcGIS, 2014). We first created a geodatabase named Leman SMakri.gdb. We then proceeded to images preparations and additional layers creation that we next used in images treatments. The main steps of images preparation and additional layers creation were as follows:
1) Using the “Editor” tool, we created a polygon defining our study area, which we used next to extract the area of interest from each bands’ raster files. For that we used the tool “Extract by Mask”, in order to cut all bands’ raster files of both Sentinel-2A and Landsat 7.
2) For each cut scenes’ raster we preceded to radiometric enhancements of levels of grey, using and modifying the histograms in each layer’s “Properties” window. This is an image modification by changing pixel brightness values in order to improve its visual impact.
3) We inserted two coordinates’ point layers for the two different sampling sites GE3 and SHL2. For that we first created two excel files containing the coordinates of the two different points. In ArcGIS we used the “Excel to Table” conversion tool to convert the excel file to a table in ArcGIS. Using the path: File→ Add Data→ Add XY Data, defined the x, y attributes according to the coordinates in the tables we created. It is also important here to define the projection coordinate system using the Edit option, in our case WGS_1984. This created two point layers containing our coordinates which we exported by right clicking on the layer and by using the path: Data→ Export Data. We saved the file as shapefile.
4) We created two buffer layers around the two different sampling sites. These layers were used for pixels statistics, as we will in detail next. To create the two buffer layers we used the “Buffer” tool. For Sentinel-2A images we created a buffer of 10 m, taking into account used bands spatial resolution of 10 m, in order to measure at least 1 pixel’s statistics. For Landsat 7 scene we used a 30 m buffer around the sampling sites points.
Further details including screen captures of main image preparation made, can be found in Appendix A6. In the next chapter we can see incorporated treatments, in order to transform and reveal measured spectral reflectance of our images.