EOC offers access to monthly cloud-free Sentinel-2 mosaics of Germany free-of-charge (Sentinel-2 L3A product). This data product has been consecutively processed since 2017, when Sentinel-2 began operations. A weighted averaging of 46 days of atmospherically corrected individual Sentinel-2 images achieves almost entirely cloud-free coverage.
First of all, the clouding influence of the atmosphere is eliminated from the image data. At the same time the MAJA atmosphere processor developed by CESBIO, CNES and DLR identifies all pixels that are completely obscured by clouds. The WASP processor (Weighted Average Synthesis Processor) developed by the French partners CESBIO, CNES and Sen2Agri selects from this cloud classification only those image pixels that are cloud-free in the time period under consideration. Depending on image quality and distance to the middle of the month, they are then weighted and combined into a new multispectral image pixel. With this approach, representative, cloud-free monthly values for 10 multispectral Sentinel-2 bands can be determined. The so-corrected data is equivalent to the values that a measuring instrument located immediately above earth's surface would have recorded. This reflection of solar irradiance from the earth's surface is known as "Bottom of Atmosphere" reflectance.
With this method it is possible to reliably remove from the image any clouds over Germany during the vegetation period. "The new product considerably increases the value of Sentinel-2 data for Germany since limitations caused by clouds are virtually eliminated", explains Dr. Pablo d’Angelo, who integrated the processor at EOC. "The monthly mosaics are also well suited to identifying changes taking place over large areas, such as during periods of drought, something that would not be noted if studying a multitude of partially cloudy individual images."
The monthly composites are now available to everyone from EOC-Geoservice in full spatial, temporal and spectral resolution. In addition to the image data, there is supplementary information for each pixel, such as mean time of recording and membership in which of four classes (land, clouds, snow, water). Torsten Heinen, who integrated the data into EOC-Geoservice, explains: "With our web-based, standardised geodata services users can selectively access regions, time intervals, and the desired spectral bands. They can incorporate these data directly into their own systems and applications without having to download them. Our services dynamically supply their applications with data in the background and can also provide a variety of map projections and output formats. It is also possible to select particular band combinations for visualising the data."
(For data access and further information use the links in the navigation column at right.)
Interactive comparison of monthly mosaics for July 2018 (left) and July 2019 (right). The 2018 drought is clearly visible. Other times and representations can be selected by clicking the relevant fields in the upper image area.