Arpa Piemonte — Multitemporal NDVI mosaic from Sentinel images (2021-2022)

The image is a mosaic in false colors of the seasonal values of the NDVI* (Normalised Difference Vegetation Index) calculated from the collection of images acquired on Piedmont by the pair of satellites Sentinel 2A-2B of the European program Copernicus, for the period autumn 2021 — summer 2022. The mosaic was generated by composing in a unique image, according to the RGB color model, the seasonal synthesis values of the NDVI index: the red (R) channel was associated with autumn 2021, green (G) spring 2022, blue (B) summer 2022. The composition of the seasonal NDVIs makes it possible to differentiate the areas for which there are one or more vegetative cycles or in any case variations in soil cover (variously colored areas) from those in which the soil is completely devoid of vegetation or with permanent water (grey to black).For the calculation of the three seasonal synthesis images of the NDVI index, the median reflectance value of each pixel has been calculated:1) From the seasonal multi-temporal stack**, for each of the bands contributing to the calculation of the NDVI, the median reflectance value of each pixel has been calculated. Two images were obtained: the synthesis image of the red band and that of the near-infrared band. 2) From the synthesis images of the individual bands produced at the previous point, the seasonal synthesis image of the NDVI index was finally calculated.The elaboration was carried out by Arpa Piemonte as part of the activities related to the national project for monitoring soil consumption 2021 conducted by the National System for Environmental Protection (SNPA) through the coordination of ISPRA and the involvement of several regional agencies. The project aims to monitor the transformations of the territory due to urban, infrastructural, commercial, etc., with the consequent loss of natural, agricultural and semi-natural soil, understood as an essential and fundamentally non-renewable environmental resource. The analysis methodology uses, for the first time on a national scale in a unitary and homogeneous way, the new Sentinel 2A/B images that, given their level of detail and the frequency of revision guaranteed by the Sentinel mission, allow to obtain overviews of the national territory in a periodic and constant way (multi-temporal analysis).The comparison between these allows to highlight the main transformations of the territory and the related phenomena of land consumption (expansion of urban settlements, new viability, shipbuilding for new works, etc.).* The NDVI is a vegetative index based on reflectance measures acquired in the spectral regions of red and near-infrared and is strongly related to chlorophyll content and, if measured over time, to the phenological cycle of plants.** Before entering the stack the individual images were suitably masked to eliminate cloud cover.