Harp Piemonte — Multitemporal mosaic NDVI from Sentinel images (2020-2021)

The image is a mosaic in false colors of the seasonal values of the NDVI* (Normalised Difference Vegetation Index) index calculated from the collection of images acquired on Piedmont by the pair of Sentinel 2A-2B satellites of the European Copernicus programme, covering the period autumn 2020 — summer 2021. The mosaic was created by composing in a unique image, according to the RGB color model, the seasonal synthesis values of the NDVI index: the red channel (R) was associated in autumn 2020, with that of green (G) in spring 2021, with blue (B) in summer 2021. The composition of the seasonal NDVI makes it possible to differentiate the areas for which there are one or more vegetative cycles or changes in the soil cover (differently coloured areas) from those where the soil is completely free of vegetation or with permanent water (gray to black shades).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 multitemporal 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 as follows: the summary image of the red band and that of the near infrared band. 2) From the summary images of the individual bands produced in 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 soil consumption monitoring project 2020 conducted by the National System for Environmental Protection (SNPA) through the coordination of ISPRA and the involvement of various regional agencies. The project aims to monitor the transformations of the territory due to urban, infrastructural, commercial expansions, 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 which, given their level of detail and the frequency of revisitation guaranteed by the Sentinel mission, allow to obtain overviews of the national territory in a regular and constant way (multi-time analysis).The comparison between these allows to highlight the main transformations of the territory and the related phenomena of land use (expansion of urban settlements, the NDVI is a vegetative index based on the measures of reflectance acquired in the spectral regions of red and near infrared and is strongly related to the content of chlorophyll and, if measured over time, to the phenological cycle of the plants.** Before merging into the stack the individual images have been appropriately masked to eliminate the cloud cover.