How trends are computed
Raw CoastSat intersections (Landsat 5/7/8/9) are cleaned in three passes: images where most transects of a beach jump together are dropped; points far from the rolling dominant shoreline position (1-yr shorth, 3.5σ, ≥15 m) are removed; monthly medians far from a 3-yr reference are removed. The mean seasonal cycle is subtracted.
Trends are Theil–Sen slopes on monthly values with 95% intervals widened for autocorrelation. Full record: all data. Since 2000: same window everywhere, for comparing beaches. Recent segment: data after the last breakpoint found by a piecewise-linear change-point fit (segments ≥ 7 yr).
Only transects on medium- and high-erodibility coast are analysed; low-erodibility and rocky coast is left out. Transects are also excluded when hidden on the CoastSat site, more than 30° off the local shore-normal, too sparse, too noisy, when the detection switches between features, or when they disagree strongly with neighbours.
Beaches are OpenStreetMap beach polygons (named where OpenStreetMap gives a name): a transect belongs to a beach when its mean shoreline point lies inside or within 100 m of the polygon, and same-name polygons within 2 km are merged. Where CoastSat casts two transects at (almost) the same place, only the one closest to shore-normal is kept. On the map transects are drawn ±100 m around their mean shoreline position. Transects on sandy coast not mapped as beach are grouped into “unmapped stretches” (neighbours within 250 m). A beach's trend is fitted to its beach-averaged series (the monthly mean of its valid transects, each expressed relative to its own median since 2000); a beach is eroding or accreting only when that trend is significant (95%), otherwise it is stable. Classes: |trend| < 0.3 stable, 0.3–1 erosion/accretion, > 1 severe/strong.
Acknowledgements
This dataset is the result of a collaboration between the University of Cagliari (Prof. Sandro de Muro), the University of Auckland (Nick Young, Giovanni Coco) and ICM-CSIC (Giovanni Coco). We acknowledge the use of CoastSat (Vos et al., 2019, Environmental Modelling & Software 122:104528). Shorelines: CoastSat web map. Beaches: © OpenStreetMap contributors (ODbL). Coastline: Natural Earth. Imagery: Esri; street map: OpenStreetMap. Built .