Enhanced monitoring of coastal change: a comprehensive validation framework for satellite imagery

Coastal change assessments have significant socioeconomic, environmental, and infrastructure implications due to the extensive impacts of climate change, such as rising sea levels, the increasing frequency and intensity of storms, as well as the consequences of human intervention.  Satellite products have been used to monitor the coast at relatively high resolution (30 m) since the 1970s through the Landsat program. However, the arrival of the EU Copernicus/Sentinels in 2014 introduced a marked increase in coastal applications thanks to the improved spatial and temporal resolutions. The research presented in this study explores the derivation of waterlines from Sentinel 2 imagery and the creation of a novel holistic approach to a validation framework. Specifically, this study aims to: a) explore the inherent waterline errors against reference datasets and begin to establish the overall uncertainty in deriving waterlines from optical satellite imagery; and b) assess the potential of these results and their suitability for coastal change applications. The results indicate an average positional error of approximately 4 meters for Sentinel images in coastal regions by evaluating the Sentinel-2 satellite images with distinct features visible in aerial orthophotography. Subsequently, the horizontal and vertical inaccuracies of the satellite-derived waterlines (SDWL) were further determined by using a GNSS line as a reference dataset. The horizontal assessment was conducted by calculating the average distance between the SDWLs and the GNSS reference lines across eighteen Sentinel-2 images corresponding to the years 2021, 2022, 2023 and 2024. These were analysed, showing a median displacement of 15 meters, and indicating an offshore trend for the satellite-derived waterlines. The vertical assessment, or height error, was computed by comparing the average height of SDWLs (as determined by the average tide gauge heights) with the reference dataset height (as measured by GNSS), resulting in a mean absolute error of 6 cm. The vertical results indicate that the SDWLs’ heights, as measured by the local tide gauges, align well with in situ local height measurements. The results of this study will aid in identifying temporal and spatial scales and resolutions at which Earth Observation products are suitable for coastal management. The initial stages of a validation framework are presented to assess the quality and applicability of satellite-derived waterlines for coastal change monitoring based on specific user requirements. Identifying the sources of error and improving uncertainty models for satellite-derived products enables better decision-making in coastal management. These analyses will demonstrate whether the outcomes remain consistent among satellite images or change according to local environmental conditions. Increasing end-user confidence in the rates of change obtained from available satellite products can provide crucial information in study areas, and at space-time resolutions previously unattainable.