Mapping Australia's dynamic coastline at mean sea level using three decades of Landsat imagery

Accurate, robust and consistent coastline mapping is critical for characterising and managing coastal change. Satellite earth observation provides an unparalleled source of freely available data for studying dynamic coastlines through time and across large spatial extents. However, previous satellite-derived shoreline mapping approaches have been challenged by two key limitations: the medium spatial resolution of freely available satellite data, and the confounding influence of tides that can obscure longer term patterns of coastal change. In this study we present Digital Earth Australia Coastlines, a new continental dataset documenting three decades of coastal change across Australia. We combine sub-pixel waterline extraction with a new pixel-based tidal modelling method to seamlessly map almost 2 million km of tide-datum shorelines along the entire Australian coast from 1988 to 2019. Our tidally-constrained median composite approach maps the dominant annual position of the shoreline at 0 m Above Mean Sea Level each year, suppressing the short-term influence of tides and sub-annual shoreline variability. Using this robust mid-term shoreline proxy, long-term coastal change rates spanning the last three decades were accurately quantified and mapped at the continental scale. We find that 22% of Australia's non-rocky coastline has retreated or grown significantly since 1988, with 16% changing at greater than 0.5 m per year. Although trends of retreat and growth were closely balanced across the Australian continent, our results highlight significant regional variability and extreme local hotspots of coastal change. Our findings provide new insights into patterns and trends of coastal change across Australia, and highlight advantages and limitations of tide modelling and composite-based methods for extracting consistent shoreline data and long-term coastal trends from earth observation data at continental scale. Digital Earth Australia Coastlines is made available to the public as free and open interactive tools and code to support future coastal research and management across Australia, and any coastal region globally with access to free and open medium resolution satellite data.