![]() ![]() They occur at all latitudes in physical habitats that are low-sloping and influenced by tides, and include ecotypes ranging from tidal rock platforms to fine-grained silts and clays 1, 2, 3, 4. Tidal flats are a globally widespread coastal ecosystem that occur at the interface between land and sea 1, 2. We expect these maps will provide a means to measure and monitor a range of processes that are affecting coastal ecosystems, including the impacts of human population growth and sea level rise. We assessed accuracy of the classification with 1,348 stratified random samples across the mapped area, which indicated overall map accuracies of 82.2% (80.0–84.3%, 95% confidence interval) and 86.1% (84.2–86.8%, 95% CI) for version 1.1 and 1.2 of the data, respectively. The composite metrics were used as predictor variables in a machine-learning classification trained with more than 10,000 globally distributed training samples. More than 1.3 million Landsat images were processed to 54 composite metrics for twelve 3-year periods, spanning four decades (1984–1986 to 2017–2019). Here we present globally consistent, spatially-explicit data of the occurrence of tidal flats, defined as sand, rock or mud flats that undergo regular tidal inundation. Assessments of the status of tidal flats, one of the most extensive coastal ecosystems, have been hampered by a lack of data on their global distribution and change.
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