Intertidal zones are coastal environments exposed to both air and water, at low and high tide consecutively, and include sandy beaches, tidal flats, rocky shores and reefs. They also characterise critical coastal habitats and ecosystems, supporting a wide range of species and ecosystem-services. Increasingly though, these dynamic environments are faced with threats from land reclaimation, coastal erosion and rising sea levels, amongst others. The ever-changing nature of the tides makes it hard to systematically capture consistent imagery of the intertidal zone, particularly across large regions and especially in remote areas of the country. Geomedian statistical techniques provide a robust method to combine tide-attributed time-series satellite imagery and produce representative and artefact free imagery ‘composites’ of Australia’s coastal high and low tide environments.
This product provides a suite of cloud-free composite Sentinel-2 satellite datasets that enable imaging of Australian coastal intertidal zones at both high and low tide. Using a geometric median (geomedian), the highest and lowest 15 % of observed tidal images in Digital Earth Australia (DEA)’s Sentinel-2 archive are combined to deliver annual snapshots of Australian coastal high and low tide environments. Spatially and temporally aligned to the DEA Intertidal product suite (https://knowledge.dea.ga.gov.au/data/product/dea-intertidal), this product (also known as DEA Tidal Composites) is an annually updated data suite, generated from rolling 3-year epochs, with a minimum of 10 m spatial resolution.
Tidal composites are produced by tidally attributing Sentinel-2 satellite images though pairing with pixel-based local tidal modelling, generated from a selected ensemble of the best performing global tide models under local conditions. The ensemble tidal modelling approach was implemented to account for the varying performance and biases of existing global ocean tide models across the complex tidal regimes and coastal regions of Australia. Tidal attribution allows the imagery archive to be sorted by tide height rather than date, so the intertidal zone can be visualised during any stage of the tide regime.
DEA Tidal Composites include 25 layers of data, separated into lowtide, hightide and quality assurance categories. Lowtide and hightide layers represent composites of the synthetic geomedian surface reflectance from Sentinel-2A, -2B and -2C analysis-ready data streams. The geomedian calculation maintains the spectral relationships between bands, ensuring that the DEA Tidal Composites product delivers robust and valid surface reflectance spectra suitable for uses such as habitat mapping and delivers cloud free and noise reduced visualisation of the shallow water and intertidal coastal regions of Australia. Quality assurance layers are provided to support interpretation of the lowtide and hightide datasets and include the tide-height thresholds above and below which associated images were included in the compositing process and the count of clear input images that contributed to each pixel in the composites.
