Geoscience Australia Sentinel-2 Tidal Composites Calendar Year Collection 3

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.

潮间带（intertidal zone）是随涨潮与退潮周期性交替暴露于空气与海水的海岸环境，涵盖沙滩、潮滩、岩岸与珊瑚礁。这类区域属于关键海岸生境与生态系统，支撑着丰富的物种多样性与生态系统服务功能。然而，这些动态变化的海岸环境正日益面临围填造地、海岸侵蚀与海平面上升等多重威胁。潮汐的周期性变化使得系统性获取潮间带的一致影像变得困难，尤其是在大范围区域乃至该国偏远地区开展此类工作时。
几何中值（geomedian）统计技术为结合潮汐标注的时序卫星影像提供了稳健的方法，可生成澳大利亚海岸高低潮环境的具有代表性且无伪影的影像合成产品。
本数据集为一套无云合成的哨兵-2号（Sentinel-2）卫星数据集，可用于拍摄澳大利亚海岸潮间带的高低潮影像。研究人员借助几何中值（geomedian）技术，将澳大利亚数字地球（Digital Earth Australia, DEA）哨兵-2号影像库中观测到的15%最高潮与15%最低潮影像进行合成，以此生成澳大利亚海岸高低潮环境的年度快照。
本数据集（又称DEA潮汐合成数据集）在空间与时间上与DEA潮间带数据集套件（https://knowledge.dea.ga.gov.au/data/product/dea-intertidal）保持对齐，采用滚动3年周期生成，每年更新一次，空间分辨率最低可达10米。
潮汐合成影像的制作流程为：将哨兵-2号卫星影像与基于像素的局地潮汐模型进行配准并赋予潮汐标注，该局地潮汐模型由筛选出的、在局地条件下表现最优的全球潮汐模型集合生成。之所以采用集合潮汐建模方法，是为了弥补现有全球海洋潮汐模型在澳大利亚复杂潮汐格局与海岸区域中表现参差不齐且存在系统性偏差的缺陷。潮汐标注可使影像库按照潮高而非拍摄日期进行排序，从而能够在潮汐周期的任意阶段可视化潮间带区域。
DEA潮汐合成数据集包含25个数据图层，分为低潮、高潮与质量保证三大类别。低潮与高潮图层对应哨兵-2A、-2B及-2C的分析就绪数据流所生成的合成几何中值地表反射率合成产品。几何中值计算过程保留了各波段间的光谱关系，确保DEA潮汐合成数据集可输出稳健且有效的地表反射率光谱，适用于生境制图等应用场景，同时可为澳大利亚浅水区与潮间带海岸区域提供无云且降噪的可视化效果。质量保证图层用于辅助解读低潮与高潮数据集，其中包含合成影像制作过程中所采用的潮高阈值，以及合成影像中每个像素对应的有效输入影像数量。