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Intertidal zones serve as a critical transition between land and ocean and are periodically inundated by tides. They are extremely important to coastal communities as it provides diverse habitats and buffers against marine hazards. However, the reduced river sediment together with sea level rise and coastal reclamation have caused the widespread loss of intertidal areas. Reliable, high-resolution, and up-to-date intertidal topography maps are key information for coastal vulnerability assessment and restoration. Existing approaches to intertidal topography construction involve significant ground surveys, with limited spatial coverage, accuracy, and efficiency, thus, restricting their potential to generalize globally. To address these issues, we propose a pixel-based approach to construct intertidal topography from dense Sentinel-2 satellite time-series and limited ground truth surveys, named as Tide2Topo. Tide2Topo differs conceptually from the widely used waterline method since it considers tidal inundation frequency as a topographic indicator rather than instantaneous waterlines. Therefore, Tide2Topo does not involve manual selection of images at different tide levels. The relationship between intertidal inundation frequency and elevation is calibrated using linear and polynomial models based on a few transect measurements instead of tidal height, preventing errors introduced by inaccurate tidal data. The proposed Tide2Topo was applied and validated in the UK and China over several complicated estuaries and bays with extensive muddy flats or sandy beaches. The results demonstrated that the topography derived from intertidal inundation frequency compared to LiDAR or UAV photogrammetric observations has a root mean square error ranging from 16 to 38 cm. The large errors were found in the tidal channel areas where the inundation frequency calculation was prone to uncertainty. Dense Sentinel-2 time-series observations ensure finer sampling of the tidal cycle, thereby n

The rapid, up-to-date, cost-effective acquisition and tracking of intertidal topography are the fundamental basis for timely, high-priority protection and restoration of the intertidal zone. The low cost, ease of use, and flexible UAV-based photogrammetry have revolutionized the monitoring of intertidal zones. However, the capability of the RTK-assisted UAV photogrammetry without ground control points, the impact of flight configuration difference, the presence of surface water in low-lying intertidal areas on the photogrammetric accuracy, and the potential of UAV/satellite Synergy remain unknown. In this paper, we used an RTK-assisted UAV to assess the impact of the above-mentioned considerations quantitatively on photogrammetric results in the context of annual monitoring of the Chongming Dongtan Nature Reserve, China based on an optimal flight combination. The results suggested that (1) RTK-assisted UAVs can obtain high-accuracy topographic data with a vertical RMSE of 3.1 cm, without the need for ground control points. (2) The effect of flight altitude on topographic accuracy was most significant and also nonlinear. (3) The elevation obtained by UAV photogrammetry was overestimated by approximately 2.4 cm in the low-lying water-bearing regions. (4) The integration of UAV and satellite observations can increase the accuracy of satellite-based waterline methods by 51%. These quantitative results not only provide scientific insights and guidelines for the balance between accuracy and efficiency in utilizing UAV-based intertidal monitoring, but also demonstrate the great potential of combined UAV and satellite observations in identifying coastal erosion hotspots. This establishes high-priority protection mechanisms and promotes coastal restoration.

Intertidal zones serve as a critical transition between land and ocean and are periodically inundated by tides. They are extremely important to coastal communities as it provides diverse habitats and buffers against marine hazards. However, the reduced river sediment together with sea level rise and coastal reclamation have caused the widespread loss of intertidal areas. Reliable, high-resolution, and up-to-date intertidal topography maps are key information for coastal vulnerability assessment and restoration. Existing approaches to intertidal topography construction involve significant ground surveys, with limited spatial coverage, accuracy, and efficiency, thus, restricting their potential to generalize globally. To address these issues, we propose a pixel-based approach to construct intertidal topography from dense Sentinel-2 satellite time-series and limited ground truth surveys, named as Tide2Topo. Tide2Topo differs conceptually from the widely used waterline method since it considers tidal inundation frequency as a topographic indicator rather than instantaneous waterlines. Therefore, Tide2Topo does not involve manual selection of images at different tide levels. The relationship between intertidal inundation frequency and elevation is calibrated using linear and polynomial models based on a few transect measurements instead of tidal height, preventing errors introduced by inaccurate tidal data. The proposed Tide2Topo was applied and validated in the UK and China over several complicated estuaries and bays with extensive muddy flats or sandy beaches. The results demonstrated that the topography derived from intertidal inundation frequency compared to LiDAR or UAV photogrammetric observations has a root mean square error ranging from 16 to 38 cm. The large errors were found in the tidal channel areas where the inundation frequency calculation was prone to uncertainty. Dense Sentinel-2 time-series observations ensure finer sampling of the tidal cycle, thereby n

The rapid, up-to-date, cost-effective acquisition and tracking of intertidal topography are the fundamental basis for timely, high-priority protection and restoration of the intertidal zone. The low cost, ease of use, and flexible UAV-based photogrammetry have revolutionized the monitoring of intertidal zones. However, the capability of the RTK-assisted UAV photogrammetry without ground control points, the impact of flight configuration difference, the presence of surface water in low-lying intertidal areas on the photogrammetric accuracy, and the potential of UAV/satellite Synergy remain unknown. In this paper, we used an RTK-assisted UAV to assess the impact of the above-mentioned considerations quantitatively on photogrammetric results in the context of annual monitoring of the Chongming Dongtan Nature Reserve, China based on an optimal flight combination. The results suggested that (1) RTK-assisted UAVs can obtain high-accuracy topographic data with a vertical RMSE of 3.1 cm, without the need for ground control points. (2) The effect of flight altitude on topographic accuracy was most significant and also nonlinear. (3) The elevation obtained by UAV photogrammetry was overestimated by approximately 2.4 cm in the low-lying water-bearing regions. (4) The integration of UAV and satellite observations can increase the accuracy of satellite-based waterline methods by 51%. These quantitative results not only provide scientific insights and guidelines for the balance between accuracy and efficiency in utilizing UAV-based intertidal monitoring, but also demonstrate the great potential of combined UAV and satellite observations in identifying coastal erosion hotspots. This establishes high-priority protection mechanisms and promotes coastal restoration.