Deriving Tidal Flat Topography Using ICESat‐2 Laser Altimetry and Sentinel‐2 Imagery.

Acquiring accurate and timely datasets on tidal flat topography is fundamental for studies, management, and policy‐making in coastal zones. In this study, we report a fusion method of freely accessed and global‐scale Sentinel‐2 (imagery) and Ice,Cloud, and Land Elevation Satellite‐2 (ICESat‐2) (laser altimetry) datasets for deriving tidal flat topography. The basic idea is to construct a model on characterizing the specific relationship between the elevations of the tidal flat (from ICESat‐2 data) and corresponding inundation frequencies (from Sentinel‐2 time‐series). The proposed method was validated for the largest tidal flat on the Chinese continental shelf with a root mean square error of 0.23 m. We highlight that this method solely relies on satellite data and does not require any in‐situ data because ICESat‐2 provides dense and accurate along‐track points as valuable prior elevation information, which enhance the ability to obtain tidal flat topography in many coastal regions even in the absence of local in‐situ data. Plain Language Summary: Global tidal flat topography is still unavailable at present, although it is the foundation of many global‐scale coastal‐related issues, such as coastal vulnerability estimation to sea level rise and flooding, coastal wetland monitoring, habitat conservation, site selection of offshore wind farms and ports, and land use planning. To generate tidal flat topography, accurate vertical and horizontal information is simultaneously acquired. The Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) satellite provides dense three‐dimensional points with high vertical accuracy, which can replace field surveying (e.g., by handheld/airborne/shipborne devices) for obtaining these three‐dimensional points. The time‐series imagery of the Sentinel‐2 satellite can map accurate inundation frequencies of tidal flats (i.e., the frequency of a location heaves out or is covered by water due to ocean tides) with high horizontal resolution. Integrating the advantages of ICESat‐2 and Sentinel‐2 makes it possible to obtain global topography for tidal flats with only satellite remotely sensed data. With freely accessed and global‐scale ICESat‐2 and Sentinel‐2 data, it can be expanded to obtain tidal flat topography in many specific coastal regions or even globally. Key Points: A method to generate tidal flat topography in coastal zones was proposed using satellite remotely sensed dataIce,Cloud, and Land Elevation Satellite‐2 provides accurate along‐track profiles as prior elevation information; therefore, in‐situ data are no longer neededGoogle Earth Engine was used to speed up implementation and can expand this method for large scale applications [ABSTRACT FROM AUTHOR]