ICESat-2 bathymetry algorithms: A review of the current state-of-the-art and future outlook

Over six years of on-orbit operations, the NASA Ice, Cloud and Land Elevation Satellite-2 (ICESat-2) has proven the value of space-based laser altimetry for monitoring the response of Earth’s surfaces to a changing climate with continuous elevation measurements. Although bathymetry is not an official science requirement for the mission, ICESat-2 has had a transformative impact on understanding nearshore bathymetry. Despite its successful, proven contributions to nearshore and coastal studies, there is currently no standardized, fully-automated algorithm for the extraction of ICESat-2 bathymetry signal. However, there are many published algorithms in the literature that present novel and innovative approaches for all aspects of the bathymetric workflow, as well as processes for conflating the ICESat-2 measurements with optical data to support bathymetric mapping. In this work, we provide a comprehensive review and synthesis of the existing algorithms and procedures comprising the main steps in an end-to-end ICESat-2 bathymetry workflow, including water surface extraction, classification of bathymetric bottom returns, refraction correction, accuracy assessment, integration with optical imagery, and ancillary steps. The review is intended to inform the development of a new Level 3 along-track data product for the ICESat-2 mission (ATL24) as a global resource for nearshore bathymetry and to aid other researchers in developing and testing their own algorithms for ICESat-2 bathymetry. We provide an assessment of the current-state-of-the-art algorithms, challenges and limitations, as well as recommended next steps for the community of researchers working on and with ICESat-2 bathymetry.