• High-water line, a geomorphic shoreline proxy, is visible in PlanetScope imagery. • Machine learning is used to automate the delineation of the HWL position. • HWL position strongly correlates with tide height during satellite flyover. • Low-tide HWL closely matches mean high water, enabling low-cost shoreline mapping. Satellite-derived shoreline mapping is a common technique for quantifying geomorphic shoreline change. However, shoreline positions derived using moderate-resolution data have questionable accuracy, are influenced by metocean conditions, and are typically based on the boundary of a binarized spectral index rather than a visible geomorphic indicator, such as the high-water line (HWL). PlanetScope (PS) can visualize the location of the HWL by detecting the spectral reflectance differences between wet and dry sediment along a sandy beach surface due to its improved spatial resolution of 3 m/pixel. The accuracy of nine HWL proxy shoreline positions is assessed by comparison to a contemporaneous mean high water (MHW) shoreline delineated across Moro Beach, CA, using a digital elevation model created from RTK GPS-corrected Unmanned Aircraft System imagery. The offset between the PS-derived HWL and UAS-derived MHW positions (Δd) was measured every 10 m in the alongshore direction using the Digital Shoreline Analysis System for each HWL dataset. A significant exponential relationship was observed between Δd and the tide height at the time of PS image acquisition, whereby the HWL shoreline was located further landward (seaward) during higher (lower) tides. The HWL shoreline, when acquired at or near low tide, was spatially coincident with the MHW shoreline, the most reliable yet cost-prohibitive shoreline proxy. PlanetScope’s advancement in spatiotemporal resolution introduces a new approach to satellite-derived shoreline mapping, one that is based on a geomorphic proxy position and a minimized influence of tide heights.
Gontz et al. (Fri,) studied this question.