Determination of water depth with high‐resolution satellite imagery over variable bottom types

TLDR

A standard passive‑sensor depth algorithm exists but requires tuning of five parameters and fails over very low‑albedo bottoms. The study develops an empirical two‑parameter ratio‑reflectance solution that works on low‑albedo features. The authors compared the standard linear transform and the new ratio transform on IKONOS imagery against lidar bathymetry, with the ratio transform based on a two‑parameter reflectance ratio. The ratio transform, tuned manually to a few chart depths, matched the linear algorithm’s performance, compensated for variable bottom types and albedo, retrieved bathymetry up to 25 m, showed greater stability across areas, performed better in turbid water, but was noisier and less effective at resolving fine morphology beyond 15–20 m, making it overall more robust than the linear transform.

Abstract

A standard algorithm for determining depth in clear water from passive sensors exists; but it requires tuning of five parameters and does not retrieve depths where the bottom has an extremely low albedo. To address these issues, we developed an empirical solution using a ratio of reflectances that has only two tunable parameters and can be applied to low‐albedo features. The two algorithms—the standard linear transform and the new ratio transform— were compared through analysis of IKONOS satellite imagery against lidar bathymetry. The coefficients for the ratio algorithm were tuned manually to a few depths from a nautical chart, yet performed as well as the linear algorithm tuned using multiple linear regression against the lidar. Both algorithms compensate for variable bottom type and albedo (sand, pavement, algae, coral) and retrieve bathymetry in water depths of less than 10–15 m. However, the linear transform does not distinguish depths .15 m and is more subject to variability across the studied atolls. The ratio transform can, in clear water, retrieve depths in >25 m of water and shows greater stability between different areas. It also performs slightly better in scattering turbidity than the linear transform. The ratio algorithm is somewhat noisier and cannot always adequately resolve fine morphology (structures smaller than 4–5 pixels) in water depths >15–20 m. In general, the ratio transform is more robust than the linear transform.