A review of active and passive optical methods in hydrography

Abstract

The history of using optical methods for hydrographic applications dates back to the mid of the last century. Since these early days, both passive and active techniques have evolved considerably and with ever new sensors and platforms available, there is unbroken scientific progress in this field capable of capturing the submerged topography of shallow water bodies. This review article gives an overview of the employed optical methods, which can be categorized into passive and active techniques. Passive methods comprise spectrally derived bathymetry as well as multimedia stereo photogrammetry, also referred to as photo bathymetry, and the active method is laser bathymetry. Another approach for categorizing optical methods is based on the used carrier platform, where satellites, crewed aircraft, uncrewed aerial systems and lately also remotely piloted underwater vehicles are used for mapping water bodies at different scales, resolution and accuracy. Another attempt to classify the approaches is whether the sensor is above or below the water table, referred to as through-water or underwater, respectively. Another focus of the paper is on data processing techniques including refraction correction, which is crucial for most optical hydrography approaches, signal processing of the radiometric content in spectrally derived bathymetry, and analysis of the backscattered waveform signal in laser bathymetry. In the recent past, machine learning techniques play an increasingly important role not only for object classification but also for regression-based depth estimation. And finally, the paper discusses the huge variety of applications ranging from mapping navigable channels for save ship navigation, disaster documentation, flood simulation, coastal protection, benthic habitat modeling, river restoration, monitoring of hydromorphological changes, maintenance of hydroelectric power plants, mapping of off-shore infrastructure, coral reef mapping to underwater archaeology. The paper presents seminal work in the field but also focuses on the multitude of recent scientific work demonstrating the unbroken relevance of optical hydrography.