Methods

Lead Line and drag wire methods are an early hydrographic surveying technique that measures bathymetric depth using ropes with depth markings and lead weights attached. These were usualy taken at fixed reference points. Measurements were often taken from a boat and depths were manually recorded.

Single and multi-beam echo sounders rely on the same underlying technology. A single beam sounder sends out a sonar (sound) beamtowards the seafloor. Bathymetric depth  is then calculated by measuring the time it takes for the sound to leave the sounder, hit the seafloor, and return.Multi-beam sounders utilise the same technlogy but emit multiple simultaneous sonar pulses in a fan shape, covering the area under the ship and out to each side covering a larger area than single-beam measurements.Image courtesy of Total Hydrographic.

Autonomous systems are the waterborne equivalent of UAVs/Drones. They often carry surveying equipment such as sounders and can be set survey tracks along which they collect data by following GPS information. Their sall size enables them to be used in hazardous andnearshore environments where maneouvarability is critical.

Small wavelengths of light are able to penetrate water bodies to limited depths. Lidar instruments with a green wavelength are well suited to collecting bathymetric depths by measuring the time between the laser transmitting and then receiving the returned pulse. The water depth is recorded by differencing the last return from a detected water surface. Traditinally, these instruments have been predominantly flown onboard airborne systems, but since 2018, the ICESat-2 satellite has been able to provide this data for nearshore waters with global distribution. Image courtesy of NASA.

Satellite Derived Bathymetry (SDB) is a means of using the reflectance measured by optical sensors to estimate water depths. As optical wavelengths penetrate water colums to different depth, the relationships between image bands can be used to infer depth. Physics-inversion-based models can be created from reflectance alone, but SDB often uses in situ data to calibrate models by building relationships between reflectance and depth. Image courtesy of GIM International.