bathymetry

The measurement of the depths of sea-bottom features in large bodies of water. Bathymetric charts indicate the depths of water in feet, fathoms, or metres, and are used to show the morphology of submarine topographic features. Detailed bathymetric mapping was only possible with the advent of continuous echo-sounding, which was first extensively used during the German Meteor Expedition (1925–7).

A bathymetric map gives the depth contours of the soil, rock, sand, etc. at the bottom of a body of water such as an ocean or a lake. In this regard, it is tool to map the topography of the floor of a body of water.

The original term was used to refer to the measurement of ocean depth.

The data used to make bathymetric maps typically comes from an echosounder (sonar) mounted beneath or over the side of a boat, 'pinging' a beam of sound downward at the seafloor. The amount of time it takes for the sound to travel through the water, bounce off the seafloor, and return to the sounder tells the equipment how far down the seafloor is.

Years ago, the occasional pings of a single-beam sounder might be averaged to make a map. In general, the wide swath, which is depth dependent, allows a boat to map more seafloor in less time by making fewer passes. The beams update many times per second (typically 1-40 Hz depending on water depth), allowing faster boat speed while maintaining 100% coverage of the seafloor. Attitude sensors correct for the boat's roll, pitch and yaw on the ocean surface, and a gyrocompass provides accurate heading information to correct for vessel yaw. Accurate sound velocity profiles (speed of sound in water) of the water column correct for refraction or "ray-bending" of the sound waves owing to non-uniform water column characteristics such as temperature, conductivity, and pressure.

Most surveys of navigable waterways in the US are performed or commissioned by the United States Army Corps of Engineers, for inland waterways, and the National Oceanic and Atmospheric Administration for oceans.