bathymetry is the underwater equivalent of altimetry. The name comes from the Greek βαθύς [vazýs] ('deep'), and μετρóν [metron] ('measure'). In other words, bathymetry is the study of the deep sea, of the third dimension of the lake or seabed. A bathymetric map (or bathymetric chart) normally shows bottom relief or terrain as isograms, and may also provide additional surface navigation information.

Originally, "bathymetry" referred to the measurement of ocean depth. Early techniques used segments of known length of heavy cable or rope, slung over the side of a ship. The biggest limitation of this technique is that it measures depth at only one point at a time, making it very inefficient. It is also very imprecise, since it is subject to the movements of the ship, the tides, and the currents that may affect the cable.

The data used today for bathymetric mapping typically comes from a sonar mounted under the keel or on the side of a ship, sending a sound wave toward the seafloor. The amount of time it takes for sound to go through the water, bounce off the bottom and return tells the team the actual depth. Years ago, each of the individual sonar pulses could be averaged to make a continuous map instead of a point measurement. Today, a wide-scan sonar can be used, consisting of dozens of simultaneous waves, very narrow and adjacent to each other, forming a fan of between 90 and 180 degrees.

The fan of sound waves formed by wide-scan sonars allows for very high resolution and precision. In general, although it depends on the depth, it allows a vessel to cover much more surface area of ​​the seabed than based on individual measurements. The waves are updated many times per second (typically 1 to 40 Hz, depending on depth), allowing the vessel to make much faster passes while maintaining 100% bottom coverage. Additional sensors correct the signal depending on the ship's pitch and motion, and a gyrocompass provides exact information on the ship's direction. Additionally, a GPS ('global positioning system') system can accurately specify the position of the vessel. Exact measurements of the speed of sound in water are also used to calculate the refraction of sound waves as they pass through layers of water with different temperatures, conductivity and pressure. A computer system processes all the data, correcting for each of the factors, as well as for the angle of each individual ray. In the end, using this massive set of data, a map is generated almost automatically.