Bathymetry is the depth of the ocean as compared to sea level. However, today it is used to describe the topography of the ocean floor and is part of the science of hydrography. Hydrography is the study and mapping of the physical features of a body of water such as a lake, pond, river, or ocean. It includes bathymetry but also takes into account tides, ocean currents, waves, the shape of the coastline, and the properties (physical and chemical) of the water.

Bathymetry data is collected by satellite, with echosounders (single beam or multi-beam), or by airborne laser measurements. Satellites collect altimetry data. This measurement of marine gravity shows generally where the ocean floor is higher or lower. The satellite sends out a radar pulse and measures the time of return. For a higher resolution picture of the seafloor, scientists use a multi-beam sonar system. These arrays are mounted to the hull of a ship and it bounces sound waves off the ocean floor. The time between sending out the signal and receiving the returning echo gives researchers the depth measurement.

It is important to map the seafloor so we can regulate extraction operations (Mining, drilling for oil), keep ships and shipping routes safe, lay new underwater pipes and cables, and protect marine species and habitats.

The data included in this map layer is from the Scripps Institution of Oceanography and was collected via satellite altimetry and published in 2009. The satellite collected altimetry data across the Earth in one-kilometer by one-kilometer boxes, called cells or pixels. Experts then used computers to stitch the cells together for the entire ocean.