Bathymetry is basically the measurement of water depth. Bathymetric surveys show depths and terrain configurations of the bottom of water bodies. It can be used for navigation safety, water volume computation, monitoring sediment changes, under water engineering construction, showing pre and post dredge conditions, oil, gas and mineral exploration, ocean pollutant studies etc.
It measures vertical distance from the ocean surface to sea floor features. The process of measuring the depth of water is called sounding.
Bathymetric surveys are usually referenced to local tidal or a vertical datum.