Hydrography involves measuring the depth of the water and fixing the position of all the navigational hazards that lie on the seafloor, such as wrecks and rocks. This is done mainly with specialised ships and boats operating echo sounders and sonars, but also using survey aircraft fitted with lasers. Useful information can also be derived sometimes from satellite observations. Hydrography also involves measuring the tide and the currents.
The most well-known application of hydrographic information is for making the nautical charts that all mariners use for navigation. Hydrographic information is required for the safe, efficient and sustainable conduct of every human activity that takes place in, on or under the sea.