What is hydrographic surveying?

In civil engineering, measurement of earth surface under water. Eventually, it leads to calculation of volume of water held (or flowing through) depressions on earth surface.

Since, underwater surfaces are not directly accessible for measurement a separate technology called sounding is used to measure the depth of surface from water surface. Sounding methods may vary from running a long string with a sink to the bed of water body to sending an ultrasonic vibration to measure the time of return.