Echo sounding is the use of sound pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves. Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the speed of sound in the water. Echo sounding is effectively a special purpose application of sonar used to locate the bottom.
As well as accident avoidance (most larger vessels will have at least a simple depth sounder), echo sounding is commonly used for fishing. Variations in elevation often represent places where fish congregate.
Most charted ocean depths use an average or standard sound speed. Where greater accuracy is required average and even seasonal standards may be applied to ocean regions. For high accuracy depths, usually restricted to special purpose or scientific surveys, a sensor may be lowered to observe the factors (temperature, pressure and salinity) used to calculate sound speed and thus determine the actual sound speed in the local water column.