An acoustic underwater survey technique used to trace the topography of submerged land surfaces. It is a method in which a sound pulse travels from the vessel to the ocean floor, is reflected, and returns. By calculations involving the time elapsed between generation of the pulse and its return and the speed of sound in water, a continuous record of sea floor topography can be made. Echo sounding depends on timing the lapse between the transmission of a short loud noise or pulse and its return from the target - in this case the bottom of the sea or lake. Most echo sounders perform these calculations mechanically, producing a graphic record in the form of a paper chart. Misleading reflections caused by the presence of undersea canyons or mountains plus variations in the speed of sound through water caused by differences in temperature, depth, and salinity limit the accuracy of echo sounding.