A vessel typically uses GPS, Radar, Decca, Loran, or optical bearings to locate itself. Simply as a routine, the echo sounder’s reading of the water’s depth is compared to what is shown on the chart. However, the under-keel clearance and depth of water need to be known when the position is not precisely known while approaching the port, crossing over a bar, near the mouth of a river, or in a poorly surveyed area. The echo sounder is helpful in this circumstance.