Multibeam sonar measures the depth of the sea floor, technically it measures the length of time it takes for the sound waves to travel from the boat to the seafloor and back as an echo. This measurement is used to figure out how deep the water is. This is much more accurate than sounding which is the process of using a rope that has weight attached to it. This method of seafloor mapping was very time consuming, especially when charting deep water. Unlike the newer method of using sonar.
