Hydrographic survey

A hydrographic survey is when special people go into the water to measure how deep it is, and make a detailed map of everything they find down there. They use special boats with machines that can send out sound waves to the bottom of the water, and the sound waves bounce back to the machines on the boat. The people on the boat can use those sound wave measurements to make a map of what the bottom of the water looks like. This is really helpful for lots of things, like building bridges or making sure ships can safely travel in the water. It's a bit like drawing a picture of what the bottom of the water looks like, so that people can understand it better and know how to use the water safely.