Surveyors pay particular attention to acquiring precise locations of shallow areas, obstructions, and aids to navigation that warn mariners. They also record tide or water level measurements to provide a vertical reference (mean lower low water) for water depths. Surveys also determine sea floor material (i.e. sand, mud, rock), which is important for anchoring, dredging, structure construction, pipeline and cable routing, and fish habitat.
Hydrographic vessels are equipped with side scan and multibeam sonars which uses sound waves to find and identify objects in the water to determine seafloor depth. NOAA’s Remote Sensing Division and commercial hydrographic contractors use aircraft equipped with lidar to measure water depths in areas with complex and rugged shorelines. Lidar is also used on some NOAA vessels to collect shoreline features at a safe distance from shore. Coast Survey and its commercial contractors survey approximately 2,000 to 3,000 square nautical miles each year.