Depths on any given NOAA chart may be based, in some areas, on hydrographic surveys conducted with leadlines prior to 1900, in other areas, on multibeam sonar surveys that attained full bottom coverage.
Over 50 percent of the depth information found on NOAA charts is based on hydrographic surveys conducted before 1940. Surveys conducted with lead lines or single-beam echo sounders sampled a small percentage of the ocean bottom. Due to technological constraints, hydrographers were unable to see between the sounding lines. Depending on the water depth, these lines may have been spaced at 50, 100, 200 or 400 meters. Today, as NOAA and its contractors re-survey areas and obtain full-bottom coverage, uncharted features (some that are dangers to navigation) are routinely discovered. These features were either: 1) not detected on prior surveys, 2) manmade objects, like wrecks and obstructions, that have appeared on the ocean bottom since the prior survey or 3) the result of natural changes that have occurred since the prior survey.