The Chart

A nautical chart is a working document. Navigators draw on it, measure from it and depend on it to keep a vessel off rocks, shoals and wrecks.

Measuring the bottom

For centuries depths were measured by lead line: a weighted line marked in fathoms (six feet). A hollow in the lead filled with tallow brought up a sample of the bottom, which helped anchoring and position-finding. Modern hydrographic surveys use single-beam and multibeam echo sounders, side-scan sonar and airborne lidar.

American charting

In 1807 Congress and President Thomas Jefferson established the Survey of the Coast, now part of NOAA's Office of Coast Survey. Its charts cover U.S. coastal waters, harbors and the Great Lakes. NOAA now produces electronic navigational charts, and its traditional paper charts are being phased out in favour of electronic and print-on-demand products.

Reading a chart

Depths are shown relative to a chart datum; on U.S. coastal charts this is mean lower low water, so depths are usually conservative. The compass rose shows true and magnetic north and the local magnetic variation. Aids to navigation follow the IALA-B system in the Americas: "red, right, returning", meaning red buoys are kept on the starboard side when returning from sea.

Electronic navigation

Electronic chart display and information systems (ECDIS) on ships and chartplotters on small boats combine charts with GPS positions, radar and AIS traffic. Watchstanders still must understand the chart, check positions by more than one method, and keep a lookout.

Programs

Maritime Crew (deck/engine), GIS / Surveying, Aquaculture & Marine Science.