Nautical chart

A map of maritime areas showing coastlines, water depths, hazards, navigation aids, and other information used for safe navigation. Its symbols, depth references, and scale are standardized for mariners.

Hydrography: The measurement and description of physical features of oceans, seas, lakes, and rivers. Nautical charts present hydrographic measurements as navigable spatial information.
Hydrographic survey: The systematic measurement of water depths and underwater features for charting and other purposes. Surveys provide much of the positional and depth data used to compile charts.
Harbor chart: A large-scale nautical chart showing detailed information for a harbor and its approaches. Large scale helps pilots and crews assess confined waters and port features.
Electronic chart: A digital chart displayed or processed electronically, not necessarily meeting official navigational standards. The broader category includes products that are not equivalent to official chart data.
Mercator projection: A cylindrical map projection that represents constant bearings as straight lines but increasingly enlarges high latitudes. Its straight rhumb lines made it useful for plotting compass courses at sea.
Sounding: A measurement of water depth at a particular location. Soundings supply the depth values printed across a chart.
Multibeam echosounder: An sonar instrument that measures water depth across a swath beneath a vessel. It can map broad seabed areas more densely than single-point depth measurements.
Sailing directions: A nautical reference describing coasts, ports, hazards, and navigation conditions in prose. They explain coastal details that charts cannot convey adequately through symbols alone.
Web Mercator: A map projection widely used for web maps that distorts scale and area, especially at high latitudes. Its familiar appearance does not make it suitable as a substitute for navigational charting.
John Harrison: An English clockmaker whose marine timekeepers enabled more reliable determination of longitude at sea. Improved longitude measurement strengthened the positional basis of maritime mapping.

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