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What is an Admiralty Chart?

Admiralty charts are hydrographic charts produced by the British Admiralty. Hydrographic charts are maps designed as aids to navigation at sea. Early charts included medieval portalans and manuscript pilot books. Later, printed charts and pilot books were published commercially. Not until 1795 was the Hydrographic Office established as a subdepartment of the Admiralty. The first chart officially published by the Admiralty was issued in November 1800. Examples of early printed and manuscript charts may be found in The National Archives, and are more fully described below.

Most British Admiralty charts delineate coastline, high and low water marks, and record depth of water as established from soundings. They record navigational hazards, such as reefs and wrecks, and navigational aids, such as lights, buoys and beacons. Most charts have a compass indicator, often an elaborate compass rose, and most have some indication of scale - either a scale bar or representative fraction or a border showing degrees of latitude and longitude.

Where direct indication of scale is absent, as on older charts, the scale may usually be determined from the appropriate published Catalogue of Admiralty Charts, Plans, and Sailing Instructions. Dates of survey and compilation are minutely recorded, as are those of the corrections continually made to maintain the accuracy and utility of the chart. It is important to note that, particularly in the early years of the Hydrographic Ofice published Admiralty charts drew on earlier surveys. In extreme cases this means that some charts may be based on surveys made more than a century earlier. For example, Admiralty chart 751, the chart of Maculla Bay which was listed in the first published catalogue of 1825, bears a survey date of 1703.

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