| Admiralty Charts |
Article Index for Admiralty |
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Information AboutAdmiralty Charts |
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Charts include: depths ( Chart Datum ), coastline, bouyage, land and underwater contour lines, seabed composition (for anchoring), hazards, tidal information (" Tidal Diamonds "), prominent land features (e.g. church towers), traffic separation schemes, RF direction finding information, lights, and in short anything which could assist navigation. Charts use the Mercator Projection so that bearings can be transferred to (or taken from) the charts directly, although allowances must be made for Magnetic Variation and Magnetic Deviation if accuracy is important. Use of this projection also has the advantage that straight lines drawn on the chart represent lines of constant bearing although in reality such lines are not straight but segments of a three dimensional " Loxodromic " Spiral known as a Rhumb Line . Corrections are released in Admiralty Notices to Mariners on a weekly basis, they are indexed by a serial number and the week and year of release, an annual summary is also released shortly before the start of the "sailing season". All charts "should" be updated on a regular basis to maintain accuracy but, there is a well known Aphorism that - "rocks don't move" and mariners are notoriously bad at keeping charts current. EXTERNAL LINK Admiralty charts Research Guide from The National Archives, UK |
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