Admiralty charts are nautical charts issued by the United Kingdom Hydrographic Office and subject to Crown Copyright. Over 3,000 charts are available and cover virtually the entire world in various levels of detail depending on the density of traffic and hazards. Large-scale charts often cover approaches and entrances to harbours, medium-scale charts cover heavily used coastal areas, and small-scale charts are for navigation in more open areas. There is also a Small Crafts Series available at even smaller scales.
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.
Passage charts at smaller scales 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.
Coastal charts and plans use a Transverse Mercator projection in order to more accurately depict the shape of the shoreline
The UK Hydrographic Office issues charts for various users. Standard charts are issued for mariners who are subject to the International Convention for the Safety of Life at Sea; folios of charts at a conventient A2 size are produced for leisure users and there are a range of digital products that complement the paper charts. However, electronic charts are not yet mandated as a legal replacement for paper charts.
The seabed and the features charted change regularly and it is vitally important that users have the most up to date information. The UK Hydrographic Office receives a vast amount of new information and sifts and filters this to identify that which is most important to its users. These updates are released in Admiralty Notices to Mariners weekly, 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 regularly to maintain accuracy and safety.