Admiralty chart
Encyclopedia
Admiralty charts are nautical chart
s 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 diamond
s"), 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
.
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, but there is a well known aphorism
that - "rocks don't move" and mariners are notoriously bad at keeping charts current. This is actually a very dangerous practice and mariners who have an accident at sea may well find that their insurance is invalid if charts have not been kept up to date.
Nautical chart
A nautical chart is a graphic representation of a maritime area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land , natural features of the seabed, details of the coastline, navigational hazards, locations of natural and man-made aids...
s issued by the United Kingdom Hydrographic Office
United Kingdom Hydrographic Office
The United Kingdom Hydrographic Office is an organisation within the UK government responsible for providing navigational and other hydrographic information for national, civil and defence requirements...
and subject to Crown Copyright
Crown copyright
Crown copyright is a form of copyright claim used by the governments of a number of Commonwealth realms. It provides special copyright rules for the Crown .- Australia :...
. 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
Chart datum
A chart datum is the level of water that charted depths displayed on a nautical chart are measured from. A chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide...
), coastline, bouyage, land and underwater contour lines, seabed composition (for anchoring), hazards, tidal information ("tidal diamond
Tidal diamond
Tidal diamonds are symbols on British admiralty charts that indicate the direction and speed of tidal streams.The symbols consist of a letter of the Roman alphabet in a rhombus, printed in purple ink...
s"), 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
Mercator projection
The Mercator projection is a cylindrical map projection presented by the Belgian geographer and cartographer Gerardus Mercator, in 1569. It became the standard map projection for nautical purposes because of its ability to represent lines of constant course, known as rhumb lines or loxodromes, as...
so that bearings can be transferred to (or taken from) the charts directly, although allowances must be made for magnetic variation and magnetic deviation
Magnetic deviation
Magnetic deviation is the error induced in a compass by local magnetic fields, which must be allowed for, along with magnetic declination, if accurate bearings are to be calculated....
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
Spiral
In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...
known as a rhumb line
Rhumb line
In navigation, a rhumb line is a line crossing all meridians of longitude at the same angle, i.e. a path derived from a defined initial bearing...
.
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
International Convention for the Safety of Life at Sea
The International Convention for the Safety of Life at Sea is an international maritime safety treaty. The SOLAS Convention in its successive forms is generally regarded as the most important of all international treaties concerning the safety of merchant ships.- History :The first version of the...
; 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, but there is a well known aphorism
Aphorism
An aphorism is an original thought, spoken or written in a laconic and memorable form.The term was first used in the Aphorisms of Hippocrates...
that - "rocks don't move" and mariners are notoriously bad at keeping charts current. This is actually a very dangerous practice and mariners who have an accident at sea may well find that their insurance is invalid if charts have not been kept up to date.