ssed under five heads, viz.
ocean, general, and coast charts, harbour plans and physical charts; for
instance, the Indian Ocean, the Mediterranean, approaches to Plymouth,
Plymouth Sound and wind and current charts. The harbour plans and coast
sheets are constructed on the simple principles of plane trigonometry by
the surveying officers. (See SURVEYING: _Nautical_.) That important
feature, the depth of the sea, is obtained by the ordinary sounding line
or wire; all soundings are reduced to low water of ordinary spring
tides. The times and heights of the tides, with the direction and
velocity of the tidal streams, are also ascertained. These MS. charts
are forwarded to the admiralty, and form the foundation of the
hydrography of the world. The ocean and general charts are compiled and
drawn at the hydrographic office, and as originals, existing charts,
latest surveys and maps, have to be consulted, their compilation
requires considerable experience and is a painstaking work, for the
compiler has to decide what to omit, what to insert, and to arrange the
necessary names in such a manner that while full information is given,
the features of the coast are not interfered with. As a very slight
error in the position of a light or buoy, dot, cross or figure, might
lead to grave disaster, every symbol on the admiralty chart has been
delineated with great care and consideration, and no pains are spared in
the effort to lay before the public the labours of the nautical
surveyors and explorers not only of England, but of the maritime world;
reducing their various styles into a comprehensive system furnishing the
intelligent seaman with an intelligible guide, which common industry
will soon enable him to appreciate and take full advantage of.

As certain abbreviations are used in the charts, attention is called to
the "signs and abbreviations adopted in the charts published by the
admiralty." Certain parts of the world are still unsurveyed, or not
surveyed in sufficient detail for the requirements that steamships now
demand. Charts of these localities are therefore drawn in a light
hair-line and unfinished manner, so that the experienced seaman sees at
a glance that less trust is to be reposed upon charts drawn in this
manner. The charts given to the public are only correct up to the time
of their actual publication. They have to be kept up to date. Recent
publications by foreign governments, newly reported dangers, changes in
cha