CHART. A chart is a marine map intended specially for the use of the seaman, to assist him to navigate seas and oceans, to sail from port to port and by its means to ascertain the position of a ship with reference to the land, the direction in which to steer, the distance to sail and the dangers to avoid. The water area on charts is studded with numerous small figures; these are the soundings, indicating in fathoms or in feet or in a combination of the two (as shown in the title of the chart) at mean low water spring tides the depth of water at any particular position. Charts show the nature of the sea bottom, the irregularity in its charac ter and give information of the greatest importance to the mariner. No matter how well the land may be surveyed a chart is prac tically valueless unless soundings are shown.
The British admiralty charts are compiled, drawn and pub lished by the hydrographic department. This department as es tablished under Earl Spencer by an Order in Council in 1795 con sisted of the hydrographer, one assistant and one draughtsman. The first hydrographer was Alexander Dalrymple, a gentleman in the East India Company's service; on his supersession in 18o8 the office of hydrographer was filled by Captain Thomas Hurd, R.N., and has since been held by officers of the Royal Navy, amongst whom may be mentioned Admirals Sir Edward Parry, Sir Francis Beaufort and Sir William Wharton. At the present time, 1928, the department consists of the hydrographer, 15 naval assistants, 23 chief cartographers, and cartographers, 57 draughts men and 38 clerical staff, apart from the numerous engravers, printers, etc., who are employed in the final stages of preparing the work for publication.
Charts prepared by the hydrographic department and published by order of the Lords Commissioners of the Admiralty are com piled from the labours of British naval officers employed in the surveying service, also from surveys made by the Royal Australian Navy, Royal Indian Marine, South African Naval Service and contributions from officers of the Royal navy and mercantile marine. Generally speaking each maritime nation is considered to be responsible for the charting of its own coastal waters and those of its dependencies ; the majority of these have efficient and well-organized hydrographic offices, carry out their own surveys and publish charts for the use of seamen. There is a free inter change of hydrographic information as between nations and foreign government charts are utilized and incorporated in various charts and publications produced by the British Admiralty, acknowl edgment of the source of the information being made in the title of the chart. Admiralty charts, which are published with the view of meeting the wants of the seaman in all parts of the world, may be classed under five heads, viz., ocean, general, coastal, harbour and physical charts ; they are constructed on either mercator or gnomonic projection according to scale and locality. After prepara tion at the hydrographic department they are engraved on copper plates which thereafter become the original printing medium and which are in constant correction due to the absolute necessity that the latest information should be available.
The depth of the sea is obtained by sounding line, wire or by sonic methods; all soundings are reduced to mean low water spring tides. The times and heights of the tides with the direction and the velocity of the tidal streams are also ascertained. The original surveys drawn by the surveyors afloat are forwarded to the Admiralty and form the basis of the published charts. The ocean and general charts are compiled and drawn at the hydrographic department ; original documents, existing charts, latest surveys and maps have all to be consulted and their compilation requires experience and judgment, for the compiler has to decide what to omit, what to insert, and to arrange the necessary work in such a manner that full information is given to the seaman. A very slight error may lead to great disaster and every symbol on the chart must be delineated with great care. No pains are spared in the effort to lay before the public the labours of the hydrographic surveyors and explorers not only of England, but of all the mari time world, to reduce their various styles into a comprehensive system and to furnish the seaman with a guide of which he may take full advantage. Certain abbreviations are used on charts and are fully described in a special publication.
There are still enormous areas which are unsurveyed or not surveyed in sufficient detail for modern requirements. Charts of these localitiesAa're usually drawn in hairline so that the experienced seaman sees at a glance that caution is necessary. The charts issued to the public are correct at the time of publication and are kept corrected for newly reported dangers, changes in character or position of lights and buoys, recent publications of foreign governments, etc., this information being supplied gratis to the public in the Admiralty notices to mariners which are published weekly. Charts are supplemented by the "sailing directions" cov ering the whole of the navigable portion of the globe, tide tables, light lists, the Admiralty list of wireless time signals, the nautical almanac, distance tables, ocean passages of the world, in which is included information regarding winds and currents, etc.