18]"/> additional revision. In the class of more thoroughly surveyed coasts should be included the Atlantic and most of the Pacific coast of the United States, Porto Rico, nearly all the coasts of Europe, Algeria, and portions of the coasts of Japan, the Philippine Islands, and India.

A large part of the world's coasts has been surveyed incompletely, but sufficiently well to permit the publication of navigational charts. This is the condition as respects most of southeastern Alaska and some other portions of the Alaskan coast, British Columbia, most of Mexico, Central America, the West Indies, Brazil and parts of Chile, the Hawaiian Islands, China, Malay Peninsula, Siam, the Dutch East Indies, Australia, New Zealand, Persia, Arabia, most of Africa, Iceland, northern Scandinavia, and Finland.

Another considerable portion of the coasts has not been surveyed, but has been covered by explorations which have been embodied in nautical charts of varied degrees of incompleteness. In this class are the north coast and considerable portions of the south and west coasts of Alaska, the Aleutian Islands, Siberia, most of the oceanic groups in the Pacific, the northern coasts of Europe and North America, Greenland, the west coast of South America, Venezuela, and Argentina.

Only a very small proportion of the total length of coasts is now entirely unexplored, and such portions are confined to the polar regions.

Chart publications of various nations. There are about eighteen nations publishing navigational charts, and adding to the information on which charts are based. Many of these nations republish to some extent the charts prepared by the others. Great Britain has kept up a series of charts covering all parts of the world and practically including in some form all information published elsewhere. This series now (1908) includes 3725 different charts, of which the annual issue is about 600,000 copies. France (1906) publishes 2948 different charts.

In the United States, charts are published by the Coast and Geodetic Survey for the coasts and tidal waters of the main country and the insular possessions, by the Hydrographic Office for oceanic areas and foreign coasts, and by the Lake Survey for the Great Lakes. The total number of different charts issued by these bureaus is about 2300, and the total annual issue is about 225,000 copies.

Systems in use on various charts.

Longitude. The first chart of New York, published by the Coast Survey in 1844, was referred to the City Hall of New York as the initial longitude, and some years ago it was the prevailing custom for each nation to use a local initial longitude. While this satisfied local pride it led to much geographical and navigational confusion. Happily the charts of all countries are now referred to Greenwich, with the following exceptions:

France refers to Paris, which is 2° 20´ 15´´ E. of Greenwich.
Spain refers to San Fernando, which is 6° 12´ 20´´ W. of Greenwich.
Portugal refers to Lisbon, which is 9° 08´ 24´´ W. of Greenwich.

Units for depths. The English fathom or foot is used for depths on the charts of Great Britain, the United States, and Japan. Russia uses the sajene of seven English feet. On the modern charts of practically all the other countries the meter is used, though on older charts various units are found.

In the first group feet are ordinarily found only on large scale or local charts of areas with moderate depths, and the other charts are in fathoms, except that on the earlier charts of the Coast and Geodetic Survey feet were used on a sanded surface inside of the three-fathom curve and fathoms on the white surface outside of that curve. Heights are stated in feet on the charts of the first group.

Plane of reference. As the depth of water varies with the tide, it is necessary for charting purposes to adopt some standard plane to which the soundings are referred. Practically all countries have adopted for this purpose a low stage of the tide, as this is obviously on the side of safety; in most cases an extreme low water is used, so that the actual depths will seldom, owing to the tide, be less than those shown on the chart. The definite reference planes used on the American charts will be mentioned later.

On nearly all charts heights are referred to mean high water, doubtless owing to this being the visible limit of the land at high tide. On topographic maps of the interior, the heights are referred to mean sea level, which plane is of course lower than the preceding by one-half the range of tide.

Symbols on charts. Fair uniformity as to general principles, with differences as to details in carrying them out, exists on the various series of charts regarding their general arrangement and the more important symbols, such as in the shading of land to distinguish from water, the use of depth curves, the representation of hills by shade or contour, the indication of shoals and dangers, and of lighthouses and buoys.

Desirability of uniformity in charts. Ships engaged in international commerce must enter foreign ports. As the information is constantly changing and charts are being corrected or improved, it is sometimes desirable for the navigator to consult the local foreign charts, and it may often be necessary for him to carry in his chart room the charts of several different countries. There are therefore important advantages in international uniformity in chart publication.

There should be a common initial longitude, and as the longitude of Greenwich has been so extensively adopted, it appears quite probable that its use may some day become universal.

A common unit for soundings and heights would be very desirable, but the fact that a large group of nations has united on the metric system, while a small group with great commercial interests retains another system, makes the attainment of uniformity difficult.

Substantial agreement as to the use of symbols on charts, particularly such as represent aids or dangers to navigation, would be desirable and doubtless feasible.

Privately published charts. Many of the earlier charts were prepared and published by private enterprise, and such charts are still published, as, for instance, the so-called "blue-back" charts printed in London. These charts have usually differed from those published by the various governments either in representing the main features in a very bold manner with little detail or in including a considerable area with many plans on a single large sheet backed for permanency. An objection to the latter is that the durability together with the high price tends to keep an old chart in use long after it is out of date. It would be financially difficult for a private firm to give the service that a government does in the matter of correcting the charts and issuing new editions, and this is an important consideration in the selection of charts.

Purpose of charts. The main purpose of charts is to furnish graphical guides to aid in taking a vessel safely from one port to another; they are maps for the use of navigators. An experienced mariner may be able to steer his vessel over a familiar course without charts, but this does not make their publication less necessary. Even such an expert pilot doubtless studied the charts in the first place; the uncertainties of the sea and the changes of information are such that his vessel's equipment should include the latest charts, and safety requires their examination. The passengers and the merchants who intrust their lives or their goods to the sea are largely dependent upon the correctness of the charts.

Besides their main purpose charts fill many other needs, among which are; for preliminary planning of