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.