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 harbor improvements and various engineering works, for defensive works and other military uses, for the fishing interests, and for general information as to the coastal regions. Charts will furnish much of interest and instruction to the traveler by sea and the dweller near the coast, who will learn to read them. Passenger steamers should more often for the interest of their patrons display charts of the waters traversed. No written or verbal description can give as clear an idea of geographical features and relations as a good map or chart.
As the charts are revised from time to time, a comparison of editions at different dates furnishes a record of the changes wrought by nature or man, and this is especially useful in studying the action in many harbor and river entrances, as well as for historical purposes.
Requirements for charts. As charts are maps of the water areas, including the adjoining land, and intended primarily for the use of mariners, they differ in important respects from topographic maps or general maps, even such as include the water areas. The main requirements for charts are these; correct and complete information, early publication of new data, clear and intelligible representation of the information, convenient arrangement as navigational instruments, and high standard of publication.
The special and sometimes difficult conditions under which charts must be used on shipboard call for good judgment throughout their preparation. Even the paper on which they are printed is of importance, in order that they may be sufficiently durable and suitable for plotting.
Information given on charts. It is evident that it is impossible to represent on a chart of any practicable scale all the features that exist on the corresponding area of the earth's surface. It is essential, therefore, that a selection be made of the classes of facts that are to be shown, as well as of the detail that is to be used for each class. The practical utility of the chart depends largely on the good judgment used in this selection. In the information shown, charts differ from maps principally in representing by soundings and curves the configuration of the bottom of the water area, and in showing ordinarily the topographic features only in the vicinity of the coast line.
The convenience of mariners should govern in the selection and arrangement of the information to be shown on charts, though they may be made useful for other purposes so long as this convenience is not lessened. The needs and preferences of navigators alone, however, differ so much that a reasonable chart must be somewhat of a compromise between conflicting views. For certain classes of navigation a boldly drawn chart showing only the dangers and a few other soundings and some landmarks might be useful. For other maritime purposes a more detailed chart would be valuable. The first, however, would fail to give facts often demanded in the navigational use of the chart, and the second if carried to an extreme would make a chart difficult to use.
FIG. 5. STATE OF ADVANCEMENT OF HYDROGRAPHIC SURVEYS OF THE COASTS OF THE WORLD, 1904. By G. W. Littlehales.
Shoals and dangers are shown either by the least depth or by rock or reef symbols. The characteristic soundings are shown on the chart, with abbreviations indicating the nature of the bottom. Depth curves are drawn, joining together points of like depth, and inclosing areas of less depth, on the same principle that contours are used on land maps; usually also the shoaler spots are made more prominent by sanding or tinting the area within them. Lighthouses, buoys, and other artificial aids to navigation are represented, with descriptive abbreviations. The coast is shown by a bold solid line for high water and a dotted line for low water. The main topographic features are represented for a moderate distance from the coast, with such detail as is useful, depending on the scale of the chart. Elevations are given in figures for prominent summits, islands, and rocks; the general configuration of hills and mountains is represented by contours on large scale charts or by hachures or shading on small scale charts. Rivers, streams, lakes, marshes, towns, roads, prominent buildings, and other important topographic features are shown by appropriate symbols. It is important that objects which may be useful in navigation as landmarks, whether natural or artificial, be plainly shown and described, if necessary to their identification, and that they should not be obscured by details of lesser importance. On the larger scale charts only, vegetation features, particularly areas covered by trees, are represented by symbols. The land area is usually clearly distinguished from the water area by a tint or stipple. Latitude and longitude are given by the projection lines and the subdivided border, or sometimes on harbor plans by a note giving the position of some one point. Brief information as to the time and range of the tides is stated in a note. Data regarding currents, whether due to tidal or other causes, are given by current arrows placed on the chart, or by explanatory notes. Compasses are for convenience printed on the charts, and data given as to the magnetic variation and its rate of change. On large scale charts scales are provided for use in measuring distances. Ranges and channel lines are given when required. The ports are indicated where storm warning signals are displayed. The areas of forbidden anchorages are shown, and when important, the positions of submarine cables. The lines dividing the high seas from inland waters are sometimes stated on United States charts. Life saving stations are given, and time balls are usually noted. Views of important features are shown on some charts.