Reading charts. A chart is a representation on paper of hydrographic and topographic information by means of various conventional methods and symbols. It is evidently important for those making use of charts to understand the system and conventions used, and to be able to interpret readily the various parts of the chart. The ability to read a chart must include an understanding of all its features, such as scale, projection, geographic position, directions, depths, plane of reference, aids to navigation, tides, currents, elevations, topography, and date of survey and publication.

Scale. For American and British charts the scale is usually expressed by the inches or fractions of an inch to the minute or degree of latitude, or by the fractional proportion of a distance on the map to the corresponding distance on the earth. These fractions are sometimes stated on the British charts, and nearly always on those of the United States Coast Survey. The chart catalogues give the scale in one or the other form. A familiarity with the meaning of scales is of value in selecting the most suitable chart, in judging of the relative uses of charts, and in estimating distances. Where the fractional scales are stated they furnish a simple means of comparing charts, as, for instance, a chart on 150000 scale will show all distances just twice as long as a chart on 1100000 scale.

The following are scale equivalents:

For use in measuring distances on large scale charts the length of one or more nautical miles is usually drawn on the chart, and sometimes scales are also given in other units. On British charts the nautical mile scale is divided into tenths (that is, cables of 100 fathoms or 600 feet length); on the American charts into quarters and eighths. Where the scale covers more than one mile the fractional divisions are shown only for the left-hand mile and the zero of the scale is placed between this and the full mile scale, so that with dividers the full miles and fraction may readily be taken off. The nautical mile in the United States is taken to be the length of a minute of arc of a great circle on a sphere whose surface equals that of the earth; this definition makes the nautical mile equal 6080.27 feet. Lecky adopts 6080 feet as the nautical mile. The length of the actual minute of latitude on the earth's surface increases from 6046 feet at the equator to 6108 feet at the poles, an increase of about one per cent. It is, however, this somewhat variable unit of length which is ordinarily used in scaling distances on the sailing charts.

On small scale charts there is usually a border scale entirely around the chart, conveniently subdivided; this serves the double purpose of facilitating the plotting or reading of positions by latitude and longitude and of furnishing a scale of minutes of latitude for use in measuring distances. On a mercator chart this scale of course varies with the latitude and it must be referred to in the mean latitude of the distance to be measured. In general practice the minute of latitude is taken as equal to the nautical mile.

Projection. On only a few charts is there a statement of the projection used. Practically all general sailing charts are on the mercator projection, which can be readily recognized by the rectangular network of meridians and parallels and the increase with latitude of the distance between the parallels. On large scale local and harbor charts the kind of projection used is not of importance to navigation, as for such limited areas the difference between projections would not affect the use of the chart. On certain small scale charts of the United States Coast Survey which are on the polyconic projection this fact is stated on the chart, and can also be readily recognized by the convergence of the meridians and curvature of the parallels. Gnomonic charts intended for taking off great circle courses are always described in their titles and are also easily recognized by the increased scale and distortion toward all the borders. Charts of the polar regions are published on several different projections, which are distinguished from the mercator by their circular or curved parallels.

Geographic position. For large scale and harbor charts the latitude and longitude of some point marked on the chart are sometimes stated on the face of the chart. For others of these, however, and for smaller scale and general charts, positions are obtained by reference to the border scale. There is a latitude scale down either side of the chart, and a longitude scale across the top and bottom. These scales are conveniently subdivided into degrees, minutes, or fractions of a minute. The minute is divided into tenths (6´´), sixths (10´´), quarters (15´´), or halves (30´´) on various charts.

Directions are indicated on charts both by the projection lines and by compass roses. Nearly all charts are now oriented with the meridian, that is, north is the top of the chart, and on a mercator chart the east and west border lines are parallel with the meridians and the north and south border lines with the parallels. Formerly many charts were not so oriented. Some of these are still in use and can readily be recognized by the diagonal or inclined direction of the projection lines with respect to the border of the chart. Of course directions must not be referred to the border lines of these diagonal charts, and scales along such border lines must not be used. Directions with respect to true north may always be referred to the projection lines of the chart, but on a polyconic or polar chart a direction must not be carried so far from any projection line as to introduce error on account of convergence of the meridians. Compass roses are placed on charts to facilitate the taking off or laying down of directions, though in some respects their use is less accurate and convenient than the use of protractors, referring to the projection lines. The British charts and many of those of the United States Coast Survey have only magnetic compasses, with degrees outside and points inside, the former graduated to 90°. These are engraved on the chart with the magnetic variation for the date of publication, or for a few years in advance, and give the annual change in the variation. Because of expense of engraving they can be changed on the charts only at intervals of some years, and until this is done allowance for the change in variation is to be made if important. The German charts and those of the United States Hydrographic Office now have a threefold compass, the outer one degrees true, the middle degrees magnetic and the inner points magnetic; the degrees in both cases are graduated to 360°, reading from north through east, south, and west; thus northwest would be stated as 315° instead of N. 45° W. Small scale charts covering extensive areas have no magnetic compasses. They sometimes have true compasses, and usually have the isogonic lines, or lines of equal magnetic variation, marked on them, from which the variation at any intermediate point can be estimated.

Depths. The unit used for depths is always stated plainly on the chart, and it is important to note this carefully, as the British, American, and Japanese charts use fathoms for some charts and feet for others, and most other countries use meters. Some of the earlier charts of the United States coast have the depths inside of the 18-foot curve in feet and outside of that curve in fathoms.

Depth curves are shown on charts in order to bring clearly to the eye the different depth areas and the limits for navigation of vessels of various drafts. The shoaler areas are usually indicated by sanding the outer limit or the entire area within the depth curve. For the curves of greater depths various standard symbols are used which vary slightly in the different series but which may readily be recognized by the soundings on either side of them. On the British charts the 1 and 3 fathom curves are usually indicated by sanding the outer edge of the areas of these depths respectively; beyond these the standard curves shown on these charts are the 5, 10, 20, and 100 fathom curves. Similar curves are used on the United States charts. The German charts show the 2, 4, 6, 10, and 20 meter and various deeper curves, and the French the 2, 5, 10, and 20 meter and deeper curves. On the United States Lake Survey charts the areas included within the 6, 12, and 18 foot curves are shaded with a blue tint, heavy along the outer edge, which brings out strongly the shoal areas.

Depth curves if clearly shown are a great aid in interpreting the hydrography and making plain the shoals and passages. The system of curves should always be understood when using a chart, and it may sometimes aid the navigator to trace out with a pencil an additional curve, if needed, beyond the draft of his vessel. The abbreviations used for the bottom characteristics are explained either on the chart or on the sheet of chart symbols, and give information which is useful in anchoring, and may be helpful in identifying a position by soundings. When a sounding is made without the lead reaching bottom, the depth obtained is sometimes shown on the chart by a short line and zero above the figure, indicating that at the depth stated, bottom was not obtained (no bottom). There are a few important symbols shown in the water area of charts. The sunken rock symbol indicates a dangerous area, or a danger having a moderate depth of water over it, or a rock the least water over which is not known; ordinarily on the United States charts the least depth will be stated when known, and the symbol omitted. The rock awash symbol indicates a rock awash at some stage of the tide, unless more definitely stated. The position of a wreck is indicated by a special symbol. P. D. (position doubtful) and E. D. (existence doubtful) are placed after soundings or rocks or other features which depend on some doubtful report not yet verified.

The following are the relations between depth units found on various charts:

Aids to navigation. Each series of charts has a definite system of representing the aids to navigation; these are similar in principle but differ as to detail. The characteristics of the lights, light-vessels, buoys, and beacons are usually explained by abbreviations placed by the side of each, and the entire system of representation is given on the explanatory sheet for the charts. Various methods of coloring lights and sectors and buoys are in use on different charts. It is evidently of importance that the user of the chart should readily understand the significance of the navigational aids as shown. For details regarding lights it is of course desirable to refer to the light lists; for the coasts of the United States detailed buoy lists are also published. Range and channel lines when shown are represented by distinctive symbols with bearings indicated. Danger ranges for the avoidance of shoals are sometimes shown. On the British charts bearings as stated on range and channel lines are magnetic; the custom varies on other charts and must be carefully noted in each case.

Plane of reference. The soundings given on the chart express the depth of water when the tide is at the height adopted for the plane of reference; this same plane is used in the tide tables, which thus will indicate the amount to be added to the soundings when the tide is above the plane, or to be subtracted when it is below. In order to be on the safe side the plane of reference adopted is always some low stage of the tide, so that there is usually more water than shown on the chart.

On the British and German charts the soundings are reduced to the mean low water of ordinary spring tides, unless otherwise stated. On the charts of the Coast and Geodetic Survey the following are the planes of reference: for the Atlantic and Gulf coasts, the mean of the low waters; for the Pacific coast, Alaska, and the Philippines, the mean of the lower low waters, except for Puget Sound and Wrangell Narrows, where planes two and three feet lower respectively have been adopted. According to the Tide Tables for 1908, at New York (Sandy Hook) the tide will fall below the plane of reference on 135 days during the year, but the extreme low tide will be only one foot below the plane. At Portland, Maine, in 1908, the extreme low water is 2.1 feet below the plane, and at San Francisco 1.5 feet. Of course when the tide is below the plane of reference the amount must be subtracted from the depths shown on the chart.

Strong winds and unusual barometric pressure may have a marked effect on the height of tide, so that it may differ appreciably from the predicted height, which is of course based on normal conditions. At Baltimore and at Willets Point observation shows that a heavy wind may reduce the tide four feet below the predicted heights.

Tides. Information regarding tides is given on all large scale charts, and additional information and predictions may be found in the Tide Tables. On the charts of the United States coast there is a small tide table giving for the high and low waters the time relations to the moon's transit and the height relations to the plane of reference. On the British charts there is a brief statement as to the tides either at the port on the chart or in the general notes; this ordinarily gives the interval in hours and minutes between the moon's meridian passage and the time of high water for the periods of full and new moon, and also the amount in feet that the spring and neap tides rise above the plane of reference, and the range of the neap tide. The following is an example of such a tide note: "H. W. F. and C. Campbellton IV^h 0^m. Springs rise 10 feet, Neaps 7 feet."

At some important ports information as to the state of the tide is given to vessels, either by means of signal balls, or by automatic tidal indicators, as at the Narrows in New York Harbor, where a large dial shows to passing vessels the height of the tide, and an arrow indicates whether it is rising or falling.

The tidal information becomes important and must be considered in navigation or in anchoring in waters where the available depth at low water approximates the draft of the vessel. In the general use of coast charts it is also important to observe the effect of the stage of tide on the appearance of many features. Rocks rising some feet above low water may be entirely submerged at high water. In some areas the aspect may be radically changed between high and low water by the baring of extensive shoals or reefs.

Currents. Information, when available, as to currents is given either by a note or by current arrows placed on the chart at the position of observation. Additional information as to certain regions is given in the United States Tide Tables. Tidal currents, flood and ebb, and currents not due to tidal action are distinguished by symbols, and the velocity is given in knots, and on some charts is indicated by the lengths of the arrows.

Complete and systematic current observations have been made in comparatively few localities because of the time and expense necessary to get the full information as to the variations of the currents with the tides and seasons. Ordinarily therefore the current arrows shown on charts indicate only the average direction and velocity, or possibly only the conditions existing at the season when the survey was made. Oceanic and coast currents are probably much less uniform than might be inferred from the current streams drawn on maps and charts. A more systematic investigation of ocean currents is required to fulfill the needs of navigation.

The tidal currents seldom turn with the tides, and there may be an interval of as much as three hours between the time of high tide or low tide and slack water. This leads to the apparent anomaly that in cases the current may be running with its greatest velocity at the time of high or low water, and may be running into a channel for several hours after the tide commences to fall. It is therefore, evidently, not safe to draw inferences as to currents solely from the tidal heights.

There are passages where the tidal currents become of the greatest importance to navigation, as, for instance, in Seymour Narrows on the inside route to Alaska, where the current velocity reaches 12 knots and the interval of apparent slack water lasts but a few minutes.

Elevations. The unit used for elevations is also stated on the face of the chart, as also the plane to which elevations are referred. On the United States charts this is generally mean high water and on British charts the high water of ordinary spring tides. Rocks and islets usually have figures shown beside them, either in brackets or underscored, which indicate the height above high water. Rocks which are bare at low water sometimes have a note "dries" or "bares" so many feet, indicating their height above low tide, although they are covered at high tide. The British charts in some regions where there is a large range of tide have underlined figures in the area between high water and low water indicating the heights above low water, or the depths of water over the bank at high water, as explained in each case.

Topography. The land area on most charts is distinguished from the water area by a stipple or tint; on some charts the topographic features have, however, been depended upon to bring out the land from the water. The solid shore line is the high-water line, and should be clear on the chart; the area between high and low water is sanded or otherwise shaded on all charts. The relief of the land is represented by hill shading or by contour lines which are the successive curves of elevation on the land. Topographic symbols are used for some of the more important features, such as cliffs, rocky ledges, buildings, bridges, trees, roads, etc. It is important for the navigator to understand the significance of the hill representation and the symbols, as they will aid him in recognizing a coast or island, and in identifying landmarks.

Date of survey and publication. There is usually an authority note on each chart showing the source of information or date of survey; if on a coast subject to change, the latter is important. On the United States Coast Survey charts the date of publication of the edition is given, and on British and other charts the date of both large and small corrections. The chart catalogues give the dates of the last editions, or the dates of extensive corrections, and this affords a means of seeing whether the copy of the chart in use is the latest edition available.