The course and distance can be computed by the form given in Bowditch, in which two sides and an included angle are given to find the other side and the (course) angle at the point of departure. The co-latitudes of the points of departure and destination and the angle between them at the pole, are respectively the sides and the included angle. However, the gnomonic chart gives the course and distance graphically.
When a chart is purchased or received from the Government offices the date of issue stamped upon it should be carefully noted. It can safely be taken for granted that the chart has been corrected up to that date and it is incumbent upon the navigator or master to seek in all Notices to Mariners subsequent to this date for any that [affect] the chart. If a Notice contains information requiring a correction the number of the chart appears in boldface type. The alterations should be made neatly with India waterproof ink, and if by the nature of the information it is impracticable to make the changes a note should be made in a conspicuous place.
Charts are printed from copper, zinc or aluminum plates and small changes easily made by hand are not changed on the plate until an accumulation of errata make it necessary, or sweeping changes of a more extensive nature takes place such as a new survey, dredged channels, etc. A chart under extensive correction is brought up to date in every particular, including the latest geographic spelling, new docks and public works. The date is noted on the right of the center margin and the dates of smaller hand corrections are indicated at the lower left corner; the figures denote the number of the weekly Notice to Mariners, in which the information is found, and the year.
The different scales of charts range from those of the world to a harbor plan. There are charts of oceans; general coasts, such as from the St. Lawrence to below New York; intermediate coasts, as from Eastport, Maine to Cape Ann; and approaches to ports, say from Cape Ann to Cape Cod for the port of Boston; and lastly there are harbor plans. Those covering large areas are known as small-scale charts while harbor charts are called large-scale charts.
A chart depends on the surveys that furnished its data, and its accuracy and reliability rests upon that survey. Even with the most careful surveys, where the lead is used to ascertain depths, there are many instances where pinnacle rocks have escaped detection by coming between the casts of the lead taken by the surveying party. These isolated rocks become points of great danger to vessels of deep draft, and it becomes a measure of safety to avoid rocky coasts and offshore patches by giving them a wide berth. Spaces devoid of soundings may well be viewed with suspicion if in reasonably shallow water, for it would appear to indicate a lack of thoroughness in the survey, at least a lack of soundings. The wire drag, a device used to sweep important areas to a certain depth, is the only sure way of discovering all the dangers of the bottom.
The aids to navigation shown on the charts are described by symbols and abbreviations as fully as possible with the limited space. All symbols are placed in the location of the aid, but in some cases the actual position may be in doubt by the nature of the symbol, for instance a buoy’s location is denoted by the ring that accompanies the symbol and not the triangle; a light vessel by the position of the dot of the light, or between them if there are two dots (lights). Buoys and light vessels often drag their moorings or go adrift entirely, especially in the winter season. It is therefore the part of wisdom to check a ship’s position by shore marks when possible and be prepared to find buoys out of position. The mechanism of a light buoy is often disarranged through various causes.
The characteristics of all lights are briefly given with the visibility and height above the sea. The charted visibility is the distance they should be seen from a vessel’s deck on which the height of the eye is fifteen feet above the sea, so, from the deck of an ordinary power boat a light will not be seen until well within the range of visibility as published, while from the deck of a large steamer the light will be seen outside its charted visibility. This refers to high-powered lights where the curvature of the earth has to be given consideration. A flashing light is one in which the flash is of less duration than the eclipse, while an occulting light has an eclipse equal to or less than the period of light. Flashes and eclipses are often grouped and receive the name of group flashing or group occulting. An alternating light is one in which two colors are shown each for an equal interval with no intervening eclipse, but if an eclipse separates the color flash from a white flash, for instance, it becomes a flashing white light varied by a red flash. It is a very common practice to insert sectors of different colors into the arc of visibility of a light in order to cover a dangerous shoal or to indicate a channel. Bearings defining these sectors are taken from seaward and not from the light. The term luminous range will be met with, and indicates the distance the power of the light can carry the visibility irrespective of an intervening horizon. A light may have a luminous range much in excess of its visibility which is limited by the horizon but in a haze or fog its penetrating power will greatly exceed that of a light of similar visibility but less luminous range. The power of a light is more commonly shown by units of a thousand candle power, thus, 5.6 indicates a power of fifty-six hundred candles. The catoptric (C.) light employs the reflecting, and the dioptric (D.) the refracting principle.
There is a large amount of useful information given on every chart that the average mariner allows to escape his notice. This failure on his part is mostly due to familiarity, or reliance on pilots with local knowledge.
The first important feature of a chart to be considered is the shoreline, which is shown as a continuous line representing the high-water mark. This, it must be borne in mind, is much changed at low water, and where the range of tide is large the shoreline is proportionately in error. Again, where the water is shallow the change is more marked than where the shores are steep-to. If account is not taken of the stage of the tide it is easy to be very much deceived.