CHAPTER XI
Charts
A difficulty was encountered when the early cartographers attempted to represent the earth’s spherical surface on a flat sheet. It can not be done, of course, without distortion being introduced in some manner. There are various methods of taking care of this error and one is adopted for one certain purpose while another scheme is used in some other work. These methods of caring for the error or distortion are known as projections, the principal being the Mercator, the gnomonic and polyconic. The Mercator projection is almost universally used for navigational purposes; the gnomonic projection facilitates the use of great circle sailing, and the polyconic is used for surveying sheets.
The Mercator chart represents the earth as though it were a cylinder instead of a sphere.
If we take the skin of one-half an orange, and assume it to represent the northern hemisphere of the earth, an attempt to forcibly bring it flat upon a table will result in the tearing or stretching of the skin. It can, however, be brought flat to the table in a regular uniform way by cutting it in a saw tooth fashion from the stem (pole) to the edge (equator), as shown in the diagram.
Fig. 13.
The shaded portions represent actual earth’s surface and the blank parts show the error introduced by using this method. In this form it is useless as a chart, so the real parts are stretched or extended each way to the dotted lines making a complete chart. It is now, however, without a vestige of accuracy in representing the bodies of land and water as they really exist. The result would be that if a round island should be in the latitude of the top of the chart it would be stretched into an elongated island lying east and west giving a very erroneous inaccurate idea of it, as shown by the east and west shading. If, however, the island had been on the equator where no east and west stretching would have occurred the island would appear in its natural shape, but the farther north or south it lies just in proportion to the latitude will it be stretched in an east and west direction. Such a condition will not serve the purposes of navigation and it becomes necessary to extend the degrees of latitude, making them appear longer and longer as the equator is departed from. This stretches the elongated east and west island in a north and south direction and brings it back approximately to its actual shape of a round island. If there was a round island in 10° N. and a similar-sized similar-shaped island in 50° N. the Mercator chart would show the northern one to be almost twice as large owing to this artificial distortion. But its relative shape would remain practically correct. It will be seen that the latitude scale on the sides of the chart carries an increasing value towards the north—on a chart where a degree is about ¼´´ long at the equator it would be about ½´´ long in 60° N. or S.
A minute of latitude is equal to a mile on this scale, but it becomes necessary to use it in the latitude in which the measurement is taken. If a course runs N. 60° E. from latitude 30° N. to 40° N. and the distance is desired, take at the middle latitude at the side of the chart a convenient multiple of distance, say 30´, on the dividers and step off the distance. Or the whole course can be taken off at once and with the points of the dividers at equal distances north and south of the middle latitude read off the number of minutes of latitude lying between them.
In very high latitudes the Mercator chart is not reliable. The distortion becomes excessive and bearings taken will not plot correctly.
All the meridians on a Mercator chart are parallel and cut the equator at right angles. They all lie in a true north and south direction. The parallels of latitude all lie east and west and are parallel to each other and at right angles to the meridians. The degrees of longitude on the globe grow smaller and smaller as the pole is approached due to the actual convergence of the meridians, but as all meridians are parallel on the Mercator chart the length of a degree must be shown the same length at the top as well as at the bottom of the chart. In just the proportion that the degrees of longitude have been lengthened artificially beyond their true length must the degrees of latitude be lengthened in each latitude. This amount is shown in Table 3, Bowditch, reckoned as the distance in miles each parallel is from the equator by the Mercator projection. Thus in latitude 40° N. the distance is 2400´ or miles, the table shows that in the construction of a Mercator chart this parallel should be increased artificially to 2607.6. These are called the meridional parts.
On a Mercator chart the ship’s course is represented by a straight line and cuts each meridian at the same angle and is called a rhumb line. For all practical purposes on short runs this rhumb line is the best to use, but it is not the shortest distance between two points. Should you be able in a course a thousand miles long, to see your port of destination your rhumb line course at the outset would not head your ship for it, but (in northern latitude) to the southward of it. However, as you proceeded the ship’s head would gradually draw towards the port and you would eventually arrive. What appears to be a straight line on this chart is really a curve on the sphere of the earth. Your line of actual vision is a great circle, and in order to follow such a bee line you must constantly change your compass course (on a long run) and describe a curve on the Mercator chart unless the ship is headed north or south or east or west along the equator, in which cases she is sailing on a great circle. The well-known Hydrographic Office Pilot Charts are on the Mercator projection and show all steamship tracks as curves, for they are great circles.
The gnomonic chart is based on a projection of the earth’s surface upon a plane tangent to any chosen point which is to be the center of the chart. The eye is assumed to be at the center of the earth looking outward to the point of tangency. It will be seen that the surface of the earth adjacent to the point of tangency will be very accurately shown on the chart, but becomes distorted gradually from the center, the sides of which show the land in such an unnatural shape that it is hardly recognizable.
With a chart on this projection great circle sailing is much simplified. The straight line between two points indicates the great circle to follow, and the course and distance is obtained by following the directions and illustrated example given on each chart. They are constructed for the different oceans and are for sale by the Hydrographic Office.
The course can be transferred to a Mercator chart by taking successive positions from the gnomonic chart and plotting them according to latitude and longitude, and joining by straight or curved lines.
In setting out on a voyage the port of destination could it be seen ahead would indicate the great circle course, and in order to continue to head directly for it, the course must be continually changed. While in the North Atlantic bound for Europe the course must be changed constantly to the east (right) in order to remain on the great circle—the straight and shortest distance.
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.
In approaching a strange harbor the chart should be scrutinized for prominent marks, and these identified as soon as possible, then the lesser objects can be picked up by their relative positions with the already identified landmarks. Among the lesser marks may be found cliffs, boulders, sandy beaches, vegetation, buildings (particularly church spires and houses with cupolas). Prominent elevations of land always serve to identify a locality. The chart shows these elevations clearly by contour lines.
A twenty-foot contour line, for instance, shows the line of the cut, should the hill be sawed off twenty feet above the sea. When contour lines are wide apart the land has a gradual slope, and as the grade becomes steeper the contour lines come closer proportionately.
The chart, wherever possible, represents the earth as seen from overhead, but in the case of vertical objects, they are of necessity shown horizontally. One of the notable cases of this is in the representation of cliffs, which in order to show their height are drawn with the side view as seen from the water.
Numbers seen on the land show the height above the high water.
From the topographical features of a chart we turn to those of hydrography. All the depths indicated on a chart are those existing at mean low water, on the Atlantic Coast, and mean lower low water on the Pacific Coast.
All the British Admiralty and most of the Hydrographic Office charts are reduced to the level of low water at ordinary spring tides. While there is usually more water to be expected than shown, it must be remembered that when the plane of mean low water is used, the low waters that, roughly speaking, come between the moon’s first and last quarters, will fall below the soundings on the charts.
The effect of an abnormal barometer and high winds must at times be borne in mind, for a continuous northwest wind will make a vital difference in the depths along the Atlantic seaboard, especially in the Delaware and Chesapeake Bays.
Very often there is information that can not be symbolized on the chart and is placed in italics in the form of a note. These are always important and should be read carefully.
Sailing directions are written to supplement the charts and preserve for the mariner a mass of information which otherwise would not reach him.
It is always well to pay attention to the current arrows, as they are a means by which the strength and direction of the tidal stream may be ascertained. The symbol of a tide rip should not be ignored by one in a small boat, as the conditions might be right to make them dangerous.
On the water areas of the chart we find contours of depths as on the land are contours of height. It is a good scheme, if one wanted to take the trouble to run a contour line indicating a depth a few feet greater than the draft of his boat, and tint the shallow water with a brush.
Large vessels of deep draft upon approaching the coast are guided by the ten-fathom curve unless the shore is very steep-to and the water very deep. The masters of such vessels would remain outside that curve until their position was well established. Lighter draft vessels are guided by the five-fathom curve in a similar manner.
When approaching the land and the landmarks are not available, the character of the bottom further assists the mariner as he sounds slowly towards the land. The kind of bottom is indicated by abbreviations which are obvious on almost every portion of the charts.
The three-fathom curve is the most important to the greatest number of navigators and for this reason is made the most pronounced. In the majority of charts it is shown by a “sanded” area within it, but in many new charts it is heavily tinted.
In changing from one chart to another while working in an unfamiliar locality, take especial note whether the soundings are in feet or fathoms.
It is an excellent practice when a vessel is brought to anchor and cross bearings taken, to estimate the radius of her swinging circle by adding the amount of chain out to the length of the vessel and with this describe a circle on the chart and note if there is any danger of tailing into shoal water at any quarter.
This has been a long voyage and I am glad to tie up and let the printers take charge. If any of my readers see places where I have stood into the shallow water of inaccuracy, I will be grateful for a passing hail that I may shift helm and get out with as little damage as possible.