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.