Navigation and Seamanship

1.4.6.1. Chart projection

A basic problem in chart making is to depict, a portion of the earth’s curved surface on a flat surface. The method used is projection or the transfer of points on the sphere (earth) to a plane surface (chart) by mathematical means. (The art and science of chart making is called cartography). The chart may show the entire world or only a small part of it. There are several hundreds of chart projections, each with some particular property that may make it desirable for some specific purpose. Of these, not more than about half a dozen have ever been of much use for navigation. The three main groups of projection are the conic, zenithal or Azimuthal and cylindrical.

The earth also being round ( a “spheroid”), it cannot be represented on a flat piece of paper without some distortion. The smaller the portion of the globe to be mapped, the less the distortion that will be present-conversely, the great the area, the greater the distortion.

For navigation, certain properties are desirable in a projection. Among them are :

1. True shape of physical features such as bodies of land or water;
2. True scale and size of various areas of land or water;
3. Correct angular as relationships;
4. Great circles as straight lines;
5. Rhumb lines as straight lines;

CYLINDRICAL PROJECTIONS- Mercator

One of the most famous map projections is the Mercator, created by a Flemish cartographer and geographer, Geradus Mercator in 1569. It became the standard map projection for nautical purposes because of its ability to represent lines of constant true direction. (Constant true direction means that the straight line connecting any two points on the map is the same direction that a compass would show.) The Mercator Projection always has the Equator as its Standard Parallel . Its construction is such that the lines of longitude and latitude are at right angles to each other – this means that a world map is always a rectangle

Advantages of a Mercators chart :

1) Rhump lines can be represented as straight lines.

2) Courses and bearings can be readily drawn and can be transferred from one part of the chart to the other without any loss of accuracy.

3) Distances can be readily measured as the scale of latitude is also the scale of distances.

4) All positions on the chart are correctly represented in their relative positions as they appear on the earth.