Worldwide, the system for recording your position at sea, or (normally) on the shore is latitude and longitude. These co-ordinates can be defined by a measurement from a vertical reference point and a horizontal reference point. This is very similar to the terrestrial grid reference system. http://www.satsig.net/lat_long.htm
In this case, the vertical reference point is the Equator. Lines of latitude are horizontal lines drawn parallel to the Equator. The lines are designated in degrees (0o to 90o) north or south of the Equator according to the angle that the line of latitude makes with the Equator at the centre of the earth. The equator is at 0o, and if you are near the north pole, you are at nearly 90o N. The island of Ireland, for example, lies between the lines of latitude 51o and 55o 30’ north of the Equator. http://en.wikipedia.org/wiki/Latitude
The horizontal reference point is the Greenwich meridian (near London). Lines of longitude are drawn vertically on the globe of the Earth and must pass through both the North and South poles (like slices of melon!). These lines are designated in degrees (Oo to 180o) east or west of the Greenwich meridian (Oo). This angle is the angle that the line of longitude makes with the Greenwich meridian at the centre of the Earth. The island of Ireland, for example, lies between the lines of longitude 5o and 11o west of Greenwich. http://en.wikipedia.org/wiki/Longitude
A position for a site is identified by giving its latitude followed by its longitude (its coordinates). For example, the Giant’s Causeway is 55o 14.4’N, 06o 30.6’W.
A chart is a map of a coastal area. As the Earth is almost a sphere, its surface is curved. Translating this curved surface onto flat maps and charts produces some distortion, but this is corrected as far as possible by the chart/map makers. This allows the lines of latitude and longitude to be displayed as almost equally spaced horizontal and vertical lines, like terrestrial ordnance survey maps.
Unlike a grid reference however, the first part of the coordinates, the latitude, is found by looking at the numbers running (increasing) up the left-hand, or right-hand side of the chart.
The second part of the coordinates, the longitude, is found by looking at the numbers running (increasing) from right to left along the bottom and top edges of the chart. (N.B. This direction is the opposite one to grid references).
To determine a latitude and longitude for your site (for example, Giant’s Causeway), measure its position on the vertical scale (latitude), then its position on the horizontal scale (longitude). It is usual to express each coordinate in degrees (latitude 55o), minutes and decimal minutes (14.4’) followed by whether it is north or south of the Equator (N) for latitude, or east or west of Greenwich (longitude 06o 30.6’W). Each degree is divided into 60 minutes. It is usual to subdivide minutes further into decimal fractions of a minute (14.4’N).