Types of charts and some terminology

Charts are made with all kinds of scale, depending on how much detail is required for navigational safety.

Sailing charts are open sea charts. Land may be depicted as a rough contour and these charts are used far from land in ocean regions.

General charts cover coastal areas outside the immediate areas of reefs or shoals (where the water depth is very low and navigation more dangerous).

Coastal charts cover areas closer to the coast and give greater detail of outlying dangers.

Harbour charts are intended for use in approaching harbours and in mooring ships to berths, usually under pilotage. They have the greatest detail.

Soundings refer to the depth of the water at a location and is marked on the chart after hydrographic surveys. Depths which have been measured are indicated by the numbers shown on the chart. Depths on charts published in most parts of the world use metres. Depth contour lines show the shape of underwater relief. Coloured areas of the sea emphasise shallow water and dangerous underwater obstructions. Depths are measured from chart datum, which is related to the local sea level.

A compass rose is an easy means (for a navigator) of finding direction on the chart and also plotting a ship’s course, besides taking bearings (angles) of objects with a compass. Called because it looks like a rose petal opened up, it is graded from 0 degrees to 360 degrees, with 0 being North, 90 being E, 180 being S and 270 being W.

Routeing and Passage Planning charts are small scale (cover large areas) and give information on winds, currents, icebergs and general information which assists us in planning a voyage at sea. These charts are not directly used for navigation.

The largest scale chart available is used since it gives us the best detail of topography and landmarks. On this, a ship’s planned course is plotted (drawn with a pencil) by the navigator. As the ship proceeds on the passage, its position (latitude and longitude) is plotted on the course line to confirm that the plan is being followed, and to steer away from any dangers.

Map projection and bearings

The Mercator projection is almost universally used in nautical charts. There are however some exceptions for very large or small scales where projections such as the gnomonic projection may be used. Since the Mercator projection is conformal, that is, bearings in the chart are identical to the corresponding angles in nature, bearings may be measured from the chart to be used at sea or plotted on the chart from measurements taken at sea. Although allowances must be made for magnetic variation and magnetic deviation if accuracy is important. Use of this projection also has the advantage that straight lines drawn on the chart represent lines of constant bearing although in reality such lines are not straight but segments of a three dimensional “loxodromic” spiral known as a rhumb line.

A bearing is the angle between the line joining the two points of interest and the line from one of the points to the north, such as a ship’s course or a compass reading to a landmark. On nautical charts, the top of the chart is always true north, rather than magnetic north, towards which a magnetic compass points. Most charts include a compass rose depicting the variation between magnetic and true north.