In this age of easy to use and highly accurate electronic navigation, conventional paper nautical charts are still regularly used by mariners. Even in cases where electronic charts have replaced them for primary use, paper charts are a reliable backup should the system fail, providing the same navigational essentials at a variety of scales.
The nautical chart is an indispensable tool for navigation. A chart is a graphic representation of an area of the sea which might also include coastlines, estuaries and islands. All cruising leisure boats should carry up-to-date paper charts. Most also carry electronic charts which are viewed on chart plotters. There are also electronic nautical chart apps which can be downloaded onto tablets and smartphones.
Charts use colours to show land, tidal drying areas, shallow areas and deep water. Actual depths of specific points are shown as numbers. Lines or depth contours join points of equal depth. Charts also show the location of rocks and other hazards, using special symbols which identify the type of hazard.
Tip: Depths of water are normally given in metres, sometimes in feet. The units are clearly shown with the title information of each chart
Latitude and longitude
Horizontal and vertical lines go across a chart at regular intervals, forming a grid. The lines are measured in degrees and are read from scales on the edges of the chart.
- Lines running across in a horizontal direction are lines of latitude and are known as parallels of latitude. They indicate how far the line is north or south of the Equator, which is at 0º latitude.
- Lines running in a vertical direction are lines of longitude and are known as meridians of longitude. They indicate how far a line is east or west of the prime meridian (0º), which runs through Greenwich Royal Observatory in the UK.
- Latitude and longitude are measured in degrees (0º), minutes (‘) and tenths of a minute. There are 60’ in 1º.
- One minute (1’) of latitude is equal to 1 nautical mile.
Traditional navigation methods use basic tools including a Breton plotter and single-handed dividers. Navigators use soft pencils to draw position marks and lines on paper charts which can easily be rubbed out after a trip.
To pinpoint a position on a chart, dividers are used to measure the distance to the nearest horizontal and vertical grid lines from the position being measured.
The latitude (distance north or south of the Equator) is found by reading it off the scale on either the right or left edges of the chart. The longitude (distance west or east of the prime meridian) is read off the scale at top or bottom edges of the chart.
To measure the distance from one object or position to another on a chart, the latitude scale at the side of the chart is used. Navigators never use the longitude scale as meridians are not parallel. The distance between them gets less as they converge towards the north and south poles.
Tip: When a position is identified and written down, the latitude is given first, longitude second. For example the latitude and longitude of Port Solent, UK is 50° 50′ 34.7676” N and 1° 6′ 3.6072” W
Just like land maps, charts vary in scale considerably. They include:
- Small scale charts (eg 1:350000) which can cover a whole sea, used for planning long voyages.
- Medium scale charts (eg 1:25000) typically covering a stretch of coastline.
- Large scale charts (eg 1:6000) possibly a detailed representation of a harbour entrance.
- Small scale equals large area, large scale equals small area. The actual scale of each chart is shown with the title information.
- When navigating into harbour, it is best to use the largest scale chart available.