It requires an understanding of only a few basic concepts to begin navigating such as the meaning of simple terms such as position, direction, distance, and depth to apply them to practical maritime navigation.
Any position on the earth can be described by its latitude and longitude which are the lines of an imaginary [ grid ] laid on the earth's surface. Grid lines running east to west and parallel to the equator are known as parallels of latitude with the equator itself being 0° latitude. Lines that run north to south, between the North and South Poles, are known as meridians of longitude.
The prime meridian or 0° longitude, runs through the Greenwich Royal Observatory in the United Kingdom from which it takes its name. Measured by the angle that they form at the [ centre ] of the earth , lines of latitude and longitude are known by degrees (°), minutes ('), and tenths of a minute there being 60' in 1°, and 360° in a circle.
The [ angle of latitude ] is measured from the centre of the earth along the prime meridian from the equator (0°), ranging from 0° to 90° north and to the south. The [ angle of longitude ] is measured at the centre of the earth along the equator from the prime or Greenwich meridian (0'), ranging from 0° to 180° east and to the west.
The latitude and longitude scales are printed at the edge of then nautical map with grid lines crossing them allowing positions to be measured and plotted. Positions are described, as latitude being the first in °N or °S, followed by longitude in °E or °W i.e., 53° 42'.7 N 5° 20.3 W.
When coastal sailing, it easier to give a position by referring to a fixed sea or landmark using direction and distance than using latitude and longitude. Direction is measured clockwise as an angle relative to north. Describing the direction of an object in relation to the [ boat's position ] , or between two objects, is done by taking a bearing the direction in which the boat is sailing, and known as a heading.
Defining a direction is relative to north but "north" can have three interpretations. True north (°T) is the direction of the geographic North Pole, being the alignment of the longitude lines on a chart. Magnetic north (°M) is the direction of the magnetic North Pole and is not the exact place as the geographic North Pole. The divergence between magnetic and true north is known as variation, altering slightly each year with the movement of the magnetic North Pole.
Compass north (°C) is the direction in which the compass points and will point to magnetic north if there is no local magnetic interference. Maritime navigation requires converting between [ true, magnetic, and compass north ] .
The nautical mile is the unit of distance used at sea and defined as [ one minute (1') of latitude ] . Because the earth is not a perfect sphere, the actual length of a nautical mile varies. It is longer at the poles and shorter at the equator but standardized to 1852m or 6076ft. At sea, the unit of speed is the knot and defined as one nautical mile per hour.
Depth and height at sea is measured in metres. Depth as shown on a nautical map is relative to a fixed datum and is usually the Lowest Astronomical Tide or [ LAT ] being the lowest water level ever expected. The water depth is higher than shown because of the height of tide. The datum from which heights are measured depends on the type of object measured.
A variety of nautical navigation terms are used in recording information on a nautical map such as bearings, headings, and other important map navigation data. There is universal recognition of these nautical navigation terms, which helps in eliminating the risk of misunderstanding. A knowledge and
understanding of these nautical navigation terms are needed to perform maritime navigation successfully.
- °T =Degrees true Suffix attached to a direction measured relative to true north, e.g, 095°T
- °M = Degrees magnetic Suffix attached to a direction measured relative to magnetic north, e.g, 135°M.
- °C = Degrees compass Suffix attached to a direction measured by the compass and not converted to °T or °M, e.g, 110°C.
- M = Nautical mile A nautical mile is equal to one degree of latitude standardized at 1852m/6076ft) and divided into 10 cables (ca) and each cable is 185m (200yd).
- Kn = Knot The unit of speed used at sea. One knot is one nautical mile per hour.
- m = Metre The standard metre is used to display depth and height on navigation charts. Metres are divided into decimetres; 7.1m is shown on navigation charts as 71.
- Fm = Fathom The old unit of depth, equal to 6ft (1.8m), sometimes found on older navigation charts. Parts of a fathom are shown in feet, e.g, 38ft is shown as 62.
Maritime navigation charts are essential to navigating at sea and is produced and supplied by the various hydrographic agencies. Originally produced for professional sea-goers, many hydrographic agencies today produce maritime navigation charts that are based on the official data but tailored to suit yachting requirements. Available in folios that cover most popular sailing areas, these maritime navigation charts also include harbour plans and information.
Being a representation of a curved surface on a flat sheet of paper, the problem is of how to best embody or project the curve of the earth while minimizing distortion of the land mass shape and size.
The [ Mercator projection ] is the popular form found on maritime navigation charts where the parallels of latitude are drawn further apart towards the poles and the meridians of longitude are drawn parallel and equidistant.
The [ Gnomonic projection ] method on other maritime navigation charts is used for small-scale ocean passage charts or very large-scale harbour navigation charts. Small-scale gnomonic nautical maps have the meridians converge at the poles, while on large-scale gnomonic nautical maps, the area is small and the meridians appear to be parallel. The projection type, Mercator or Gnomonic, used on a particular maritime navigation chart is shown close to the title.
Various scaled marine charts are available.
- Small-scale marine charts cover whole seas or oceans and are used for planning and plotting positions on long passages.
- Medium-scale marine charts are usually cover sections of coastline and contain information of coastal and offshore features around the departure and destination points.
- Large-scale marine charts cover small areas in greater detail and are essential when entering an unfamiliar harbour or navigating a intricate stretch of water.
The marine chart scale along with other details, such as the units used and the last correction date are shown close to its title.
Marine charts use a range of [ nautical chart symbols ] depicting important features and potential hazards. Have a common knowledge of nautical chart symbols as well as keep a reference guide on board.
Maritime navigation charts are produced from information obtained from surveys conducted at regular intervals and the frequency is dependent on the importance of the area to shipping. Man-made and geographic features change so the chart is updated with published corrections or a new version is issued. Regular corrections are issued by the marine chart authorities enabling an update of the marine chart.
Maritime navigation charts are available in electronic form and in combination with a [ marine chartplotter ] , allow maritime navigation to be prepared on a screen and also incorporate other instrument data.
Marine chartplotters display nautical map information on screen and allow the identification of positions, courses, and distances by directing the cursor. Marine chartplotters have a zoom function as well as overlaying the position shown by the GPS, along with comparing the radar image with the nautical map details. The disadvantages of marine chartplotters are their cost and susceptibility to power failure.
Navigators should be proficient in their operation and accurately interpret the information. Do not assume digital systems are accurate, but check the information using another source. Relying on digital navigational systems is not an alternative to learning to manually navigate so keep paper navigation charts on board as a back-up.