Rene Westerhuis - Skippers Cockpit Navigation Guide

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Position, latitude and longitude

The grid of latitudes and longitudes on a nautical chart is necessary for determining location and distance. Thanks to this system of co-ordinates, any place on Earth can be represented as a geographical position. To achieve this, the Earth is covered with a network of imaginary lines: parallels and meridians. The most important and largest (zero) parallel is the equator and the most important (zero) meridian runs through Greenwich according to international agreements.

Latitude is the distance north or south of the equator. The North Pole = 90 north latitude (NL), and the South Pole = 90 south latitude (SL). Longitude is the distance east or west of the Prime Meridian (Greenwich). 180 west = 180 west longitude (WL) and 180 east = 180 east longitude (EL). All meridians (lines of longitude) are the same length but the distance between them decreases towards the North and South Poles. Parallels (lines of latitude) maintain an equal distance between them but decrease in length towards the North and South Poles. On a chart, the shape of the Earth is projected onto a cylinder and unfolded to a flat surface, on which parallels and meridians remain at right angles to each other, so the meridians do not run towards each other. To be able to draw a path as a straight line on the map, the parallels towards the North and South Pole are also stretched. This method of the expanding map or Mercator projection, in which the scale slowly expands, is applied to all British charts.

Digital marine charts

Electronic marine charts come in two types: raster charts and vector charts. A raster chart (RNC Raster Navigational Chart) is an exact copy of a paper chart, built-up from pixels and often digitised with the help of a scanner. A vector map (ENC Electronic Navigational Chart) is built-up in layers from a digital file (database) with components, co-ordinates and information.

Co-ordinate format

On all British charts, the minutes are divided into tenths along the chart edge. With GPS receivers and electronic charts, different and sometimes rather confusing, ways of noting positions may be used. In the examples below, the second method corresponds best to the paper nautical chart but still requires a little thought to be carried out!

Sector light on multi-coloured chart. The elevation is 21 metres. The nominal range of the white light is 18 nautical miles. The range of the green light is 12 nautical miles and the red light is in between

Notice to Mariners

In accordance with international regulations, laid down in SOLAS (International Convention for the Safety of Life at Sea), nautical products, including sea charts, must be kept up-to-date. Therefore, the UK Hydrographic Office (UKHO) publishes weekly Notices to Mariners (NMs) containing a comprehensive list of corrections to charts. The NMs are free to access and can be found at www.admiralty.co.uk/maritime-safety-information/admiralty-notices-to-mariners