Nautical chart
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Contents
Overview
A nautical chart is a graphic representation of a sea area and adjacent coastal regions. Depending on the scale of the chart, it may show:
- depths of water and heights of land (topographic map)
- natural features of the seabed
- details of the coastline
- navigational hazards
- locations of natural and human-made aids to navigation
- information on tides and currents
- local details of the Earth's magnetic field
- human-made structures such as harbours
- buildings and bridges
Nautical charts are essential tools for marine navigation; many countries require vessels, especially commercial ships, to carry them. Nautical charting may take the form of charts printed on paper or computerized Electronic Navigational Chart (ENC).
Recent technologies have made available paper charts which are printed "on demand" with cartographic data that has been downloaded to the commercial printing company as recently as the night before printing. With each daily download, critical data such as Local Notices to Mariners are added to the on-demand chart files so that these charts are up to date at the time of printing.
Electronic and paper charts
Conventional nautical charts are printed on large sheets of paper at a variety of scales. Mariners will generally carry many charts to provide sufficient detail for the areas they might need to visit. Electronic Navigational Charts (ENCs), which use computer software and electronic databases to provide navigation information, can augment or in some cases replace paper charts, though many mariners carry paper charts as a backup in case the electronic charting system fails.
Sources and publication of nautical charts
Nautical charts are based on hydrographic surveys. Hydrographic data for many areas of sea may be dated and not always reliable.
Nautical charts are issued by power of the national hydrographic offices in many countries. These charts are considered "official" in contrast to those made by commercial publishers. Many hydrographic offices provide regular, sometimes weekly, manual updates of their charts through their sales agents. Individual hydrographic offices produce national chart series and international chart series. Coordinated by the International Hydrographic Organization (IHO), the international chart series is a worldwide system of charts ("INT" chart series), which has been developed with the goal of unifying as many chart systems as possible.
There are also commercially published charts, some of which may carry additional information of particular interest, e.g. for yacht skippers.
Depth sounding
Depths are measured in a variety of ways. Historically the sounding line was used. In modern times, echo sounding is used for measuring the seabed in the open sea. When measuring the safe depth of water over an entire obstruction, such as a shipwreck, the minimum depth is checked by sweeping the area with a length of horizontal wire. This ensures that difficult to find projections, such as masts, do not present a danger to vessels navigating over the obstruction.
Chart correction
The nature of a waterway depicted by a chart may change, and artificial aids to navigation may be altered at short notice. Therefore producers of nautical charts also provide a system to inform mariners of changes that affect the chart. Chart corrections and notifications of new editions are often provided by way of Notice to Mariners, Local Notice to Mariners, Summary of Corrections, and Broadcast Notice to Mariners.
Various and diverse methods exist for the correction of ENCs.
Map projection, positions, and bearings
The Mercator projection is used on the vast majority of nautical charts. Since the Mercator projection is conformal, that is, bearings in the chart are identical to the corresponding angles in nature, courses plotted on the chart may be used directly as the course-to-steer at the helm.
The gnomonic projection is used for charts intended for plotting of great circle routes.
Positions of places shown on the chart can be measured from the longitude and latitude scales on the borders of the chart, relative to a geodetic datum (such as WGS 84).
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 compass points. Most charts include a compass rose depicting the variation between magnetic and true north.
Details on a nautical chart
Many countries' hydrographic agencies publish a "Chart 1", which explains all of the symbols, terms and abbreviations used on charts that they produce for both domestic and international use. Each country starts with the base symbology specified in IHO standard INT 1, and is then permitted to add its own supplemental symbologies to its domestic charts, which are also explained in its version of Chart 1. Ships are typically required to carry copies of Chart 1 with their paper charts.
Pilotage information
The chart uses symbols to provide pilotage information about the nature and position of features useful to navigators, such as sea bed information, sea mark, and landmarks. Some symbols describe the sea bed with information such as its depth, materials as well as possible hazards such as shipwrecks. Other symbols show the position and characteristics of buoys, lights, lighthouses, coastal and land features and structures that are useful for position fixing. The abbreviation "ED" is commonly used to label geographic locations whose existence is doubtful.
Colours distinguish between man-made features, dry land, sea bed that dries with the tide, and seabed that is permanently underwater and indicate water depth.
Depths and heights
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 the chart datum, which is related to the local sea level. The chart datum varies according to the standard used by each national Hydrographic Office. In general, the move is towards using lowest astronomical tide (LAT), the lowest tide predicted in the full tidal cycle, but in non-tidal areas and some tidal areas Mean Sea Level (MSL) is used.
Heights, e.g. a lighthouse, are generally given relative to mean high water spring (MHWS). Vertical clearances, e.g. below a bridge or cable, are given relative to highest astronomical tide (HAT). The chart will indicate what datum is in use.
The use of HAT for heights and LAT for depths, means that the mariner can quickly look at the chart to ensure that they have sufficient clearance to pass any obstruction, though they may have to calculate height of tide to ensure their safety.
Tidal information
Tidal races and other strong currents have special chart symbols. Tidal flow information may be shown on charts using tidal diamonds, indicating the speed and bearing of the tidal flow during each hour of the tidal cycle.
