Nautical chart
A nautical chart or hydrographic chart is a graphic representation of a sea region or water body and adjacent coasts or banks. Depending on the scale of the chart, it may show depths of water and heights of land, 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, and 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 charts. 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.
Data sources
Nautical charts are based on hydrographic surveys and bathymetric surveys. As surveying is laborious and time-consuming, hydrographic data for many areas of sea may be dated and are sometimes unreliable. 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. All depths on charts is measured with respect to a datum/reference level. This ensures that difficult to find projections, such as masts, do not present a danger to vessels navigating over the obstruction.Publication
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, the international chart series is a worldwide system of charts, which is being 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.
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, old or uncorrected charts should never be used for navigation. Every producer of nautical charts also provides a system to inform mariners of changes that affect the chart. In the United States, chart corrections and notifications of new editions are provided by various governmental agencies by way of Notice to Mariners, Local Notice to Mariners, Summary of Corrections, and Broadcast Notice to Mariners. In the U.S., NOAA also has a printing partner who prints the "POD" NOAA charts, and they contain the very latest corrections and notifications at the time of printing. To give notice to mariners, radio broadcasts provide advance notice of urgent corrections.A good way to keep track of corrections is with a Chart and Publication Correction Record Card system. Using this system, the navigator does not immediately update every chart in the portfolio when a new Notice to Mariners arrives, instead creating a card for every chart and noting the correction on this card. When the time comes to use the chart, he pulls the chart and chart's card, and makes the indicated corrections on the chart. This system ensures that every chart is properly corrected prior to use. A prudent mariner should obtain a new chart if he has not kept track of corrections and his chart is more than several months old.
Various Digital Notices to Mariners systems are available on the market such as Digitrace, Voyager, or ChartCo, to correct British Admiralty charts as well as NOAA charts. These systems provide only vessel relevant corrections via e-mail or web downloads, reducing the time needed to sort out corrections for each chart. Tracings to assist corrections are provided at the same time.
The Canadian Coast Guard produces the Notice to Mariners publication which informs mariners of important navigational safety matters affecting Canadian Waters. This electronic publication is published on a monthly basis and can be downloaded from the Notices to Mariners Web site. The information in the Notice to Mariners is formatted to simplify the correction of paper charts and navigational publications.
Various and diverse methods exist for the correction of electronic navigational charts.
Limitations
In 1973 the cargo ship MV Muirfield struck an unknown object in the Indian Ocean in waters charted at a depth of greater than 5,000 metres, resulting in extensive damage to her keel. In 1983, HMAS Moresby, a Royal Australian Navy survey ship, surveyed the area where Muirfield was damaged, and charted in detail a previously unsuspected hazard to navigation, the Muirfield Seamount. The dramatic accidental discovery of the Muirfield Seamount is often cited as an example of limitations in the vertical geodetic datum accuracy of some offshore areas as represented on nautical charts, especially on small-scale charts.A similar incident involving a passenger ship occurred in 1992 when the Cunard liner Queen Elizabeth 2 struck a submerged rock off Block Island in the Atlantic Ocean. In November 1999, the semi-submersible, heavy-lift ship Mighty Servant 2 capsized and sank after hitting an uncharted single underwater isolated pinnacle of granite off Indonesia. Five crew members died and Mighty Servant 2 was declared a total loss. More recently, in 2005 the submarine USS San Francisco ran into an uncharted seamount about 560 kilometres south of Guam at a speed of, sustaining serious damage and killing one seaman. In September 2006 the jack-up barge Octopus ran aground on an uncharted sea mount within the Orkney Islands while being towed by the tug Harold. £1M worth of damage was caused to the barge and delayed work on the installation of a tidal energy generator prototype. As stated in the Mariners Handbook and subsequent accident report: "No chart is infallible. Every chart is liable to be incomplete".
Map projection, positions, and bearings
Historically the first projection, invented by Marinus of Tyre ca. AD 100 according to Ptolemy, was what is now called equirectangular projection. While it is very convenient for small seas like the Aegean, it is unsuitable for seas larger than Mediterranean or an open ocean, even though early explorers had to use it for want of a better.The Mercator projection is now 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. NOAA uses the polyconic projection for some of its charts of the Great Lakes, at both large and small scales.
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.
However, the use of the Mercator projection has drawbacks. This projection shows the lines of longitude as parallel. On the real globe, the lines of longitude converge as they approach the north or south pole. This means that east–west distances are exaggerated at high latitudes. To keep the projection conformal, the projection increases the displayed distance between lines of latitude in proportion; thus a square is shown as a square everywhere on the chart, but a square on the Arctic Circle appears much bigger than a square of the same size at the equator. In practical use, this is less of a problem than it sounds. One minute of latitude is, for practical purposes, a nautical mile. Distances in nautical miles can therefore be measured on the latitude gradations printed on the side of the chart.