Editing Nautical chart (section)

== 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.

=== Labels ===
[[File:Nautical-chart-1.jpg|left|thumb|Automatically labeled nautical chart]]

Nautical charts must be labeled with navigational and depth information. There are a few commercial software packages that do [[automatic label placement]] for any kind of map or chart. Modern systems render electronic charts consistent with the IHO S-52 specification, issued by the [[International Hydrographic Organization]] (IHO).<ref>{{Cite web| title=Specifications for chart content and display aspects of ECDIS | url=https://iho.int/uploads/user/pubs/standards/s-52/S-52%20Edition%206.1.1%20-%20June%202015.pdf | archive-url=https://web.archive.org/web/20210430104905/https://iho.int/uploads/user/pubs/standards/s-52/S-52%20Edition%206.1.1%20-%20June%202015.pdf | archive-date=2021-04-30}}</ref> 

=== Pilotage information ===
[[File:NOAA chart 25664 1976 detail.png|right|thumb|Detail of a United States NOAA chart, showing a harbour area]]

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 [[navigational hazards]] such as [[shipwreck]]s. Other symbols show the position and characteristics of [[navigational aid]]s such as [[buoy]]s, [[light characteristic|lights]], [[lighthouse]]s, 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 human-made features, dry land, sea bed that dries with the tide, and seabed that is permanently [[underwater]] and indicate water depth.

=== Depths and heights ===
[[File:British Admiraly chart colours.svg|thumb|upright=1.3|Use of colour in British [[Admiralty chart]]s]]

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. Older charts, as well as those published by the United States government, may use [[Foot (length)|feet]] or [[fathom]]s. Depth [[contour line]]s 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 trend 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 race]]s and  strong currents have special chart symbols. Tidal flow information may be shown on charts using [[tidal diamond]]s, indicating the speed and bearing of the tidal flow during each hour of the tidal cycle.