Editing Nautical chart (section)

== Map projection, positions, and bearings ==
[[File:Fernão Vaz Dourado 1571-1.jpg|thumb|A pre-Mercator nautical chart of 1571, from Portuguese cartographer [[Fernão Vaz Dourado]] ({{Circa|1520|1580}}). It belongs to the so-called ''plane chart'' model, where observed latitudes and magnetic directions are plotted directly into the plane, with a constant scale, as if the Earth's surface were a flat plane (Portuguese National Archives of Torre do Tombo, Lisbon)]]
Historically the first projection, invented by [[Marinus of Tyre]] ca. AD 100 according to [[Ptolemy]], was what is now called [[equirectangular projection]] (historically called plane chart, ''plate carrée'', {{langx|pt|carta plana quadrada}}). 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 map|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 distance|great circle routes]]. [[NOAA]] uses the [[polyconic projection]] for some of its charts of the [[Great Lakes]], at both large and small scales.<ref>See, for example, NOAA 14860 - Lake Huron 1:500,000 and NOAA 14853 Detroit River 1:15,000.</ref>

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 (navigation)|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 (navigation)|course]] or a compass reading to a landmark. On nautical charts, the top of the chart is always [[true north]], rather than [[North Magnetic Pole|magnetic north]], towards which a compass points. Most charts include a [[compass rose]] depicting the [[Magnetic declination|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 (north–south distances) 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.<ref>[http://www.sailingissues.com/navcourse2.html Nautical charts] on sailingissues.com</ref>