 | |
| Map of Earth | |
| Longitude (λ) | |
|---|---|
| Lines of longitude appear vertical with varying curvature in this projection; but are actually halves of great ellipses, with identical radii at a given latitude. | |
| Latitude (φ) | |
| Lines of latitude appear horizontal with varying curvature in this projection; but are actually circular with different radii. All locations with a given latitude are collectively referred to as a circle of latitude. | |
| The equator divides the planet into a Northern Hemisphere, a Southern Hemisphere and has a latitude of 0°. |  |
Latitude
The latitude of a place on the earth's surface is the angular distance north or south of the equator.[2] Latitude is usually expressed in degrees (marked with °) ranging from 0° at the Equator to 90° at the North and South poles.[2] The latitude of the North Pole is 90° N, and the latitude of the South Pole is 90° S.[2] Historically, mariners calculated latitude in the Northern Hemisphere by sighting the North Star Polaris with a sextant and sight reduction tables to take out error for height of eye and atmospheric refraction. Generally, the height of Polaris in degrees of arc above the horizon is the latitude of the observer.
Longitude
Similar to latitude, the longitude of a place on the earth's surface is the angular distance east or west of the prime meridian or Greenwich meridian.[2] Longitude is usually expressed in degrees (marked with °) ranging from 0° at the Greenwich meridian to 180° east and west. Sydney, Australia, for example, has a longitude of about 151° east. New York City has a longitude of about 74° west. For most of history, mariners struggled to determine precise longitude. The problem was solved with the invention of the marine chronometer. Longitude can be calculated if the precise time of a sighting is known.
Modern technique
Most modern navigation relies primarily on positions determined electronically by receivers collecting information from satellites. Most other modern techniques rely on crossing lines of position or LOP.[3] A line of position can refer to two different things: a line on a chart and a line between the observer and an object in real life.[4] A bearing is a measure of the direction to an object.[4] If the navigator measures the direction in real life, the angle can then be drawn on a nautical chart and the navigator will be on that line on the chart.[4]
In addition to bearings, navigators also often measure distances to objects.[3] On the chart, a distance produces a circle or arc of position.[3] Circles, arcs, and hyperbolae of positions are often referred to as lines of position.
If the navigator draws two lines of position, and they intersect he must be at that position.[3] A fix is the intersection of two or more LOPs.[3]
If only one line of position is available, this may be evaluated against the dead reckoning position to establish an estimated position.[5]
Lines (or circles) of position can be derived from a variety of sources:
- celestial observation (a short segment of the circle of equal altitude, but generally represented as a line),
- terrestrial range (natural or man made) when two charted points are observed to be in line with each other,[6]
- compass bearing to a charted object,
- radar range to a charted object,
- on certain coastlines, a depth sounding from echo sounder or hand lead line.
There are some methods seldom used today such as "dipping a light" to calculate the geographic range from observer to lighthouse
Methods of navigation have changed through history.[7] Each new method has enhanced the mariner’s ability to complete his voyage.[7] One of the most important judgments the navigator must make is the best method to use.[7] Some types of navigation are depicted in the table.
| Modern navigation methods | ||
| Illustration | Description | Application |
|---|---|---|
 | Dead reckoning or DR, in which one advances a prior position using the ship's course and speed. The new position is called a DR position. It is generally accepted that only course and speed determine the DR position. Correcting the DR position for leeway, current effects, and steering error result in an estimated position or EP. An inertial navigator develops an extremely accurate EP.[7] | Used at all times. |
 | Pilotage involves navigating in restricted waters with frequent determination of position relative to geographic and hydrographic features.[7] | When within sight of land. |
 | Celestial navigation involves reducing celestial measurements to lines of position using tables, spherical trigonometry, and almanacs. | Used primarily as a backup to satellite and other electronic systems in the open ocean.[7] |
| Electronic navigation covers any method of position fixing using electronic means, including: | ||
 | Radio navigation uses radio waves to determine position by either radio direction finding systems or hyperbolic systems, such as Decca, Omega and LORAN-C. | Losing ground to GPS. |
 | Radar navigation uses radar to determine the distance from or bearing of objects whose position is known. This process is separate from radar’s use as a collision avoidance system.[7] | Primarily when within radar range of land. |
 | Satellite navigation uses artificial earth satellite systems, such as GPS, to determine position.[7] | Used in all situations. |
The practice of navigation usually involves a combination of these different methods.
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