Latitude, Longitude and Ship Positions

A ship’s position on the Earth’s surface is described using two coordinates: latitude and longitude. Latitude indicates a location north or south of the Equator, while longitude indicates a location east or west of the Prime Meridian through Greenwich. Both values are needed to identify a position precisely, and they must be read in the correct order with their directions.

Latitude and longitude

The Equator is at 0° latitude; the geographic poles are at 90° north and south. Lines of latitude run parallel to the Equator. Their degree value increases from 0° to 90° north or south. Lines of longitude connect the poles. The Prime Meridian is the reference point at 0° longitude; positions are given east or west of it. Together, latitude and longitude pinpoint a location: latitude alone does not describe a specific place, and neither does longitude alone.

A written coordinate usually gives the latitude first, with N or S, followed by the longitude, with E or W. A direction letter and a sign are two different ways of indicating the same direction. When western longitudes are expressed as negative decimal degrees, the minus sign identifies the western hemisphere; W should not also be interpreted as a sign. Data formats may differ, so make sure you read the expected format.

Degrees, minutes and seconds

Coordinates can be written in degrees, minutes and seconds (DMS), degrees and decimal minutes, or decimal degrees. These units use a sexagesimal system: 60 minutes make one degree, and 60 seconds make one minute. To convert to decimal degrees, divide the minutes by 60 and the seconds by 3,600, then add them to the degree value.

Example: 54° 30′ 00″ north is 54 + 30/60 + 0/3600, or 54.5° north. 10° 15′ 30″ east is 10 + 15/60 + 30/3600, or 10.258333° east. The coordinate pair is therefore 54.5° N, 10.258333° E. NOAA data formats illustrate the same principle with an example: 40.75° equals 40°45′00″; in its decimal format, western longitudes are shown with a negative sign.

Don’t confuse degrees with decimal degrees: 12.5° means twelve and a half degrees, or 12°30′. It does not mean 12°5′. With decimal minutes, however, the decimal part represents a fraction of a minute. So before copying a coordinate, check the unit symbols rather than comparing only the digits. Leading zeros may also be part of the required format in technical displays.

Coordinates on maps and displays

When reading a ship’s position from a display, ask four questions: Is the first value the latitude? Are a direction or sign shown? Are minutes and seconds written as sexagesimal units, or is the coordinate in decimal format? Which reference and coordinate system does the display specify? Only with this information can you reliably compare two position reports or plot them on a nautical chart.

Coordinates locate the point. A nautical chart adds the local information needed to understand the surrounding area. This is the difference between water depth and position: depth describes the body of water, while latitude and longitude identify a location on the Earth’s surface. A navigable channel is a passage shown on the chart that vessels can travel through. Introductory information about nautical charts and navigation displays can help you combine coordinates with other chart information.