Latitude (abbreviation: Lat., φ, or phi) is the angle from a point on the Earth's surface and the equatorial plane, measured from the center of the sphere. Lines joining points of the same latitude are called parallels, which trace concentric circles on the surface of the Earth, parallel to the equator. The north pole is 90° N; the south pole is 90° S. The 0° parallel of latitude is designated the equator, the fundamental plane of all geographic coordinate systems. The equator divides the globe into Northern and Southern Hemispheres.
Longitude (abbreviation: Long., λ, or lambda) is the angle east or west of a reference meridian between the two geographical poles to another meridian that passes through an arbitrary point. All meridians are halves of great circles, and are not parallel. They converge at the north and south poles.
A line passing to the rear of the Royal Observatory, Greenwich (near London in the UK) has been chosen as the international zero-longitude reference line, the Prime Meridian. Places to the east are in the eastern hemisphere, and places to the west are in the western hemisphere. The antipodal meridian of Greenwich is both 180°W and 180°E.
In 1884, the United States hosted the International Meridian Conference and twenty-five nations attended. Twenty-two of them agreed to adopt the location of Greenwich as the zero-reference line. San Domingo voted against the adoption of that motion, while France and Brazil abstained. To date, there exist organizations around the world which continue using historical prime meridians before the acceptance of Greenwich and the ill-attended conference became common-place.
For example, Baltimore, Maryland (in the USA) has a latitude of 39.3° North, and a longitude of 76.6° West. So, a vector drawn from the center of the Earth to a point 39.3° north of the equator and 76.6° west of Greenwich will pass through Baltimore.
This latitude/longitude "webbing" is known as the conjugate graticule.
In defining an ellipse, the vertical diameter is known as the conjugate diameter, and the horizontal diameter — perpendicular, or "transverse", to the conjugate — is the transverse diameter. With a sphere or ellipsoid, the conjugate diameter is known as the polar axis and the transverse as the equatorial axis. The graticule perspective is based on this designation: As the longitudinal rings — geographically defined, all great circles — converge at the poles, it is the poles that the conjugate graticule is defined. If the polar vertex is "pulled down" 90°, so that the vertex is on the equator, or transverse diameter, then it becomes the transverse graticule, upon which all spherical trigonometry is ultimately based (if the longitudinal vertex is between the poles and equator, then it is considered an oblique graticule).
Degrees: a measurement of angle
There are several formats for writing degrees, all of them appearing in the same Lat, Long order.
- DMS Degrees:Minutes:Seconds (49°30'00"N, 123°30'00"W)
- DM Degrees:Decimal Minutes (49°30.0', -123°30.0'), (49d30.0m,-123d30.0')
- DD Decimal Degrees (49.5000°,-123.5000°), generally with 4-6 decimal numbers.
DMS is the most common format, and is standard on all charts and maps, as well as Global Positioning Systems (GPS) and geographic information systems (GIS). DD is the most convenient if a need for calculation or computation might arise, avoiding the complexity and likely introduction of errors by mixed radix degree minute second arithmetic.
- The French Institut Géographique National (IGN) still displays a latitude and longitude on its maps centred on a meridian that passes through Paris
- Haswell, Charles Haynes (1920). Mechanics' and Engineers' Pocket-book of Tables, Rules, and Formulas. Harper & Brothers. Retrieved on 2007-04-09.