What Is Latitude and Longitude?

Latitude is the angle north or south of the equator (−90° to +90°); longitude is the angle east or west of the prime meridian (−180° to +180°). Together they uniquely name any point on Earth's surface, with latitude written first per ISO 6709.

Latitude and longitude form a geographic coordinate system: a spherical-coordinate convention defined on the WGS 84 reference ellipsoid, the surface that civilian GPS, almost every web map, MGRS, UTM and any modern geodetic calculation are referenced to. The system has been the global location language for several centuries, and modern coordinates inherit a long chain of decisions — the 1884 choice of Greenwich, the 1929 fixing of the nautical mile, the 1984 satellite-era redefinition of the prime meridian, the 2014 update of the WGS 84 spec. This article defines the two axes precisely, gives the surface distance per degree at real latitudes, identifies the datum behind every modern coordinate, and addresses the misconceptions that trip new readers up.

Definition

Latitude is the angle between the equatorial plane and the surface normal of the reference ellipsoid at the point; longitude is the angle east or west of a chosen prime meridian, measured in the equatorial plane. Together the pair names a unique direction from the centre of the ellipsoid, and the surface point in that direction is the location. Both are angles, not distances — this is the single most-important fact in the whole topic, and the source of every common misconception listed at the foot of this page.

The angle written as "latitude" on civilian GPS, modern web maps, MGRS and UTM is geodetic latitude — the angle between the equatorial plane and the line perpendicular to the WGS 84 ellipsoid at the point. Geocentric latitude, measured from the ellipsoid's centre to the surface point, differs by up to 11.5 arcminutes near 45° latitude. On the ground, that 11.5′ corresponds to roughly 21 km of horizontal position if the two definitions are accidentally swapped.

How latitude is measured

Latitude is reckoned from 0° at the equator to ±90° at the poles, with positive values north and negative values south (alternatively a trailing N or S). The Empire State Building's 40.7484° N puts it just under halfway from the equator to the North Pole. Lines of constant latitude are called parallels; each is a circle whose plane is parallel to the equator, with circumference shrinking toward each pole and degenerating to a single point exactly at ±90°.

The 1.12 km equator-to-pole swing is a consequence of the ellipsoid's flattening. WGS 84's polar radius is 21.384 km shorter than the equatorial radius, so the local radius of curvature is paradoxically larger at the poles than at the equator (a smaller body curves over more sharply in cross-section). The variation is invisible in everyday distance work, but it accumulates: a 1,000 km surveying baseline crossing 40° of latitude needs the ellipsoidal correction or it accrues several metres of error.

How longitude is measured

Longitude is the angle measured east (positive, or trailing E) or west (negative, or trailing W) of the prime meridian, in the equatorial plane. By international convention since the 1884 International Meridian Conference, the meridian passes through Greenwich, England — specifically through the IERS Reference Meridian, which superseded the original Royal Observatory line for high-precision work in 1984. Eastern-hemisphere longitudes are positive; western longitudes are negative.

The IERS Reference Meridian lies 102.478 m east of the brass strip set in the Royal Observatory courtyard for the Airy Transit Circle, the meridian the 1884 conference adopted. The 5.31-arcsecond shift was finalised in 1984, when the international community switched from astronomical determination of longitude (which relies on local vertical and is perturbed by mass anomalies) to satellite-based determination (which depends on the geocentre and is free of those biases). Two distinct prime meridians therefore exist on the ground in Greenwich today; modern GPS reports the IERS one.

The progression follows cos(φ) almost exactly — the slight deviation comes from the ellipsoidal correction, never more than 0.34% across the full range. The collapse from 111 km at the equator to zero at the pole is the single biggest source of distortion in flat maps: it is why high-latitude landmasses appear stretched east-west on a Mercator projection, why flight paths into Arctic destinations sweep through many lines of longitude in a few minutes, and why GPS receivers struggle to provide a stable bearing within a few kilometres of the geographic poles.

The coordinate pair

A pair (latitude, longitude) names a unique point on the WGS 84 ellipsoid. The Empire State Building, for example, sits at (40.7484° N, 73.9857° W) — written latitude-first per ISO 6709:2022. The same point can be written in several distinct formats, all carrying the same information; the choice of format is domain-driven, not informational.

The precision of a decimal-degrees coordinate is set by the number of decimal places, and each decimal translates directly to ground distance. Six decimal places of latitude correspond to about 11 cm at the equator and slightly less at higher latitudes for longitude; three decimal places correspond to about 111 m — a city block. Beyond what the underlying measurement can actually resolve, additional decimals are false precision: the digits exist but the receiver did not produce them.

Civilian GPS resolves about 4.9 m (95%) under open sky per the 2020 SPS Performance Standard, which lines up with five decimal places of useful precision. The sixth decimal place is honest only if the data source is differential GPS, real-time-kinematic (RTK) GPS, surveyed control points, or cadastral-grade records; on a phone receiver it is bits without a backing signal. The DMS ↔ decimal converter losslessly round-trips between the three angular formats; /tools/utm-converter and /tools/mgrs-converter handle the projected ones.

Datum: WGS 84 is the modern default

A latitude/longitude pair is not fully specified without a datum — a definition of Earth's shape (the reference ellipsoid) and the orientation of the coordinate axes against the planet. Civilian GPS, nearly all web maps, MGRS and UTM use the WGS 84 reference frame published by NGA. Without the implicit-or-explicit datum, the two numbers describe a direction but not a position.

Earlier datums — NAD 27 in North America, OSGB36 in Britain, Tokyo Datum in Japan, Pulkovo 1942 across the former Eastern bloc — can differ from WGS 84 by 100 m to several hundred metres at any given point. The numerical coordinates look the same; the position on the ground is not. For consumer maps the shift is invisible; for surveying, legal-boundary work, aviation and any modern engineering it is consequential.

Why latitude usually comes first

ISO 6709:2022 fixes the order: latitude first, longitude second. Most software, all paper maps and most everyday tools follow the standard. The natural reading order ("up-down before left-right") agrees with it, and the historical primacy of latitude — the one-arcminute-equals-one-nautical-mile relationship from the 17th-century charts — reinforced the same direction.

Worked example: the Empire State Building

The Empire State Building, on Fifth Avenue in Manhattan, sits at 40.7484° N, 73.9857° W. Decomposed:

• 40.7484° N is 40.7484 degrees north of the equator. The parallel at 40.7484° N (an east-west circle, circumference about 30,338 km, half-way down the Northern Hemisphere) passes through the building.
• 73.9857° W is 73.9857 degrees west of the IERS Reference Meridian. The meridian at 73.9857° W (a great circle joining the two poles) passes through the building.

All six refer to the same physical point on the WGS 84 ellipsoid. The DMS ↔ decimal converter round-trips between the first three; /tools/utm-converter, /tools/mgrs-converter and /tools/plus-codes handle the projected and encoded formats.

Common misconceptions

Related pillars

This is the foundation pillar of the coordinates topic. The other seven pillar concepts on Coordinately:

• Coordinate formats explained — the six ways the same lat/lon can be written (DD, DMS, DDM, UTM, MGRS, Plus Codes)
• How GPS works — the satellite system producing every modern coordinate
• What is a map projection? — the geometry that turns lat/lon into the maps you see
• What is a geodetic datum? — the reference frame coordinates need to point at real places
• Time zones explained — how longitude maps to civil time
• History of latitude and longitude — 2,300 years from Eratosthenes to GPS
• Great-circle distance — the shortest path between any two coordinates