Longitude
Definition
Longitude is the geographic coordinate that specifies a location’s east–west position on Earth, measured as the angle between the prime meridian (0° at Greenwich) and the meridian passing through the point. Values range from −180° to +180°. Unlike latitude, the physical length of a degree of longitude varies with latitude, shrinking to zero at the poles. Precise longitude depends on the reference ellipsoid and timekeeping because the Earth rotates; historically, marine chronometers enabled safe ocean navigation. On modern systems, geodetic longitude is part of a reference frame (e.g., WGS84) that evolves over time with tectonics, so high-precision users track epochs and velocities. Longitude underlies global tiling schemes, time zones, and great-circle navigation. When combined with latitude, it enables unambiguous addressing of any place on the planet. Provide explicit methods, QA notes, and version history so others can reuse the layer responsibly.
Application
Aviation and maritime navigation use longitude and latitude for routing. Web maps use lon/lat in APIs for geocoding and tile addressing. Surveyors stake boundaries; astronomers track celestial events tied to longitude. Emergency services locate calls; researchers analyze longitudinal gradients in ocean salinity or socioeconomic indicators.
FAQ
Why is 0° longitude at Greenwich?
International agreement in 1884 established the Greenwich meridian for consistency; other prime meridians were used historically but standardization simplified navigation and mapping.
Why does a degree of longitude represent different distances?
Meridians converge toward the poles. At the equator ~111 km; mid-latitudes less; near the poles, almost zero. Projections and buffers must account for this.
Can longitude be expressed beyond ±180°?
Yes, some systems use 0–360° for convenience (e.g., planetary mapping). Convert carefully when mixing datasets to avoid wraparound artifacts.
Does longitude change at a fixed monument?
Slightly over years due to plate motion and reference-frame updates. High-precision applications use coordinates with an epoch or transform to a current frame.