Latitude

Latitude forms a fundamental component of the geographic coordinate system used to describe precise positions on the Earth and other celestial bodies. Defined as an angular measure north or south of the Equator, it provides a means of locating any point on the planet’s surface when paired with longitude. The system of parallels and meridians, collectively known as the graticule, offers a structured reference network for navigation, mapping, and a wide variety of scientific applications.

The Concept of Latitude

Latitude specifies the north–south position of a point on the Earth. It is expressed as an angle between the equatorial plane and a line that is perpendicular to the reference surface at the given point. On the idealised spherical Earth, this perpendicular line is radial, whereas on the ellipsoidal Earth it is the normal to the ellipsoid. Values range from 0° at the Equator to 90° North at the North Pole and 90° South at the South Pole. Parallels, the lines representing constant latitude, run east–west and are parallel to the Equator.
In common geographical usage, latitude is specified either in degrees, minutes, and seconds, or in degrees and decimal minutes. For example, a position such as 50° 39.734′ N falls within this navigational system. Although the term “latitude” is generally understood to mean geodetic latitude, several related definitions exist for more specialised applications.

Background and Reference Surfaces

Determining accurate latitude requires an underlying model of the planet’s shape. Two key conceptual layers guide this:

• The Geoid: A surface representing mean sea level across the globe, extended beneath landmasses. This irregular form approximates Earth’s gravitational potential and serves as a physical reference surface.
• The Reference Ellipsoid: A mathematically defined, smooth surface approximating the geoid but easier to employ in calculations. Modern mapping systems usually rely on an oblate ellipsoid of revolution because Earth is slightly flattened at the poles and bulged at the Equator due to rotation.

The correspondence between a point on the Earth’s surface and the reference ellipsoid is established by projecting the point along the ellipsoidal normal. Its latitude is taken as the latitude of this corresponding point on the ellipsoid. The choice of ellipsoid influences coordinate values, which means an explicit coordinate reference system (CRS) must be stated for high-precision work. The ISO 19111 standard emphasises this necessity, noting that without full CRS information, latitude and longitude values may be ambiguous.

The Graticule

The network formed by the intersection of parallels and meridians is known as the graticule. Meridians are lines of constant longitude and run from pole to pole. The angle between any meridian plane and the meridian through Greenwich defines longitude. When combined with parallels, the graticule enables precise location referencing. In many illustrations, meridians may be spaced at regular angular intervals, such as every 6°, and parallels every 4°, creating a systematic grid.

Determining Latitude

Latitude can be obtained using different methods:

• Celestial Navigation: The meridian altitude method enables navigators to determine latitude by measuring the altitude of a celestial object at its highest point in the sky.
• Geodetic Techniques: Modern land-based surveying requires knowledge of the Earth’s gravitational field to correctly align theodolites.
• Satellite Navigation: Global Positioning System (GPS) technology determines latitude by calculating positions relative to satellite orbits, which themselves depend on accurately modelled gravitational fields.

The scientific discipline concerned with studying the Earth’s shape and gravitational field is geodesy, which provides the theoretical basis for latitude determination.

Latitude on a Spherical Earth

When Earth is modelled as a sphere, definitions become mathematically simpler. The Equator forms a great circle perpendicular to the Earth’s axis of rotation. Parallels are smaller east–west circles parallel to this plane. Spherical latitude is defined as the angle between the equatorial plane and the radial line connecting the point to the centre of the sphere.
Although the spherical model is accurate enough for basic cartography, navigation, and introductory studies, it does not reflect the true geometry of the Earth, whose polar flattening must be taken into account for precise measurements.

Named Latitudes

Certain parallels hold special significance due to Earth’s axial tilt:

• Tropic of Cancer – approx. 23.5° N
• Tropic of Capricorn – approx. 23.5° S
• Arctic Circle – approx. 66.5° N
• Antarctic Circle – approx. 66.5° S

These arise from the axial tilt, the angle between the equatorial plane and the orbital plane (the ecliptic). The tilt determines seasonal variations in solar altitude. At the solstices, regions within the polar circles experience continuous daylight or night, while only locations within the tropics have the Sun directly overhead at any time.

Latitude and Map Projections

On map projections, the representation of parallels and meridians varies widely. For example:

• Mercator Projection: Parallels appear as horizontal straight lines; meridians as vertical straight lines.
• Transverse Mercator Projection: Both parallels and meridians appear as complex curves; neither is consistently horizontal or vertical.

This diversity arises because projections aim to preserve certain geometric properties—such as shape, area, or distance—which necessitates distortions in the representation of latitude and longitude lines.

Latitude on an Ellipsoidal Earth

A more accurate model of Earth is the oblate ellipsoid. Isaac Newton first predicted such a shape in 1687, arguing that a rotating fluid body in equilibrium must bulge at the Equator. Later measurements confirmed this.
An ellipsoid of revolution is defined by rotating an ellipse around its minor axis, giving rise to two essential parameters:

• Semimajor axis (equatorial radius)
• Flattening (or equivalently, polar radius or eccentricity)

Flattening quantifies how much the poles are compressed relative to a perfect sphere. The eccentricity, derived from flattening, is another key parameter used in geodetic calculations.
Reference ellipsoids—such as WGS84, used in GPS—are defined using precise values for these parameters. Because Earth-based coordinate systems rely on specific ellipsoids, the latitude of a point varies slightly depending on the chosen model. This necessitates datum transformations when converting between mapping systems based on different reference ellipsoids.

Practical Significance of Coordinate Systems

Latitude and longitude together form a global referencing framework for navigation, mapping, aviation, oceanography, environmental monitoring, and numerous scientific disciplines. When combined with altitude, they establish a three-dimensional location within a defined coordinate reference system.
While casual applications—such as general map reading or everyday navigation—can rely on unstated reference ellipsoids, scientific and engineering applications require explicit CRS definitions to ensure accuracy. GPS devices typically integrate transformation algorithms to display positions on national or local grid systems linked to specific datums.