Latitude

Latitude is the angular distance of a position from the equator to the nearest pole. However, latitude can acquire different meanings if inappropriately applied to other locations on Earth.

Often expressed in degrees from the equator (positive north and negative south), latitude can [Page 1763]be expressed in a variety of units (radians, grads, semicircles, arc seconds, or mils) and formats (+/-or N/S, to indicate the hemisphere; degrees: minutes and fractional minutes; or degrees: minutes: seconds and fractional seconds). Since one minute of latitude is equal to one nautical mile latitude, tick marks are used on charts as a local graphic scale. Without reference to a specific geodetic datum, latitude values will point to different places on Earth.

If Earth were perfectly spherical, then the general definition for geographic latitude might suffice—that is, the angle from the equatorial plane for a line from the center of mass of the spherical Earth to a position. Earth and its gravity field are slightly elliptical in shape. The distance from the center of Earth to the equator is about 21 km (kilometers) longer than the distance from the center to either pole. A consequence of this flattened spherical shape, an ellipsoid of rotation, is that the force of gravity is only exactly toward the center of mass of Earth along the equator or at either pole. Everywhere else, the plumb bob, the bubble level, or the horizon at sea, the references for local level, are affected by the ellipsoidal gravity surface that is perpendicular to a line from a position toward but slightly away from the center of the Earth. The angle between the equatorial plane and a line perpendicular to the ellipsoid surface that passes through a position is the geodetic latitude.

There are other latitudes. If one measures position with respect to a sphere that has the same surface area as a reference ellipsoid, the north-south angle is the authalic latitude. Geocentric latitude is the angle, similar to geographic latitude for a sphere, between the equatorial plane and the center of the ellipsoid. There is a parametric (or reduced) latitude, defined as the angle between the equatorial plane and a line from the center of the ellipsoid perpendicular to a surface defined by a sphere tangent to the ellipsoid along the equator or the latitude on a sphere for which the parallel has the same radius as the parallel of geodetic latitude on the ellipsoid through the same point. Conformal latitude is a transformation of geodetic latitude that allows a conformal mapping using spherical geometry. Isometric latitude is proportional to the parallels on an ellipsoidal Mercator projection. Rectifying latitude is the angular distance along a line of longitude from the equator to a point on Earth.

Other latitudes have been defined on Earth, including astronomic, auxiliary equidistant, geomagnetic, magnetic, and transverse. Surveyors use the word for the y value in a traverse defined in a plane coordinate system.

The importance for geography is that these latitudes are represented by significantly different angles, making their appropriate selection and use critical for positioning and analysis.

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