This is of great importance in accurate applications, such as a Global Positioning System (GPS), but in common usage, where high accuracy is not required, the reference ellipsoid is not usually stated. Since there are many different reference ellipsoids, the precise latitude of a feature on the surface is not unique: this is stressed in the ISO standard which states that "without the full specification of the coordinate reference system, coordinates (that is latitude and longitude) are ambiguous at best and meaningless at worst". Latitude and longitude together with some specification of height constitute a geographic coordinate system as defined in the specification of the ISO 19111 standard. The latitude of a point on the actual surface is that of the corresponding point on the reference surface, the correspondence being along the normal to the reference surface, which passes through the point on the physical surface. Lines of constant latitude and longitude together constitute a graticule on the reference surface. The definitions of latitude and longitude on such reference surfaces are detailed in the following sections. The simplest choice for the reference surface is a sphere, but the geoid is more accurately modeled by an ellipsoid.
The second step is to approximate the geoid by a mathematically simpler reference surface. In the first step the physical surface is modeled by the geoid, a surface which approximates the mean sea level over the oceans and its continuation under the land masses. Two levels of abstraction are employed in the definitions of latitude and longitude.
On its own, the term latitude should be taken to be the geodetic latitude as defined below. Latitude is used together with longitude to specify the precise location of features on the surface of the Earth. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude is an angle (defined below) which ranges from 0° at the Equator to 90° (North or South) at the poles. In geography, latitude is a geographic coordinate that specifies the north– south position of a point on the Earth's surface. In this example meridians are spaced at 6° intervals and parallels at 4° intervals. The graticule shows the latitude and longitude of points on the surface. The circles parallel to the equator are lines of constant latitude, or parallels. The lines from pole to pole are lines of constant longitude, or meridians. A graticule on the Earth as a sphere or an ellipsoid.
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