Your GPS coordinates depend on which ellipsoid the map chose
Latitude measures how far north or south a point sits, from 90°S at the South Pole through 0° at the Equator to 90°N. Parallels run east-west as circles. Without naming the reference ellipsoid, the same spot can sit 140 metres from where another datum places it.
In geodesy and cartography, latitude specifies north-south position as an angle from −90° to 90°. Lines of constant latitude, called parallels, run east-west parallel to the equator. Together with longitude, latitude forms a coordinate pair locating any surface point. The term usually means geodetic latitude: the angle between the normal to the ellipsoid and the equatorial plane.
Defining latitude requires two modeling steps. First the physical surface is approximated by the geoid, mean sea level extended under continents. Then a simpler reference surface—typically an oblate ellipsoid—replaces it. Isaac Newton showed in 1687 that a rotating self-gravitating fluid in equilibrium forms an oblate ellipsoid, confirmed by 18th-century geodetic measurements along meridian arcs.
Many reference ellipsoids exist, so latitude is not unique without specification. GPS devices use WGS84, with equatorial radius 6,378,137.0 m and inverse flattening 298.257223563. The Eiffel Tower's geodetic latitude on WGS84 is 48.8583° N, but the same coordinates on datum ED50 mark a point 140 metres away. Handsets transform WGS84 into local datums automatically.
On a sphere, the equator is 0°, the North Pole 90°N, the South Pole 90°S. Four other parallels matter: tropical circles at axial tilt i and polar circles at 90° − i. Only between the tropics can the Sun stand directly overhead at the zenith. Map projections distort how parallels and meridians appear; on Mercator they are horizontal and vertical, on Transverse Mercator both become complicated curves.
Source: Latitude
