What is latitude?

Globe showing the Equator.
Globe showing the Equator.

Equator
We can imagine the Earth as a sphere, with an axis around which it spins. The ends of the axis are the North and South Poles. The Equator is a line around the earth, an equal distance from both poles. The Equator is also the latitude line given the value of 0 degrees. This means it is the starting point for measuring latitude. Latitude values indicate the angular distance between the Equator and points north or south of it on the surface of the Earth.





Latitude and Longitude Video


Types of Latitude

Isometric latitude
The isometric latitude, ψ (not to be confused with the geocentric latitude notation), is very important in the development of the ellipsoidal versions of the normal Mercator projection and the Transverse Mercator projection. The name "isometric" arises from the fact that at any point on the ellipsoid equal increments of ψ and longitude λ give rise to equal distance displacements along the meridians and parallels respectively. The graticule defined by the lines of constant ψ and constant λ, divides the surface of the ellipsoid into a mesh of squares (of varying size). The isometric latitude is zero at the equator but rapidly diverges from the geodetic latitude, tending to infinity at the poles. The conventional notation is given in Snyder[2[[http://en.wikipedia.org/wiki/Latitude#cite_note-snyder-1|]]]:15:
Astronomical latitude
A fundamentally different measure of latitude is the astronomical latitude, which is the angle between the equatorial plane and the normal to the geoid (i.e. a plumb line). This is the only "latitude" that is not based on the spheroid being used to approximate the Earth's surface. It differs from the geodetic latitude only slightly (usually not more than a few thousandths of a degree) due to the slight deviations of the geoid from the reference ellipsoid.
Conformal latitude
The conformal latitude, χ, gives an angle-preserving (conformal) transformation to the sphere.
chi(phi)=2arctanleft[left(frac{1+sinphi}{1-sinphi}right)left(frac{1-esinphi}{1+esinphi}right)^{!textit{e}};right]^{1/2}-frac{pi}{2};!
chi(phi)=2arctanleft[left(frac{1+sinphi}{1-sinphi}right)left(frac{1-esinphi}{1+esinphi}right)^{!textit{e}};right]^{1/2}-frac{pi}{2};!
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This expression is sometimes given in the form
chi(phi)=2arctanleft[tanleft(frac{phi}{2}+frac{pi}{4}right)left(frac{1-esinphi}{1+esinphi}right)^{!textit{e}/2};right]-frac{pi}{2};!
chi(phi)=2arctanleft[tanleft(frac{phi}{2}+frac{pi}{4}right)left(frac{1-esinphi}{1+esinphi}right)^{!textit{e}/2};right]-frac{pi}{2};!
psi(phi)=lnleft[tanleft( frac{pi}{4}+frac{phi}{2}right) right]+frac{e}{2}lnleft[ frac{1-esinphi}{1+esinphi} right]
psi(phi)=lnleft[tanleft( frac{pi}{4}+frac{phi}{2}right) right]+frac{e}{2}lnleft[ frac{1-esinphi}{1+esinphi} right]