Calculations often represent angles by small letters of the Greek alphabet, and that way latitude will be represented by $$\lambda$$ (lambda, Greek L), and longitude by $$\phi$$ (phi, Greek F). Here is how they are defined.
Charts used in ocean navigation often use the OPPOSITE notation — $$\lambda$$ for LONGITUDE and $$\phi$$ for LATITUDE. The convention followed here resembles the one used by mathematicians in 3 dimensions for spherical polar coordinates (http://www.phy6.org/stargaze/Scelcoor.htm#q65).