with x(λ0) = 0 and y(0) = 0, gives x(λ) and y(φ). The value λ0 is the longitude of an arbitrary central meridian that is usually, but not always, that of Greenwich (i.e., zero). The angles λ and φ are expressed in radians. By the integral of the secant function,NIST. See Sections 4.26#ii and 4.23#viii
