A method is given for producing world maps of plasma frequency that includes variation with height above the surface of the earth as well as variations with latitude, longitude, and local mean time. The maps, based on a numerical analysis of ionospheric data, are expressed in a functional form suitable for computer calculation of the value of plasma frequency and any of its partial derivatives at a given position and instant of time. The representation of vertical profiles of plasma frequency is based on a technique introduced by Lanczos for accelerating the convergence of trigonometric approximations. Diurnal and geographic variations are represented by methods previously developed for ionospheric mapping. An illustration is given using the monthly average value of plasma frequency for November 1966 from vertical profiles at 36 stations.