A three-dimensional ionospheric electron density (Ne) model (L.-C. Tsai, C. H. Liu, T. Y. Hsiao, and J. Y. Huang, Radio Sci., 44, doi:10.1029/2009RS004154, 2009) has been named the TaiWan Ionospheric Model (TWIM) and constructed from vertical Ne profiles retrieved from FormoSat3/COSMIC GPS radio occultation measurements. We further improved the TWIM by including global distributed ionosonde foF2 and foE data and qualifying COSMIC Ne profiles based on the empirical orthogonal function (EOF) classification. The improved TWIM also exhibits vertically-fitted α-Chapman-type layers, with distinct F2, F1, E, and D layers, and surface spherical harmonics approaches for the fitted layer parameters including peak density, peak density height, and scale height. These results are useful in investigation of near-Earth space and large-scale Ne distribution. This way the continuity of Ne and its derivatives is also maintained for practical schemes for providing reliable radio propagation predictions. We have presented a numerical and step by step ray-tracing method (L.-C. Tsai, C. H. Liu, and J. Y. Huang, Radio Sci., 45, doi:10.1029/ 2010RS004359, 2010) on the TWIM and including an Earth-centered magnetic dipole and horizontal Ne gradient effects. The ray-tracing methodology has potential applicability to ionospheric correction as applied to GPS positioning. In this paper, ionospheric corrections on both single-receiver and double-receiver L1 GPS code measurements using the TWIM have been presented and evaluated. Its performances with respect with the ionospheric corrections using other ionospheric models are also presented and compared.