Global ionospheric maps (GIMs) are still commonly used to represent total electron content (TEC). However, a large number of permanent GPS receivers provide significant data for monitoring ionosphere. The GIMs supply accurate TEC results, although spherical models are not able to fit local ionospheric perturbations as a source of local receiver data. Therefore, the use of station-based TEC computation becomes more preferred and convenient. The station-based TEC approach is established on weighting averaging or modeling vertical TECs. Using this approach to reveal ionospheric activity depends on geometric distribution of ionospheric piercing points (IPP). The most effective parameter for examination of station-based TEC estimation is a function of relative geometry between receiver and satellite. In this study, a quality term has been used to define, for TEC estimation according to distribution of IPPs, similar to geometric dilution of precision (GDOP). The quality term has been described for the first time in this field and it was named as R-TEC (reliability of total electron content).