Accurate modelling of the ionosphere is crucial for GNSS-based applications, mainly positioning and navigation, to mitigate the effect of ionospheric disturbances. Increasing volumes of GNSS data, which are acquired from continuously operating terrestrial GNSS receivers, pave the way of calculating high-precision and high-resolution models of the Vertical Total Electron Content (VTEC) of the ionosphere. However, since the GNSS data distribution is rather inhomogeneous, appropriate, i.e., data adapted evaluation procedures have to be applied. Within the project OPTIMAP (Operational Tool for Ionospheric Mapping And Prediction) a global VTEC model has been developed as a series expansion in terms of tensor products of trigonometric and polynomial B-spline functions for longitude and latitude representation, respectively. This method enables (1) to estimate the unknown model parameters, i.e. the B- spline series coefficients, sequentially by the assimilation of GNSS observations using a Kalman filter, (2) to handle the inhomogeneous data distribution, i.e. data gaps, appropriately due to the localizing properties of B-spline functions and (3) to control the spectral content, i.e. the maximum degree, of the VTEC representation efficiently by performing a multi-scale analysis. This presentation focusses on the quality and the validity of OPTIMAP. First, we present results of the differenced STEC analysis applied to the OPTIMAP VTEC solutions. Then, we show how the OPTIMAP results coincide with satellite altimetry. Finally and in particular, we compare OPTIMAP VTEC maps with IGS VTEC maps.