In a cognitive radio system, awareness of the prevailing radio environment of the primary network, such as digital terrestrial television, is important since it governs the emission limits for the secondary system devices. Field strength estimates of the primary network are typically computed using terrain based radio propagation models, or by using univariate interpolation techniques to approximate the field strength from measurement samples. However, the limited geographical information or small numbers of measurement samples sets a restriction on the accuracy of the field strength estimates. In this paper, we propose a multivariate interpolation method for incorporating field measurements into radio propagation models. The proposed method shows considerable increase in estimation accuracy compared to previously presented methods. The model is verified using field measurement data from an operational digital terrestrial television network.