In this paper, we present the calculation of the effective height of the Gaisberg tower using several models in which the Gaisberg Mountain is represented by either simplified geometrical shapes (hemisphere and hemiellipsoid), or by using the actual 3D topography. The procedure to estimate the effective height based on the comparison of the electric field at the top of the tower located on flat and mountainous terrains is studied in detail for the three considered representations of the mountain. For each case, the electric field is computed numerically using the finite element method. We show that the use of the actual 3D topography of the mountain surface results in a very low value of the effective height in the range of 200 - 300 m, which results in an underestimation of the total number of flashes to the tower. The obtained results give some concerns about the applicability of this method of estimation for the effective height.