The geoid model represents part of the national coordinate system. It can be used for the purpose of GNSS-levelling, but the use of geoid heights also improves incorporation of terrestrial observations into the state coordinate system. In GNSS levelling tasks, geoid heights are obtained from the geoid model, but with terrestrial observations the deflection of the vertical is also needed. Determination of geoid heights from the geoid model is a simple engineering task; however, determination of deflection of the vertical is not so common in geodetic practice. The purpose of this paper is to present the local method of establishing the deflections of vertical with the help of a plane, which is calculated on the basis of interpolated geoid heights. The coefficients of the plane give the deflection of the vertical in the point of gravity. This means that, given a known geoid, we can calculate the deflection of the vertical at any point in the region of Slovenia. Comparison of calculated deflections with the measured deflections was performed in order to estimate the accuracy of the proposed procedure. The procedure was tested in the geodetic network with four points.