The global geopotential models do not contain the very high frequencies of the Earth’s external gravity field. This is called omission error. The omission error become more important in mountainous areas (areas with highly variable topography). This work consists in reducing the omission error (refinement) on Bouguer gravity anomalies derived from the global geopotential model EGM2008 using the spectral enhancement method. This method consists in computing the residual terrain effects and then coupling them to the gravimetric signal of the global geopotential model. To compute the residual terrain effects, we used the Residual Terrain Model (RTM) technique. A reference surface (ETOPO1) developed up to degree 2190 (maximum degree of the EGM2008 model) and a detailed elevation model (AW3D30) were needed to perform it. The computation was done with TC program of the GRAVSOFT package. The topography of the study area is taken with a constant density of 2670 kg/m3. For the inner and outer zones, the respective integration radius of 10 km and 200 km have been chosen. We obtained very important RTM values ranging from -53.59 to 34.79 mGal. These values were added to the gravity anomalies grid of the EGM2008 model to improve their accuracy at high frequencies. On a part of the Cameroon Volcanic Line and its surroundings (mountainous area), we made a comparison between the residual Bouguer anomalies before and after refinement. We get differences ranging from -37.40 to 26.40 mGal. We concluded that the impact of omission error on gravimetric signatures is more felt on areas with a high variable topography, especially on the Cameroon Volcanic Line and around the localities of Takamanda, Essu, Dumbo and Ngambe. This shows the great influence that topography has on these gravity anomalies when it is not taken into account. We can conclude that in preparing a global geopotential model, a high resolution DTM has to be used to decrease the omission error. This means the degree of expansion has to increase in order to take the higher frequencies into account. The new refined Bouguer anomalies grid can be used in addition to terrestrial gravity anomalies in the study area, especially in mountainous areas where gravimetric data are very sparse or non-existent.