def get_element_centroids(self):
        """return the central points of all elements

        Returns
        -------
        Nx2 array
            x/z coordinates for all (N) elements
        """
        centroids = np.vstack((
            np.mean(self.grid['x'], axis=1), np.mean(self.grid['z'], axis=1)
        )).T

        return centroids