def METHOD_NAME(self, dem_parameter):
    '''
    Get bottom surface for each roof
    :param dem: DEM object
    '''
    self.bottomsurface = copy.deepcopy(self.topsurface)
    for i in range(0, self.surface_num):
        base_height = get_height_from_dem(self.bottomsurface[i].point_cor,
                                          dem_parameter)
        self.bottomsurface[i].point_cor[:, 2] = base_height
    for i in range(0, self.surface_num):
        for j in range(0, self.surface_num):
            if i != j:
                relationship_flag = check_relation(
                    self.topsurface[i].point_cor, self.topsurface[j].point_cor)
                if relationship_flag == 1:
                    base_height1 = get_height_from_lower_surface(
                        self.topsurface[i].point_cor, self.topsurface[j].point_cor)
                    self.bottomsurface[j].point_cor[:, 2] = base_height1