In the method some existing surface of a bearing course (120) of a traffic route is first chosen as the reference level (100), in relation to which the target level (150) of a surface of a bearing course to be formed is determined. For example the upper surface of a surface layer of a traffic route can be chosen as the reference level. After this the forming procedure of a bearing course is performed, in which the surface of a bearing course is brought to the target level. Typically the forming procedure of a bearing course comprises spreading, levelling and compacting of a soil layer on the surface of the reference level. The location of a traffic route is determined in the terrain as plane coordinate points (x n , y n ) and the elevation z n of an existing bearing course chosen as the reference level is determined in each coordinate point (x n , y n ) as cross sectional figures extending from the unchanged terrain on the first side of the traffic route over the surface of the traffic route to the unchanged terrain on the second side of the traffic route. The coordinate points (x n , y n ) of the location are determined with a satellite positioning method. The measurement data is saved as spatial coordinate points (x n , y n , z nr ) into a given value data file (242), which is used in the later stages of the forming of the bearing course. The elevations z nm of the level of the upper surface of the bearing course formed and the elevations z nr of the reference level are determined in the coordinate point (x n , y n ) of the traffic route with laser scanning. The laser scanning can be performed form a vehicle (300) travelling along the traffic route and/or a construction machine participating in the forming of the bearing course, travelling along the traffic route.