1. Field of the Invention
The invention relates to a gradient information calculating system capable of acquiring road gradient information, a vehicle running control system, a navigation system, and a gradient information calculating method.
2. Description of the Related Art
Vehicles, such as hybrid vehicles, which can generate electric power through regenerative braking, or the like, are able to run with high efficiency by performing driving control according to the gradients of roads. For example, the engine power is reduced and electric power is generated on downward slopes, whereas the vehicle is driven basically with the engine, with the aid of an electric motor, on upward slopes. On the basis of this concept, it is known to control the vehicle according to an energy consumption advance plan, in which the amount of energy consumed is planned in advance, based on gradient information associated with each road, so as to further improve the fuel economy.
However, a map DB (data base) stored in a navigation system contains gradient information for only a part of the roads, such as access roads to highways, and the thus limited gradient information is not sufficient for making of the energy consumption advance plan. If it is planned to acquire gradient information of all of the links (roads) through measurements, or the like, and store them in the map DB, the cost is expected to be enormous, which makes the plan impractical.
In the meantime, the navigation system calculates vehicle location information, utilizing the GPS (Global Positioning System). The GPS enables the navigation system to detect the three-dimensional location of the vehicle, and thus enables the system to detect the elevation of the vehicle. However, the accuracy of commercially available GPS radio waves is intentionally reduced, and therefore, the GPS radio waves are not suitable for detecting the elevation. Also, it is required to capture four or more GPS satellites in principle for detecting the elevation; therefore detection of the elevation is less likely to be accomplished while the vehicle is running in urban areas.
In view of the above background, there has been proposed a technology of calculating gradient information of links by using grid elevation data with a grid spacing of 50 m (meter), which is released by the Geographical Survey Institute of Japan (see, for example, Japanese Patent No. 3985622). However, it is difficult to use the grid elevation data as they are for all roads, since the grid elevation data indicate an elevation of each of sections (meshes) into which a map is divided by latitudes and longitudes in a grid pattern, regardless of the presence or absence of a road in each section. Japanese Patent No. 3985622 discloses a navigation system that estimates gradients of roads, with reference to locations (specific point data), such as bridges or tunnels, where data of elevations, gradients, etc. of the actual roads are known, and grid elevation data.
However, the navigation system of Japanese Patent No. 3985622, which uses the specific point data stored in map data and the grid elevation data, assures the accuracy of elevations only at around the specific point data, and there is a limit to the roads for which the gradients can be accurately estimated. Namely, it is difficult to acquire accurate gradient information for roads that are spaced apart from the specific point data.