(i) Field of the Invention
The present invention relates to a vehicle-navigation system which is mounted on a vehicle and which navigates a vehicle to a destination along a route with the aid of an image display or the like.
(ii) Description of the Prior Art
In order to reduce the load of a driver during driving, various navigation devices have been investigated, and some devices have already been mounted on vehicles. Among these devices, there is a vehicle-navigation device for leading a vehicle to a destination along a route. This vehicle-navigation device has a mechanism for informing the driver of a route to be selected at a junction, such as an intersection. The device which is nowadays put to practical use is an image display device attached to an instrument panel of the vehicle, whereby the driver is informed of the route.
In such a vehicle-navigation device, a scale for a map display is set by a driver of a vehicle to display the map at the desired scale. However, setting the scale must be done by the driver, and each time the vehicle gets closer to the destination and more detailed information is required, scaling up of the map (the enlargement of the map) must be carried out by the driver.
Japanese Patent Laid-open No. 123084/1992 describes a device for automatically carrying out such an adjustment of the scale as mentioned above. In this device, the scale of the map display can be set so as to display a present position and a destination together.
However, in the conventional device, the map must be scaled down in order to display a starting point and a destination point together, when these two points are a large distance away from each other. Therefore, when the vehicle is near to the starting point, the driver must change the scale manually so as to scale up the map.
Furthermore, in searching the route, the number of roads to be searched is limited by the capacity of a memory and a processing speed. For this reason if even narrow roads are searched, it is necessary to memorize such narrow roads, and a long period of time is required to search the narrow roads. In general, the roads to be searched are limited to trunk roads such as superhighways, toll roads, national roads and prefectural roads. Therefore, if no trunk road is present in the vicinity of the starting point, a route search can begin at a point that is distant from the starting point on occasions. In a case, the conventional device displays the map at such a low scale so as to include the destination. However, the route from the present position to the route search beginning point is required by the driver, but in the conventional device, the scale of the map is so low that the above-mentioned route cannot be easily recognized.