1.  Field of the Invention
The present invention relates to a navigation system that distributes map information for informing progress of a movable body over a network, a system thereof, a method thereof, a program thereof, and a recording medium storing the program. 
2.  Description of Related Art
Communication-based navigation systems conventionally known are that acquire map information stored in sever units via communication by using mobile communication terminals such as an on-board or portable navigation apparatus, a PDA (Personal Digital Assistant), a mobile phone, a PHS (Personal Handyphone System), and a portable personal computer so as to search for information on a current position and routes to a destination location, and to retrieve information on nearby shops.  In such communication-based navigation system, since the management of map information with enormous data volume is centralized at a server unit, a mobile terminal need not to have a storage section for storing the map information.  Thus, simplification of the configuration and reduction in size and weight are easily realized while most-updated map information is readily provided.  For the purpose of smooth operation and management of the data, the map information is constituted by a plurality of digitalized map files that are divided into a plurality of areas. 
Typically, when displaying a map on a display section, the display section uses display-oriented data having element data relating to elements that constitutes the map.  In other words, the map is smoothly displayed with the use of data dedicated to display functions.  In a navigation system, when overlaying a current position of a vehicle onto a displayed map, map matching processing is performed in order to display the current position in a manner such that the vehicle travels on a road in the map.  It is known that the map matching processing is also performed, with the use of matching data where a road is represented by point information and segment information that connects the point information, in order to appropriately correct the current position on the basis of the matching data and the current position of the vehicle separately measured and overlay the corrected current position onto the map displayed by the display-oriented data. 
By the way, when traveling according to a travel route, the travel route acquired from the server unit often needs to be changed because of constructions, traffic accidents or traffic jams.  With an arrangement where the server unit re-searches for a travel route each time the travel route is changed, since it takes time to acquire the re-searched route, prompt and smooth navigation might not be provided.  There is a well-know arrangement to overcome this disadvantage in which a server unit distributes an optimum travel route as well as other travel routes in advance considering every points that might cause route changes to eliminate the needs for rerouting at the server unit upon each route change. ( See Patent Document 1). 
The server unit according to the Patent Document 1 acquires information on a current position and a destination from a terminal unit and searches for the optimum travel route.  Then, the server unit recognizes intersections on which route changes might be occurred and searches for travel routes from the intersections to the destination.  The server unit sends to a terminal unit the optimum travel route and other travel routes which can be used when the vehicle deviates from the optimum travel route.  The terminal unit travels according to the optimum travel route.  When the terminal unit deviates form the travel route, the navigation is continued on the basis of the other routes acquired in advance. 
[Patent Document 1]
Japanese Translation of a PCT Application 2001-519029 (Pages 6 to 28)
However, the server unit according to the Patent Document 1 distributes the optimum travel route and the plurality of travel routes, which may not be in actual use, to be used when the terminal unit deviates from the travel route.  Thus, a communication load as well as communication time are increased while the terminal unit is required to have an ability to receive and process enormous volume of information.  These facts result in disadvantages such as limitation of available types of the terminal unit.