Abstract:
A map data distribution server that is connected to a network and distributes map data via the network is provided. The map data distribution server holds first map data and second map data that is of a newer version than the first map data and holds a plurality of update data including an instruction to update the first map data for each of partitions of a given size on a map. The map data distribution server decides whether a predetermined condition is fulfilled upon receiving an update request to update the first map data in a first partition out of the partitions to the second map data from a terminal communicating with the map data distribution server via the network, selects one of the plurality of update data relevant to the first partition, based on the result of the decision, and transmits the selected update data to the terminal.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2011-160047 filed on Jul. 21, 2011, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND 
       [0002]    The present invention relates to a technique for providing a service using map data to a user and, in particular, to a technique for distributing data to update map data stored on a terminal device. 
         [0003]    Services using map data such as a navigation service for motor vehicles or pedestrians are widely provided. Map data is normally displayed on the screen of a terminal possessed by a user who receives such a service. Besides, by displaying various kinds of additional information overlaying the map, convenience of users can be improved. A typical example of additional information is so-called VICS (Vehicle Information and Communication System, a registered trademark) information, that is, information on traffic congestion and restriction among others. Other examples of additional information that is beneficial for users are information on facilities recommended to users, weather information (for example, information on a road that may be flooded due to a so-called guerilla-like torrential rain or the like), etc. 
         [0004]    On the other hand, map information stored on a terminal is updated to reflect a change due to new construction, abolition, or the like of a real road, facility, or the like. In this regard, although map data over a widespread area (e.g., all over Japan) may be updated in a lump, such update takes considerable time because of a large amount of data to be updated. Thus, in late years, inter alfa, a method in which update is confined to a partition for which update is required and update is performed on a per-partition basis (see, e.g., Japanese Unexamined Patent Application Publication No. 2003-65770) and a method of updating only data for differences (differences between old version map data and new version map data) for each partition are known. 
       SUMMARY 
       [0005]    When map information is updated on a per-partition basis as described above, in one page of a map displayed on a terminal, latest map information may be displayed in some regions, while old map information may be displayed in other regions. In such a case, as the above-mentioned additional information, by displaying an overlay of information indicating a region where an old version of map is displayed and information showing a difference between new and old versions of map information (for example, information showing the position and shape or the like of a road that has newly been opened to traffic) in a region where an old version of map is displayed, among others, it is possible to induce the user to update the map. 
         [0006]    In order to provide such additional information so that various kinds of terminals existing in the market can display that information in an overlay manner, it is necessary to adopt a common reference method in which such information can be handled by any of these terminals. For example, it is easily implemented to display an overlay of additional information based on coordinates (e.g., latitudes and longitudes) (for example, additional information is associated with coordinate values indicating a position where it should be displayed on the map and the terminal displays the additional information in the position indicated by the coordinate values). This is because the latitudes and longitudes and the like can be used in common by terminals such as general navigation devices. Thus, as for information that is displayed based on coordinates, such as information on facilities recommended to users, a terminal&#39;s function that displays information downloaded from a map sensor or Web site is implemented. 
         [0007]    However, if the method of displaying additional information based on coordinates is adopted, additional information having a shape or an area, such as a road or a specific region, cannot be correctly overlaid on the map in some cases. For example, the shape of a road that is defined by a map stored on a terminal may differ depending on the map version. Hence, for example, in a case where traffic congestion information that is displayed so as to overlay a road is associated with coordinate values indicating the shape of the road on which it should be displayed, the traffic congestion information will be displayed in a position out of the road, unless the shape defined by the coordinate values matches the shape of the road defined by the map data stored on the terminal. The same may occur with the shape or the like of a specific region. In order to prevent this, it is desirable that additional information is associated with information identifying a road link or a region (e.g., a prefecture or the like) on the map (for example, typically, a road link ID) stored on the terminal. 
         [0008]    However, it is difficult in practice to display additional information based on such association. This is because, for example, a road link ID corresponding to a section of the same road differs for each type of map stored on each terminal and information on the position and shape of a region such as a prefecture is not assigned even information identifying them. 
         [0009]    Such a technique has already been developed that, for example, an information center provides traffic congestion information associated with a road link ID and a terminal displays an overlay of the traffic congestion information on the map. This is implemented by providing an individual terminal with a conversion table that maps between road ink IDs which are transmitted from the information center and road link IDs in the map data stored on the terminal. Furthermore, even after a road link is added or deleted in a new version of map data to reflect a change of a real road, the information center continues to provide traffic congestion information associated with a road link ID in an old version in order to allow the traffic congestion information to be displayed on an old version of map for a certain period even after the version is replaced by a new one. Consequently, the cost for implementing such a method increases. 
         [0010]    On the other hand, shape information on a prefecture or the like can also be updated on a per-parcel basis. Here, the parcel is a manageable unit of map data, which corresponds to a partition of a given size. However, in this case, because it is necessary to update all background information such as building shapes, green spaces, and rivers, for each of a large number of parcels, the amount of update data increases to a great extent. 
         [0011]    Alternatively, a method is also conceivable in which overlay information of a distinct scheme, not based on map data on the terminal, is transmitted from the map center or the like to the terminal. However, in this case, the terminal needs to have a plurality of display systems that completely differ from each other as applications and, thus, the cost of this method also increases. 
         [0012]    The present invention has been contrived in view of the problems discussed above and is intended to provide additional information at low cost and in an easy-to-use form. 
         [0013]    A typical aspect of the present invention is outlined as follows. A map data distribution server that is connected to a network and distributes map data via the network is provided. The map data distribution server includes an interface connected to the network, a processor connected to the interface, and a storage device connected to the processor. The map data distribution server holds first map data and second map data that is of a newer version than the first map data and holds a plurality of update data including an instruction to update the first map data for each of partitions of a given size on a map. The map data distribution server decides whether a predetermined condition is fulfilled upon receiving an update request to update the first map data in a first partition out of the partitions to the second map data from a terminal communicating with the map data distribution server via the network, selects one of the plurality of update data relevant to the first partition, based on the result of the decision, and transmits the selected update data to the terminal. 
         [0014]    According to an aspect of the present invention, it is possible to provide additional information that is displayed on a map in an overlay manner at low cost and in an easy-to-use form. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram representing an architecture of a map data distribution system according to an embodiment of the present invention; 
           [0016]      FIG. 2A  is a diagram illustrating a table mapping between parcel ID and prefecture code, held by a map distribution server in the embodiment of the present invention; 
           [0017]      FIG. 2B  is a diagram illustrating a table mapping between terminal ID and prefecture code for which terminal has update right, held by the map distribution server in the embodiment of the present invention; 
           [0018]      FIG. 2C  is a diagram illustrating a table mapping between parcel ID and update element, held by the map distribution server in the embodiment of the present invention; 
           [0019]      FIG. 3A  is a diagram illustrating a table mapping between terminal ID and prefecture code for which terminal has update right, held by the map distribution server in the embodiment of the present invention; 
           [0020]      FIG. 3B  is a diagram illustrating a table mapping between prefecture code and update element for hatching, held by the map distribution server in the embodiment of the present invention; 
           [0021]      FIG. 4  is a diagram illustrating an update element management table held by the map distribution server in the embodiment of the present invention; 
           [0022]      FIG. 5  is a first flowchart of a map data update process that is performed in the map data distribution system of the embodiment of the present invention; and 
           [0023]      FIG. 6  is a second flowchart of a map data update process that is performed in the map data distribution system of the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the following, an embodiment of the present invention will be described in conjunction with the drawings. 
         [0025]      FIG. 1  is a block diagram representing an architecture of a map data distribution system according to an embodiment of the present invention. 
         [0026]    The map data distribution system of the present embodiment includes a map distribution server  10  connected to a network  40  and a navigation terminal  20  that communicates with the map distribution server  10  via the network  40 . 
         [0027]    The map distribution server  10  is a computer including a processor  17 , a memory  18 , a network interface (I/F)  15 , a management data storage unit  11 , an update data storage unit  12 , and a map data storage unit  16  interconnected with each other. 
         [0028]    The processor  17  executes programs stored in the memory  18 . 
         [0029]    The memory  18  is, for example, a semiconductor memory and stores, inter alfa, programs that are executed by the processor  17 , data that is referenced by the processor  17 , and data that has been output from the processor  17 . At least, an update data extraction module  13  and an update data management module  14  which are programs to be executed by the processor  17  are stored in the memory  18  in the present embodiment. Processes that are implemented by executing the programs by the processor  17  will be discussed later. In the following description, as for wording that, for example, the update data extraction module  13  executes a process; actually, the processor  17  executes the process according to a program such as the update data extraction module  13 . 
         [0030]    The network I/F  15  is connected to the network  40  for communication with the navigation terminal  20  among others. 
         [0031]    The management data storage unit  11 , update data storage unit  12 , and map data storage unit  16  each may be independent storage devices like hard disk drives (HDD) or flash memories or may be allocated storage areas within a single storage device. 
         [0032]    Map data is stored in the map data storage unit  16 . The map data includes coordinate values and the like that represent the positions and shapes of components (planimetric features) of a map. Real planimetric features are updated occasionally. One example hereof is updating a road network such as new construction, abolition, or replacement of a road. The map data may include information on planimetric features at a point of time in the past and information on planimetric features at a later point of time (in other words, old version map data and new version map data). 
         [0033]    Generally, map data is managed in units of rectangular partitions of a given size. These rectangular partitions are called parcels or meshes. Each parcel is assigned a unique identifier (parcel ID). 
         [0034]    Map update data in units of planimetric features (including map information and road network information) is stored in the update data storage unit  12 . For example, if a navigation terminal  20  has a certain version of map data, the map update data is data that is created and transmitted to update this map data to a new version of map data and, in particular, includes at least data for differences between these versions. Specifically, an update element including at least information instructing to update map data is stored in the update data storage unit  12 . 
         [0035]    Map data and update elements will now be explained. 
         [0036]    Map data is used for a navigation terminal  20  to search for a route, display a map and a searched route, and guide the user (driver) along the searched route. Principal data included in map data is road data, background data, and guidance data. The road data describes a road network by connections between nodes which correspond to intersections and links which correspond to road sections connecting the intersections. This data is used for a route search and displaying roads on screen by the navigation terminal  20 . The background data mainly represents the positions and shapes or the like of planimetric features other than roads (e.g., green spaces, ponds, rivers, railway tracks, buildings, etc.). This data is used for displaying a map on screen by the navigation terminal  20 . Specifically, the background data includes data such as points, lines, or polygons that represent individual planimetric features and may further include text that describes the name of a place, building, or the like. The guidance data includes information that is displayed on the screen of the navigation terminal  20 , as required to guide the user of the navigation terminal  20 ; for example, information representing a detailed shape of an intersection. 
         [0037]    All of the road data, background data, and guidance data in the present embodiment may be basically the same as used in existing navigation systems and, therefore, detailed description thereof is dispensed with. However, in some cases, the background data in the present embodiment may include additional information which will be described later. 
         [0038]    An update element includes differences between old version map data and new version map data and, more specifically, information instructing to update the map data from an old version to a new version. For example, as information instructing to update road data, an update element may include information that instructs to add nodes and links corresponding to a newly constructed road, delete links and nodes corresponding to an abolished road, and change the positions and shapes or the like of links and nodes corresponding to a replaced road. Further, as information instructing to update background data, an update element may include information that instructs to add an object (e.g., a graphic symbol such as a point, line, or polygon) showing a newly constructed planimetric feature (e.g., a building or the like) on the map, delete an object showing a removed planimetric feature, and alter an object showing a planimetric feature whose shape or the like was altered. Description on an update element to update guidance data is dispensed with. 
         [0039]    Update elements are normally managed parcel by parcel. For example, when the navigation terminal  20  transmits a map distribution request for a parcel to the map distribution server  10 , the map distribution server  10  transmits an update element relevant to the parcel back to the navigation terminal  20  ( 52 ). The navigation terminal  20  updates the map data for the parcel according to the received update element. 
         [0040]    However, as for road data, in some cases, parcel by parcel update may result in a break or the like of a road at the boundary of a parcel. Updating only a portion of the nodes and links corresponding to a continuous road may result in a situation that normal data reference cannot be made because of inconsistency between updated data and data not updated. Hence, as for road data, update may be performed for each road that is continuous, instead of parcel by parcel update. In each case, information instructing to update background data is managed parcel by parcel. Accordingly, the present embodiment is applicable in each case. 
         [0041]    Information for management of map update data is stored in the management data storage unit  11 . For example, inter alia, information indicating a version of map update data (that is, a generation of difference) and a parcel ID indicating a position where the map update data should be applied are stored in the management data storage unit  11 . Besides, in the management data storage unit  11  in the present embodiment, at least, a table mapping between parcel ID and prefecture code  200 , a table mapping between terminal ID and prefecture code for which terminal has update right  220 , a table mapping between parcel ID and update element  240 , a table mapping between terminal ID and prefecture code for which terminal has update right  300 , and a table mapping between prefecture code and update element for hatching  320  are stored. Details on these tables will be described later (see  FIGS. 2A-2C  and  FIGS. 3A and 3B ). 
         [0042]    The navigation terminal  20  is a mobile terminal that performs, inter alia, a route search and showing the search result to guide the user to a destination. For example, it is an in-vehicle navigation terminal mounted in a motor vehicle or a pedestrian navigation terminal carried by a pedestrian. In the latter case, the navigation terminal  20  may be a mobile phone or the like with application software for navigation embedded in it. 
         [0043]    In general, a map data distribution system includes a plurality of navigation terminals  20  which are, however, not shown in  FIG. 1 . Although, in the present embodiment, the navigation terminal  20  is shown as a typical example of a terminal device, the present invention can also be applied to any type of terminal device that stores and uses map information. 
         [0044]    The navigation terminal  20  in the present embodiment includes a processor  28 , a memory  29 , a network I/F  21 , a screen input/output I/F  25 , a management data storage unit  26 , and a map data storage unit  27 . 
         [0045]    The processor  28  executes programs stored in the memory  29 . 
         [0046]    The memory  29  is, for example, a semiconductor memory and stores, inter alia, programs that are executed by the processor  28  and data that is referenced by the processor  28 . In the memory  29  in the present embodiment, a distribution request module  22 , a map update module  23 , and a navigation processing module  24  which are programs to be executed by the processor  28  are stored. 
         [0047]    The distribution request module  22  transmits a map update data distribution request to the map distribution server  10 . The map update module  23  updates map data stored in the map data storage unit  27 , using map update data distributed. These processes will be described later (see  FIG. 5  and  FIG. 6 ). In the following description, as for wording that processes are executed by the distribution request module  22 , map update module  23 , and navigation processing module  24 ; actually, the processor  28  executes the processes according to the respective programs. 
         [0048]    The navigation processing module  24  searches for a route to a destination and guides the user along the route, based on map data stored in the map data storage unit  27 . Such route search and guidance can be performed by a publicly known method as in an existing in-vehicle navigation terminal and, therefore, detailed description thereof is dispensed with. 
         [0049]    The screen input/output I/F  25  includes a screen on which map information or the like is displayed to the user and an input device that accepts input from the user. The screen input/output I/F  25  may be comprised of, for example, a liquid crystal display device and a keyboard or the like or may be a liquid crystal display device with a touch pad. 
         [0050]    The network I/F  21  is connected to a radio communication device  30 . The radio communication device  30  performs radio communication with a radio base station  31  connected to the network  40 . The network I/F  21  performs communication with the map distribution server  10  via the radio communication device  30 , radio base station  31 , and network  40 . Transmitting a map data distribution request and receiving map update data, which will be described later, are implemented by this communication. 
         [0051]    The radio communication device  30  may be built in the navigation terminal  20 . 
         [0052]    The management data storage unit  26  and the map data storage unit  27  each may be independent storage devices like hard disk drives (HDD) or flash memories or may be allocated storage areas within a single storage device. 
         [0053]    Map data that is referenced by the navigation processing module  24  is stored in the map data storage unit  27 . This map data is updated according to map update data transmitted from the map distribution server  10 . 
         [0054]    Information for management of map data is stored in the management data storage unit  26 . In the management data storage unit  26 , for example, inter alia, information mapping between latitude/longitude and parcel ID, which indicates a parcel to which the position of the navigation terminal  20  itself belongs, and information identifying map update data applied to the map are stored. 
         [0055]    The network  40  is connected to the Internet  41 . The map distribution server  10  may acquire a new version of map data via the Internet. 
         [0056]      FIG. 2A  is a diagram illustrating a table mapping between parcel ID and prefecture code  200 , held by the map distribution server  10  in the embodiment of the present invention. 
         [0057]    The table mapping between parcel ID and prefecture code  200  contains entries of parcel ID  201  and prefecture code  202 . 
         [0058]    The parcel ID  201  is information identifying each parcel. 
         [0059]    The prefecture code  202  is a code identifying a prefecture that falls within each parcel. As the prefecture code  202 , for example, prefecture codes defined by Japanese Industrial Standards X0401 may be used or other identifiers may be used. 
         [0060]    In the example of  FIG. 2A , entries “10”, “11”, and “12” for the prefecture code  202  corresponding to a value “0001” of the parcel ID  201  are registered. This means that at least a part of a prefecture identified by the prefecture code “10”, at least a part of a prefecture identified by the prefecture code “11”, and at least a part of a prefecture identified by the prefecture code “12” fall within a parcel identified by the parcel ID “0001”. 
         [0061]      FIG. 2B  is a diagram illustrating a table mapping between terminal ID and prefecture code for which terminal has update right  220 , held by the map distribution server  10  in the embodiment of the present invention. 
         [0062]    The table mapping between terminal ID and prefecture code for which terminal has update right  220  contains entries of terminal ID  221  and a list of prefectures for which the terminal has an update right  222 . 
         [0063]    The terminal ID  221  is information identifying each navigation terminal  20 . 
         [0064]    The list of prefectures for which the terminal has an update right  222  is a list of codes identifying prefectures for which each navigation terminal  20  has an update right of map data. For example, if the user of a navigation terminal  20  purchases an update right of map data for a prefecture, a code identifying the prefecture for which the user purchased the update right is added to the list of prefectures for which the terminal has an update right  222  corresponding to the terminal ID  221  of the navigation terminal  20 . In the example of  FIG. 2B , a code identifying one prefecture is registered in the list of prefectures for which the terminal has an update right  222  corresponding to each terminal ID  221 ; however, actually, list consisting of codes identifying a plurality of prefectures may be registered for each terminal. 
         [0065]    In the present embodiment, an update right of map data is managed on a per-prefecture basis (that is, the user can purchase an update right of map data for each prefecture). The table mapping between parcel ID and prefecture code  200  ( FIG. 2A ) and the table mapping between terminal ID and prefecture code for which terminal has update right  220  ( FIG. 2B ) are referenced in order to decide whether the update requesting terminal has an update right for a prefecture falling within a parcel for which an update was requested. However, an update right of map data may be managed for each region (e.g., country) other than prefecture. In that case, instead of a prefecture identifying code, a code identifying a particular region is stored in the table mapping between parcel ID and prefecture code  200  and the table mapping between terminal ID and prefecture code for which terminal has update right  220 . 
         [0066]      FIG. 2C  is a diagram illustrating a table mapping between parcel ID and update element  240 , held by the map distribution server  10  in the embodiment of the present invention. 
         [0067]    The table mapping between parcel ID and update element  240  contains entries of parcel ID  241  and update element ID  242 . 
         [0068]    The parcel ID  24  is information identifying each parcel and corresponds to the parcel ID  201  in the table mapping between parcel ID and prefecture code  200  ( FIG. 2A ). 
         [0069]    The update element ID  242  is information identifying an update element that is applied to update the map data of each parcel from an old version to a new version. An update element corresponding to each update element ID will be described later with reference to  FIG. 4 . 
         [0070]    In the table mapping between parcel ID and update element  240 , only such an update element is registered that is transmitted to a navigation terminal  20  having an update right of map data. An update element that is transmitted to a navigation terminal  20  not having an update right of map data is registered in a table mapping between prefecture code and update element for hatching  320  which will be described later. 
         [0071]      FIG. 3A  is a diagram illustrating a table mapping between terminal ID and prefecture code for which terminal has update right  300 , held by the map distribution server  10  in the embodiment of the present invention. 
         [0072]    The table mapping between terminal ID and prefecture code which terminal has update right  300  contains entries of terminal ID  301 , a list of prefectures for which the terminal has an update right  302 , and a list of prefectures for which the terminal does not have an update right  303 . 
         [0073]    The terminal ID  301  and the list of prefectures for which the terminal has an update right  302 , respectively, correspond to the terminal ID  221  and the list of prefectures for which the terminal has an update right  222  in the table mapping between terminal ID and prefecture code for which terminal has update right  220  ( FIG. 2B ). 
         [0074]    The list of prefectures for which the terminal does not have an update right  303  is a list of codes identifying prefectures for which each navigation terminal  20  does not have an update right of map data. Specifically, codes identifying prefectures not registered in the list of prefectures for which each navigation terminal  20  has an update right  302  are registered in the list of prefectures for which the terminal does not have an update right  303 . 
         [0075]    The table mapping between terminal ID and prefecture code for which terminal has update right  300  is created based on the table mapping between terminal ID and prefecture code for which terminal has update right  220 . Thus, the map distribution server  10  does not need to hold both the table mapping between terminal ID and prefecture code for which terminal has update right  220  and the table mapping between terminal ID and prefecture code for which terminal has update right  300  at the same time. Eventually, it is sufficient for the map distribution server  10  to hold only the table mapping between terminal ID and prefecture code for which terminal has update right  300 . Alternatively, the map distribution server  10  may hold the table mapping between terminal ID and prefecture code for which terminal has update right  220  instead of the table mapping between terminal ID and prefecture code for which terminal has update right  300  and, as necessary, may create a list that corresponds to the list of prefectures for which the terminal has an update right  302  based on the list of prefectures for which the terminal has an update right  222 . 
         [0076]      FIG. 3B  is a diagram illustrating a table mapping between prefecture code and update element for hatching  320 , held by the map distribution server  10  in the embodiment of the present invention. 
         [0077]    The table mapping between prefecture code and update element for hatching  320  contains entries of prefecture code  321  and an update element ID list  322 . 
         [0078]    The prefecture code  321  is a prefecture identifying code and corresponds to, for example, one of the prefecture identifying codes registered in the list of prefectures for which the terminal does not have an update right  303 . 
         [0079]    The update element ID list  322  is a list of the identifiers of update elements for hatching (that is, each of such update elements is used to update map data in a parcel that falls within a prefecture for which the navigation terminal  20  does not have an update right of map data). Details on the update elements for hatching will be described later (see  FIG. 4 ). 
         [0080]    Although the table shown in  FIG. 3B  only maps between prefecture codes and update elements for hatching, information that associates each update element for hatching with a parcel for which the update element is applied (that is, a parcel whose map data is updated according to the update element) may further be preserved in this table. 
         [0081]      FIG. 4  is an update element management table  400 , held by the map distribution server  10  in the embodiment of the present invention. 
         [0082]    Upon acquiring a new version of map data, the map distribution server  10  creates an update element for updating old version map data to the new version map data. The created update element is registered in the update element management table  400  and stored in the update data storage unit  12 . 
         [0083]    The update element management table  400  contains entries of update element ID  401  and update data  402 . 
         [0084]    The update element ID  401  uniquely identifies each update element. 
         [0085]    The update data  402  is update data contained in each update element. This update data specifically includes information instructing to add, delete, or alter an object in each parcel. A format of such update data may be the same as applied in existing navigation systems and, therefore, detailed description thereof is dispensed with. 
         [0086]    Update elements that are registered in the update element management table  400  are not only update elements that are used when a navigation terminal  20  has an update right (that is, those corresponding to the entries of the update element ID  242 ), but also update rights that are used when a navigation terminal  20  does not have an update right (that is, those listed in the update element ID list  322 ). 
         [0087]    For example, an update element ID “11” is registered in the table mapping between parcel ID and update element  240 . Thus, the update element identified by the update element ID “11” is transmitted to a navigation terminal  20  that has the relevant update right and used. On the other hand, an update element ID “140” is registered in the table mapping between prefecture code and update element for hatching  320 . Thus, the update element identified by the update element ID “140” is transmitted to a navigation terminal  20  that does not have the relevant update right and used. According to the present embodiment, for each parcel, at least two update elements, that is, an update element to be transmitted to a navigation terminal  20  that has the relevant update right and an update element to be transmitted to a navigation terminal  20  that does not have the relevant update right (that is, an update element for hatching) are prepared and either is selected and transmitted according to condition (see  FIG. 5  and  FIG. 6 ). 
         [0088]    An update element to be transmitted to a navigation terminal  20  that has the relevant update right (that is, an update right registered in the table mapping between parcel ID and update element  240 ) includes information instructing to update road data, information instructing to update background data, and information instructing to update guidance data in order to update the map data from an old version to a new version, as described previously. 
         [0089]    On the other hand, an update element for hatching, in principle, does not include information for updating the map data from an old version to a new version, because it is to be transmitted to a navigation terminal  20  that does not have an update right. That is, an update element for hatching, in principle, does not include information instructing to update road data and information instructing to update guidance data. However, as for information instructing to update background data, an update element for hatching, in principle, does not include information representing differences between old and new map data (that is, information instructing to add, delete, or alter an object corresponding to a planimetric feature), but includes an instruction to add a display showing that an update right is not granted as additional information. 
         [0090]    For example, an update element for hatching includes, inter alia, an instruction to add a graphic object showing a prefecture for which the update right is not granted. The graphic object showing a prefecture for which the update right is not granted is the one displayed to show a prefecture whose map data cannot be updated because the navigation terminal  20  that requested an update does not have its update right; for example, it is hatching displayed overlaying a region of the prefecture. Hatching may be of any kind such as, e.g., a particular color, tone, or pattern, which is displayed over the region of the prefecture. While hatching may be displayed so as to be superposed over a map that has already been displayed, it is desirable that it is displayed as an overlay of a translucent color or the like so that the map being displayed before hatched is visible. Such hatching is displayed for the purpose of inducing the user of the navigation terminal  20  to purchase the relevant update right. Accordingly, an update element for hatching may further include, inter alia, text informing the user that the update right is not granted and text advising the user to purchase the update right. 
         [0091]    In a case where a newly added road exists in a prefecture for which the update right is not granted, an update element for hatching may further include an instruction to add a graphic object showing the position and shape or the like of the road. The position and shape or the like of such a road is only displayed as a background image. Because road data about that road is not added, a route search relevant to the road cannot be performed. However, by displaying such a road, it is possible to motivate the user of the navigation terminal  20  to purchase the relevant map update right. 
         [0092]    It is also conceivable that displaying a new road in a region for which the update right is not granted as above is implemented by using road data, not background data. However, in that case, a problem arises in which road data that can be used for a route search is provided to even a navigation terminal  20  that does not have the relevant update right. Besides, in order to add road data, an update of its related road data may become necessary to maintain consistency in some cases. Hence, the above display is difficult to implement by updating road data such as adding a part of any given road, as long as an existing framework of a map data update function is used. 
         [0093]    On the other hand, as for background data, no considerations need to be taken for relations between objects (e.g., consistency or the like), as long as the identifiers of the objects included in the background data are managed not to overlap. Thus, by providing hatching to show a region for which the update right is not granted and information to display a road or the like in the region as background data, it is possible to easily implement displaying information as to whether the terminal has the update right in the framework of the map data update function that existing navigation terminals have. 
         [0094]      FIG. 5  and  FIG. 6  are flowcharts of a map data update process that is performed in the map data distribution system of the embodiment of the present invention.  FIG. 5  is a flowchart in a case when the navigation terminal  20  has an update right and  FIG. 6  is a flowchart in a case when the navigation terminal  20  does not have an update right. 
         [0095]    First, the distribution request module  22  in the navigation terminal  20  transmits a map distribution request including information identifying a parcel whose map data is to be updated (parcel ID targeted for update), the current map version stored on the navigation terminal  20 , and information identifying the navigation terminal  20  (terminal ID) to the map distribution server  10  (step  501 ). 
         [0096]    The map update data management module  14  in the distribution server  10  identifies a prefecture targeted for update by searching the table mapping between parcel ID and prefecture code  200  for a match on the parcel ID targeted for update, as the key, included in the received map distribution request (step  502 ). 
         [0097]    Then, the update data management module  14  searches the table mapping between terminal ID and prefecture code for which terminal has update right  300  for a match on the terminal ID, as the key, included in the received map distribution request. By comparing the search result and the prefecture targeted for update identified at step  502 , the update data management module  14  checks whether the terminal has an update right of map data for the prefecture targeted for update (step  503 ). 
         [0098]    As the result of the check at step  503 , if it has been decided that the navigation terminal that transmitted the map distribution request has the update right of map data for the prefecture targeted for update (step  504 ), the update data management module  14  searches the table mapping between parcel ID and update element  240  for a match on the parcel ID targeted for update as the key and obtains an update element ID associated with the parcel ID targeted for update (step  505 ). 
         [0099]    Then, the update data management module  14  searches the update element management table  400  for a match on the update element ID obtained at step  505  as the key and transmits the thus obtained update element to the navigation terminal  20  (step  506 ). 
         [0100]    The map update module  23  in the navigation terminal  20  updates the map data according to the received update element (step  507 ). Specifically, the map update module  23  performs adding, deleting, or altering an object in the parcel targeted for update according to an instruction included in the received update element. 
         [0101]    Since steps  601  to  603  in  FIG. 6  are the same as steps  501  to  503  in  FIG. 5 , respectively, their descriptions are dispensed with. Unless otherwise specified, the descriptions on the steps  501  to  503  apply as-is to the steps  601  to  603 . 
         [0102]    As the result of check at step  603 , if it has been decided that the navigation terminal that transmitted the map distribution request does not have the update right of map data for the prefecture targeted for update (step  604 ), the update data management module  14  searches the table mapping between prefecture code and update element for hatching  320  for a match on the prefecture code of the prefecture targeted for update as the key and obtains an update element ID associated with the prefecture targeted for update (step  605 ). 
         [0103]    Then, the update data management module  14  searches the update element management table  400  for a match on the update element ID obtained at step  605  as the key and transmits the thus obtained update element (that is, update element for hatching) to the navigation terminal  20  (step  606 ). 
         [0104]    Hatching to show that the update right is not granted may be displayed only over the parcel targeted for update. However, if an update right is managed on a per-prefecture basis, hatching displayed over the entire region of a prefecture for which the terminal does not have an update right is regarded as more helpful to make the user aware that the update right is not granted. Hence, if the parcel targeted for update is included in the prefecture for which the terminal does not have the update right, the update data management module  14  obtains update elements for hatching related to all parcels within the prefecture and transmits them to the navigation terminal  20 . 
         [0105]    The map update module  23  in the navigation terminal  20  updates the map data according to the received update elements (step  607 ). This procedure is the same as step  507  in  FIG. 5 . 
         [0106]    The map data update process then terminates. 
         [0107]    If a plurality of prefectures are identified as those targeted for update at step  502  and if a subset of them is judged as a prefecture for which the terminal has the update right (step  504 ), but the remaining subset is judged as a prefecture for which the terminal does not have the update right (step  604 ), the steps  505  to  507  are executed for the parcel including a prefecture for which the terminal has the update right and the steps  605  to  607  are executed for a prefecture for which the terminal does not have the update right. 
         [0108]    By the way, generally, the boundary of a parcel does not match the boundary of a prefecture. Consequently, it may happen that a plurality of prefectures fall within one parcel and a subset of them is judged as a prefecture for which the terminal has the update right and the remaining subset is judged as a prefecture for which the terminal does not have the update right (step  503 ). For example, if a navigation terminal  20  that has an update right for prefecture A, but does not have an update right for prefecture B transmits a map distribution request for a parcel targeted for update that includes the boundary between both prefectures (step  501 ), the update data management module  14  may decide that the terminal has the update right for the entire parcel targeted for update (step  503 ) and may execute the steps  505  and  506 . In this case, update elements for hatching related to the parcel targeted for update are not transmitted. 
         [0109]    Alternatively, in the above case, for the parcel targeted for update, the update data management module  14  may execute both the procedure to follow if it has been decided that the terminal has the update right (steps  505  and  506 ) and the procedure to follow if it has been decided that the terminal does not have the update right (steps  605  and  606 ). In this case, road data and background data or the like in the entire parcel are updated, according to an update element associated with the parcel targeted for update. Further, hatching or the like is displayed over prefecture B for which the terminal does not have the update right, according to an update element for hatching associated with the parcel. If the hatching is translucent, the underlying background being displayed is visible. 
         [0110]    It is undesirable that hatching continues to be displayed for a long time, even if it is translucent, since it may hinder the visibility of the underlying background being displayed. Hence, the navigation terminal  20  may terminate the display of hatching at suitable timing. Specifically, information representing a time limit of display may be included in an update element for hatching that is distributed by the map distribution server  10 . After the navigation terminal  20  starts to display hatching, according to updated map data at step  607 , it may stop the hatching display upon the elapse of the time limit of display and display the underlying map (not updated) again. 
         [0111]    At the above step  501 , a map distribution request may be transmitted at any timing. For example, it may be transmitted periodically or in response to a user&#39;s command. Alternatively, for each parcel, the navigation terminal  20  may count the frequency at which the user actually has passed across the parcel (e.g., the number of times the user has passed across the parcel for a predefined period) and may transmit an update request for the parcel when the passage frequency for the parcel has exceeded a predetermined value. 
         [0112]    Alternatively, the navigation terminal  20  may transmit a map distribution request that includes information representing the passage frequency per parcel as described above. The update data management module  14  in the map distribution server  10  having received this map distribution request, if it has been decided that the terminal does not have the update right for the parcel targeted for update (step  604 ), may further decide whether the passage frequency included in the map distribution request exceeds a predetermined value. Only if it has been decided that the passage frequency exceeds the predetermined value, the update data management module  14  may execute the steps  605  and  606 . If the passage frequency included in the map distribution request does not exceed the predetermined value, no update element is eventually selected and the map data for the parcel targeted for update is not updated. Updating map data of a prefecture that the user passes across more often is regarded as more beneficial for the user. By displaying hatching over only a region for which the passage frequency is high, it is possible to preferentially induce the user to purchase an update right that is more beneficial. 
         [0113]    While, in the foregoing embodiment, a display (e.g., hatching) or the like to show a prefecture for which the terminal does not have an update right has been discussed as an example of additional information, the present invention can also be applied to other additional information. Another example of additional information is a display of weather information. For example, the map distribution server  10 , in addition to primary update elements (that is, update elements that are registered in the table mapping between parcel ID and update element  240 ), may create a second update element including display data for showing a road having a possibility of being flooded in case of a so-called guerilla-like torrential rain occurring as background data. If weather condition fulfills a certain condition (for example, the precipitation in an area surrounding or upstream of the parcel targeted for update exceeds a predetermined value), then the map distribution server  10  may transmit the second update element to the navigation terminal  20 . 
         [0114]    According to the embodiment of the present invention described hereinbefore, by handling additional information like display data, for example, for showing a region for which the update right is not granted, as background data, it is possible to provide additional information at low cost and in an easy-to-use form by using the framework of the map data update function that existing navigation terminals have.