Various navigational functions, involving active safety, driver assistance and location-based services require data that is not considered to be part of a map database and is likely supplied by a vendor other than that of the map provider. Such data needs to be cross-referenced with the entities and attributes of the main database. However, since the auxiliary data is not necessarily compiled with the main database some other means is needed to establish cross-referencing, which is referred to as attaching the auxiliary data. Two common approaches are function-specific referencing tables and generic referencing.
Function-specific referencing tables
Function-specific referencing tables provide a means for attaching function-specific data to a map-data base produced by any participating supplier. Such a table is collaboratively produced to support a specific function or class of functions involving location-based service, active-safety or advanced driver assistance. It will generally consist of a list of map elements of a specific type (e.g., links, intersections, point-of-interest locations, etc.) along with identifying attributes (e.g., street names, longitude/latitude coordinates, etc.). Additionally, each entry in the table is assigned a unique identifier. The set of entries in a table are generally selected, through consensus of all interested parties. As a practical matter the result will represent a small subset of the elements of the given type that are available in the map databases and will consist of those that are more important to the application area. After a table is formulated, it is the task of each participating supplier to determine and cross-reference the elements in their map-database that correspond to the table entries.
Figure 2: TMC locations in Metro Detroit
A widely used example is the TMC standard for location-code tables for referencing traffic data. TMC, which stands for Traffic Message Channel[8], is part of the Radio Data System (RDS), which is implemented as a sub-carrier modulation of a commercial FM broadcast signal. The TMC tables primarily provide references to point locations along major roads corresponding to intersections with other roads. A table entry identifies a point location using both contextual information (such as, region, road and section of road, name of intersection) and approximate longitude/latitude coordinates.
Identifiers assigned to entries in a table are 16-bit integers and therefore have a range of 65536 values. This is too few to cover the world, so separate tables are formulated for each country or region of a country. For a given metropolitan region, only intersections along freeways, arterials and some major roads are included. This is illustrated in the following figure for the Detroit metro area. The coverage is intended for providing traffic advisory information on high-use roads. Traffic-based route planning, on the other hand, requires coverage of all or almost all major roads and, therefore, is not adequately supported by TMC location code tables as they are currently formulated.
Generic referencing
Generic referencing is an attempt to attach data to any map database by discovering reference information through a form of map matching. The function-specific data items are assigned to elements, such as points, links or areas, that likely only approximate the corresponding map elements in a specific map database. A search of the map database is made for the best fit. To enhance the search process neighboring elements are strategically appended to each given element to help ensure that the correct solution is found in each case. For example, if the map element is a link connecting two intersections, then one or both cross streets could be appended for the sake of the search. Hopefully, this makes an incorrect match unlikely. Although the procedure is quite heuristic, a proposed standard called Agora outlines the strategy for choosing neighboring elements to append.
Function-specific referencing tables
Function-specific referencing tables provide a means for attaching function-specific data to a map-data base produced by any participating supplier. Such a table is collaboratively produced to support a specific function or class of functions involving location-based service, active-safety or advanced driver assistance. It will generally consist of a list of map elements of a specific type (e.g., links, intersections, point-of-interest locations, etc.) along with identifying attributes (e.g., street names, longitude/latitude coordinates, etc.). Additionally, each entry in the table is assigned a unique identifier. The set of entries in a table are generally selected, through consensus of all interested parties. As a practical matter the result will represent a small subset of the elements of the given type that are available in the map databases and will consist of those that are more important to the application area. After a table is formulated, it is the task of each participating supplier to determine and cross-reference the elements in their map-database that correspond to the table entries.
Figure 2: TMC locations in Metro Detroit
A widely used example is the TMC standard for location-code tables for referencing traffic data. TMC, which stands for Traffic Message Channel[8], is part of the Radio Data System (RDS), which is implemented as a sub-carrier modulation of a commercial FM broadcast signal. The TMC tables primarily provide references to point locations along major roads corresponding to intersections with other roads. A table entry identifies a point location using both contextual information (such as, region, road and section of road, name of intersection) and approximate longitude/latitude coordinates.
Identifiers assigned to entries in a table are 16-bit integers and therefore have a range of 65536 values. This is too few to cover the world, so separate tables are formulated for each country or region of a country. For a given metropolitan region, only intersections along freeways, arterials and some major roads are included. This is illustrated in the following figure for the Detroit metro area. The coverage is intended for providing traffic advisory information on high-use roads. Traffic-based route planning, on the other hand, requires coverage of all or almost all major roads and, therefore, is not adequately supported by TMC location code tables as they are currently formulated.
Generic referencing
Generic referencing is an attempt to attach data to any map database by discovering reference information through a form of map matching. The function-specific data items are assigned to elements, such as points, links or areas, that likely only approximate the corresponding map elements in a specific map database. A search of the map database is made for the best fit. To enhance the search process neighboring elements are strategically appended to each given element to help ensure that the correct solution is found in each case. For example, if the map element is a link connecting two intersections, then one or both cross streets could be appended for the sake of the search. Hopefully, this makes an incorrect match unlikely. Although the procedure is quite heuristic, a proposed standard called Agora outlines the strategy for choosing neighboring elements to append.
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