In the UML model, two different walkthroughs lead to information about location.
ALERT-C location referencing system
This section describes the ALERT-C referencing system and gives details of the contents of ALERT-C tables. Since this version of the DATEX II specification does not include the exchange of ALERT-C Location tables this is provided as general support information.
Note
Reminder: An ALERT-C localisation is a tabular address defining a locating element belonging to this table.
Only the reference is transmitted between the Supplier and the Client. It is up to the client to look it up in the table for the descriptive elements corresponding to the referenced location using the ALERT-C code. To have a unique ALERT-C coding at the European level, the reference must comprise three elements (triplet):
- Country Code,
- Table number,
- Location number
Two distinct locations at the European level cannot have the same triplet.
Warning
Caution: currently certain codes for country table references were not allotted; they are thus invalid.
It is the European standard EN ISO 14819-3 that defines the rules of constitution of the ALERT-C location tables.
The following table gives indications on the contents of the various common attributes used by the classes related to the ALERT-C location.
Code |
Definition |
Possible values |
alertCLocationCountryCode |
Country code defined by RDS (IEC 62106). It uses only 15 values (1 hexadecimal digit: 1 to 9 and A to F). It doesn’t identify a specific country because values are shared by several countries. |
F (France)1 (Germany)
1 (Germany)
E (Spain)
5 (Italy) etc…
etc…
|
alertCLocationTableNumber |
Number allowed for a country CountryCode + TableNumber fully identifies the table |
17 to 32 (France)
17 to 24 (Spain)
1 to 16 (Italy)
5 (Italy) etc…
etc…
|
alertCLocationTableVersion |
Version number of the ALERT-C location table in a country. |
|
alertCDirectionCoded |
It is the concerned direction. Positive direction corresponds to the direction used when following positive linkage in the table |
both
negative
positive
unknown
|
alertCDirectionNamed |
String for the direction name |
|
alertCDirectionSense |
Indicates for circular routes (i.e. valid only for ring roads) the sense in which navigation should be made from the primary location to the secondary location, to avoid ambiguity. The value TRUE indicates the positive RDS direction, i.e. direction of coding of road. |
true (ALERT-C positive direction)
false (ALERT-C negative direction)
|
alertCLocationName |
Location name (redundant because already in the table) |
|
offsetDistance |
The non-negative offset distance from the ALERT-C referenced point to the actual point. |
|
specificLocation |
Unique code within the ALERT-C location table which identifies the specific point, linear or area location. |
Number between 1 and 63487 |
Examples can be found here.
Alert-C Point location
A point will be used to locate certain phenomena where dimension length is negligible such as for example accidents, incidents (broken down vehicles), obstacles, …. DATEX II proposes two methods of localization for the points, called method 2 and method 4 (the previous DATEX method 1 and method 3 have been abandoned):
- In method 2, the point is defined by the tabular address of the ALERT-C point;
- In method 4, the point is defined by the tabular address of the ALERT-C point, supplemented by the distance between this ALERT-C point and the point to be located.
In an ALERT-C location table an ALERT-C point includes a type and a point name as well as the name of intersecting roads at the point. Moreover, a reference towards the road (or the segment of road which carries it) is systematically added to the point and makes it possible to attach the name/number of road to this point. Consequently, all points are attached to a road (and only one).
Alert-C Linear location
Linear will be used to locate certain phenomena or operator actions where dimension length is relevant, such as for abnormal traffic flow (queues, …), the bad condition of roads (e.g. road surface), …
DATEX II proposes three methods of localization for linear:
- In method 2, the linear one is defined by two points (primary location and secondary location), each point being defined only by the tabular address of the corresponding ALERT-C point;
- In method 4, the linear one is defined by two points (primary location and secondary location), each point being defined by the tabular address of the corresponding ALERT-C point, supplemented by the distance between this ALERT-C point and the end of the linear location.
- In method “linear by code”, the linear is defined only by the tabular address of the corresponding linear ALERT-C location (road, street or segment).
The primary location is the nearest defined downstream point (for methods 2 or 4). In an ALERT-C location table an ALERT-C linear location includes a type, the names of the origin and end of linear section as well as the associated road number/name.
Alert-C Area location
In an ALERT-C location table an area location includes a name and its type can be found starting from the contents of the location table.
It can be used to locate certain surface phenomena like weather phenomena. It can also be used to provide an indication of the destination (when it corresponds to a city for example).
Gml
The package “Gml” includes four class with their attributes for the definition of a linear location which specializes a location of a traffic object. This method complies with EN ISO 19136.
GmlLinear
The class “GmlLineString” instantiates a GML line string and has three attributes:
- “posList” which contains the ordered list of the two- or three-dimensional point coordinates,
- “srsName” which specifies the Coordinate Reference System (CRS) used to interpret the coordinates
- “srsDimension” which provides the number of coordinates for each point belonging to the LineString. This value is either 2 or 3. If it is not provided, the default value is “2”.
Generally, the sequence in the coordinates is “latitude” then “longitude”.
The order of the points forming the line string provides the direction of the geographic feature.
GMLArea
The class “GmlMultiPolygon” instantiates a case of GML_MultiSurface where all the surfaces belonging to the set are polygons (class “GmlPolygon”). When this area is defined by several polygons (and not only one), each of them may be associated with a name (attribute “gmlAreaName”) to distinguish them.
Each polygon is defined by an exterior linear ring and zero to several interior linear rings that define interior patches. A linear ring (class “GmlLinearRing” equivalent to GM_LinearRing) is a specific, not self-intersecting closed curve derived from a line string, i.e. where the last tuple is identical to the first one. Such linear ring is composed of at least four positions. This linear ring is not self-intersecting to avoid creating fragmented (non-compact) areas since several exteriors can be attached to an instance of the “GmlMultiPolygon” class.
Linear referencing system of locations
This section describes the linear referencing system used by DATEX II. It accommodates all the actual linear referencing systems adopted by road operators in Europe. It can be used for systems based on a distance measured from the linear element origin: kilometre point or its US equivalent mile point or true mileage as well as for the UK system named “chainage”, hectometre-points, or link offset. It also accommodates systems where there exist intermediate points from which the distance is measured: systems based on kilometre-post, milepost, reference post as well as cross-streets or control sections. It is fully compliant with a draft CEN ISO standard on the same topic (EN ISO 19148) being adopted, with the two following restrictions:
- Distances are only expressed in metres according to the DATEX II principle on units, unlike the draft standard where it is permissible to express them with different units.
- The lateral/vertical offset features are not implemented
Reminder: This system is based on predefined elements that need to exchanged and agreed by the Supplier and the Client before being used in a DATEX II exchange. Only the reference is transmitted between the Supplier and the Client. It is up to the client to look up the descriptive elements corresponding to the referenced location using the provided elements.
Such a linear referencing system includes three elements:
- general information on the location itself:
- administrative area including the location,
- geographic direction: e.g.: northbound, westbound, clockwise, …
- relative traffic flow direction of the considered location regarding the underlying
- referenced linear element direction: aligned, opposite, both, unknown
- height grade of linear section: above grade, at-grade, below grade
- Expression of distance of the referenced point: this expression of distance can be absolute (expressed from the origin of the road element), relative to an upstream intermediate point named “from referent”, or interpolative (expressed by a percentage along the linear element and calculated from the linear element origin).
In case of relative method, the “from referent” may also be associated with a “toward referent” (downstream), both bracketing the considered point.
In case the considered point is bracketed by two referents, the direction defined as going from the “from referent” to the “toward referent” overrides the general direction of the linear element (see below). The location direction is thus compared to this direction.
A referent is typified: boundary, intersection, reference marker, landmark, road node.
Finally, each referent may be associated with ETRS89 geographic coordinates (latitude, longitude).
- The linear element underlying the location. It includes several attributes:
- road number,
- road name, one of the two attributes shall be provided,
- linear element nature (optional): road, road section, slip-road, other
- reference model which identifies the road network reference model which segments the road network according to specific business rules. It may be e.g. a graph describing the considered road network.
- reference model version: version of the referenced road network model
This underlying linear element is defined either with an identifier string (e.g. looking up in a map database) or as an ordered sequence of points with one at each extremity. These points are defined as referent. In this latter case, the sequence order provides the reference linear element direction.
Point along a linear element
A point will be used to locate certain phenomena where dimension length is negligible such as for example accidents, incidents (broken down vehicles), obstacles, …. In case of a point referencing, the general information is provided by the class “PointAlongLinearElement”.
Linear within a linear element
Linear will be used to locate certain phenomena or operator actions where dimension length is relevant, such as for abnormal traffic flow (queues, …), the bad condition of roads (e.g. road surface), …
This linear is defined by its two extremities: “from” point and “to” point. Unlike some other referencing based on points like ALERT-C, the order between “from” point and “to” point is only defined downstream by considering the underlying linear element direction. If the impacted traffic direction is aligned, the attribute “directionRelativeOnLinearSection” is used. The general information is provided by the class “LinearWithinLinearElement”.
No area referencing
This referencing system cannot be used for referencing areas.
OpenLR
OpenLR point location references are defined in more detail in the OpenLR whitepaper version 1.5.
OpenLR Line Location Reference
An OpenLR line reference is defined as a sequence of OpenLR base Location Reference points forming a route/itinerary on a road network.
Be aware that OpenLR lines can comprise more than one linear in terms of the DATEX II definitions of a linear. It would harm the basic concepts of on-the-fly location referencing as supported by OpenLR when an implementation is made by splitting the openLR lines up according to the DATEX II definition of a linear.
Therefore, it is recommended to include only one OpenLR linear from the beginning to the end of a trajectory.
OpenLR Area Location reference which
OpenLR Area Location reference is a set of ordered X,Y coordinates (ETRS89) together forming a geometric area, which has to be one of the following types:
- circle location reference: defined by a centre point in coordinates, with a radius of the circle
- rectangle location reference: defined by the coordinates of the lower left (south-west) and upper right (north east) corners of the rectangle
- grid location reference: defined by a rectangles and the number of columns and rows that form the grid within this rectangle
- polygon location reference: defined by the coordinates of at least 3 corners of the polygon.
- closed line location reference: an area framed by road segments. This area is defined by openLR lines