Point Reference (Formation) [PRF]
- Wolfgang Scheucher
- POROSNICU Eduard
- Manfred Beckmann
AIXM Model Overview
The following AIXM class diagram indicates the features/properties that might be relevant for the encoding of point references, provided as distance and/or angle indications.
In AIXM a PointReference may be defined for a SegmentPoint. This will be coded by the SegmentPoint.facilityMakeup property. A SegmentPoint is the generalisation for EnRouteSegmentpoint and a TerminalSegmentPoint.
A SegmentPoint is associated with a segment, this may be for example the start or end of a RouteSegment or the start end or arc centre of of a SegmentLeg of a terminal procedure, or the holding point of a HoldingPattern. A special case of using the PointReference class is to define the outbound leg of a HoldingPattern.
The point reference is coded as part of the feature that uses the segment point and not the segment point itself (e.g. the formation data (distance and bearing) are coded as part of the RouteSegment feature and not the DesignatedPoint feature).
The SegmentPoint is then located at a SignificantPoint (see topic SignificantPoint (Overview) for more information). The SegmentPoint.pointChoice property is used to code that information.
For defining a formation (i.e. point reference) for this significant point an AngleIndication and DistanceIndication is specified. In general, a point reference will be defined for SegmentPoint that is located at a DesignatedPoint (but not for other significant points such as navaid or runway centre line point although the model would allow it).
Open Question: Point reference only for DPN
The defined AngleIndication and/or a DistanceIndication uses a SignificantPoint as referenced the facility (in general this will be a Navaid) that defines the formation parameter (i.e. the angle or the distance receptively) in regard to the designated point.. from one or more referenced facilities. This may be in case of an unnamed point which is just defined by a bearing and distance and does not have coordinates. This information may also be provided for named points with defined coordinates.
The AngleIndication.fix and a DistanceIndication.fix property is used to reference the DesigantedPoint the formation is defined for.
Also, the PointReference.point property is used to reference the DesignatedPoint.
Coding guidelines
For the enroute environment, such information may be found published in "ENR ATS Routes" and on en-route and terminal procedure charts. The distance and the angle may be provided from the same or different significant points.
AIP context
The example below shows the publication of a distance and angel indication for the designated point "RODON" from the navaid "POD".
The example below shows the publication (chart) of a distance and angel indication for the designated point "KODAP" from the navaid "BOR".
The DistanceIndication.distance attribute carries the value of the distance between the SegmentPoint and the facility used as reference.
In most cases this will be a Navaid, but it may also be any other SignificantPoint, such as another DesignatedPoint. For this purpose the DistanceIndication.type attribute may be used to indicate if the distance is from distance measuring equipment ('DME') or just a simple 'GEODETIC' distance. In case the distance is measured based on another type of Navaid (e.g. a TACAN), the value 'OTHER' has to be used in conjunction with the Navaid type, e.g. 'OTHER:TACAN'.
The AngleIndication.angle attribute carries the value of the angle between the SegmentPoint and the used facility.
The attribute AngleIndication.angleType is used to define if the angle is a radial ('RDL') or a bearing ('MAG' or 'TRUE'). 'RDL' shall only be used in case the angle indication is related to a VOR or TACAN. For all other cases, either 'MAG' or 'TRUE' shall be used. The other values available for that attribute do not make sense in the context of the encoding of a formation and shall not be used.
Only in case both magnetic and true bearing is provided also the trueAngle attribute may be used to carry the value for the true bearing. In this case the angleType attribute has to be 'MAG' and the angle attribute will carry the value for the magnetic bearing.
The AngleUse class may be used to defines if the angle indication provides the course guidance for the segment.
AIXM defines the AngleIndication.indicationDirection as follows:
Code indicating the if the angle is TO or FROM the Navaid or Designated Point that is used.
PANS-AIM defines the Formation Bearing as follows
The bearing from the reference VOR/DME...
Hence, the AngleIndication.indicationDirection shall be encode with 'FROM'. This has to be taken into account when providing the AngleIndication.angle.
PANS-AIM also requires accuracy values for the formation bearing and distance. As in AIXM no dedicated attributes are provide this information has to be encoded as Note for the corresponding feature.
Open Question DPN_7
Or shall this be encoded as Metadata?
In addition, the PointReference class may be used to specify for what purpose the defined AngleIndication and/or DistanceIndication was established
The data type CodeReferenceRoleType for the corresponding attribute role provided the following values:
| Value | Description |
|---|---|
| 'INTERSECTION' | An intersection is where two or more "things" cross. An intersection will be the intersection of two angle indications or two distance indications (DME/DME fix). The indications must reference different navaids. |
| 'RECNAV' | The recommended navaid providing information for the segment. Used for ARINC leg types that need navaid information other then what is supplied by standard angle and distance indications. |
| 'ATD' | Along Track Distance. Defined the distance along the guidance course to/from another significant point. |
| 'RAD_DME' | Defined by a bearing (angle indication) and a distance (distance Indication) from a navaid. |
| 'OTHER' | Other |
PANS-AIM does not require a role for PointReference to be encoded.
AIXM 5.1.1
The AIXM definition for 'INTERSECTION' is not in line with the one of ANNEX 11.
An intersection is where two or more "things" cross. An intersection will be the intersection of two angle indications or two distance indications (DME/DME fix). The indications must reference different navaids.
A Significant Point is defined by ICAO Annex 11
Note.— There are three categories of significant points: ground-based navigation aid, intersection and waypoint. In the context of this definition, intersection is a significant point expressed as radials, bearings and/or distances from ground-based navigation aids.
Also, for some values abbreviation are used, whereas for others not.
PANS-AIM requires an Accuracy for the Formation - Bearing and Distance.
AIXM 5.1.1 does not provide a dedicated attribute. A corresponding Note for the distance and/or angle attribute may be used to code that information
Coding rules for formation (angle & distance indication)
| Identification | Data Encoding Rule | Justification | Data Verification Rule (UID) | Remarks |
|---|---|---|---|---|
| PRF-301 | The clockwise measured angle defined by the North, the position of the SignificantPoint providing the bearing and the SignificantPoint to which it refers must be consistent (within 5 degrees) with the value of the AngleIndication.angle. | System (EAD) Rule | TBD | Open Question DPN_9 Is this EAD rule relevant for the AIP Data Set? |
| PRF-302 | The calculated distance between the position of the SignificantPoint providing the distance indication and the SignificantPoint to which it refers must be consistent (within 1 KM) with the value of DistanceIndication.distance. | System (EAD) Rule | TBD | Open Question DPN_10 Is this EAD rule relevant for the AIP Data Set? |
Coding Examples
The figure below illustrates the formation concept by giving an example:
The designated point is defined by an angle indication of 149 degrees and a distance indication of 68.2 KM from the VOR/DME "BOR".
More coding examples can be found in the AIP Data Set - Specimen (DONLON):
| No. | Description | XPath Expression |
|---|---|---|
HPT-EX-05 DPN-EX-05 AIN-EX-01 DIN-EX-02 NAV-EX-01 | Formation for a fix used on a holding point | //aixm:HoldingPatternTimeSlice[@gml:id =HPT_BOR_029_10_208_15DME'] | //aixm:DesignatedPointTimeSlice[@gml:id ='BOR10'] | //aixm:DistanceIndicationTimeSlice[@gml:id ='DIN_BOR_10DME'] | //aixm:AngleIndicationTimeSlice[@gml:id ='AIN_BOR029'] | //aixm:NavaidTimeSlice [@gml:id ='NAV_BOR'] |
RSG-EX-03 DIN-EX-03 AIN-EX-02 DPN-EX-02 NAV-EX-07 | Formation for a fix used as significant point for a route segment. | //aixm:RouteSegmentTimeSlice[@gml:id ='RSG_UL123_ABOLA_ILURU'] //aixm:DistanceIndicationTimeSlice[@gml:id ='DIN_WOB_337'] | //aixm:AngleIndicationTimeSlice[@gml:id ='AIN_WOB_015'] | //aixm:DesignatedPointTimeSlice[@gml:id ='ABOLA'] | //aixm:NavaidTimeSlice [@gml:id ='NAV_WOB'] |