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model by adopting a lowerCamelCase style:
• removing the '_' character;
• converting the first token to lower case (it indicates a data type);
• converting the first character of the following tokens to upper case;
• converting the remaining characters of the other tokens to lower case.
For example, the CODE_TYPE_NORTH attribute of the VOR entity corresponds to the <codeTypeNorth>
child element of the VorType. Using an XML Schema editor like XMLSpy1, it is possible to represent
in a diagram the definition of an AIXM feature and its attributes. For example, the diagram representing
the VOR feature is included here.
1http://www.xmlspy.com
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Figure 3.1. VOR Feature Diagram
3.3. Feature Identification
As described above, AIXM employs natural keys in order to uniquely identify feature instances. For
example, the unique identifier of a VOR is composed of the position of the station (latitude and longitude)
plus the radio identification. The unique identifier of a feature is declared as a separate complex type in
the AIXM-Features.xsd sub-schema. For example, the unique identifier of the VOR feature is declared
as the VorUidType complex type. The declaration of the VorType contains a child element named
<VorUid>, having as type the VorUidType.
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Figure 3.2. DME Feature Diagram
The AIXM Schema also allows for the use of an artificial unique identifier (surrogate key). The declaration
of every "FeatureUid" complex type contains an optional attribute named 'mid' of type xsd:string
long, which can be used as artificial unique identifier in a given context. This context can be a dataset
or an application domain.
An example of a dataset could be an AIXM encoded export of a complete aeronautical database. Unique
artificial identifiers could be generated when the dataset is created and cease to exist after the dataset is
processed by the receiving system. Such an artificial identifier will have a transient nature.
An application domain could result from an agreement between two or more systems to use AIXM with
artificial identifiers. How exactly the artificial unique identifiers are constructed and resolved in order
to be persistent remains to be agreed between such systems.
The European AIS Database (EAD), which employs AIXM as main data exchange format, will use
natural keys as unique identifiers. The values of the 'mid' attributes, if present in an AIXM-Update file
received by the EAD, will be ignored. However, the EAD will fill-in the 'mid' attributes for all AIXMUpdate
messages generated by the EAD2.
How features are uniquely identified has consequences on two aspects of the Schema:
• how relationships are encoded and
2Due to internal EAD reasons, a number of features will 'borrow' the 'mid' value from their parent entities. At the time of the
writing of this document, this was foreseen to be the situation for the following 'geometry type' features: Abd, Apg, Fpg, Tsg,
Tlg, Rpg. They will use the same 'mid' value as their parent Ase, Apn, Fpa, Tsa, Tla and Rpa respectively. Please check Appendix
A for the meaning of these 3-letter abbreviated feature names.
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• how updates are encoded.
3.4. Feature Relationships
Relationships are encoded by including the natural key of the related feature.
For example, the collocation relationship between a DME and a VOR is encoded by a child element of
with name <VorUid> and of type VorUid in the DmeType. A DME feature instance would contain at
this place the natural key of the collocated VOR: its position (latitude and longitude) and its radio identification.
The same mechanism is applied for all relationships encoded in the AIXM Schema. As a rule, the name
of a child element that corresponds to a relationship usually ends in (but sometimes only contains) "Uid".
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Chapter 4. AIXM Schema Files
4.1. Inclusions
The diagram below indicates how the five four sub-schema files, which compose the AIXM Schema,
are included in each other. For example, the AIXM-Update.xsd schema needs to include the AIXMUpdate-
DataTypes.xsd file and the AIXM-Features.xsd file, which includes the AIXM-DataTypes.xsd
file.
Figure 4.1. AIXM Schema File Inclusions
4.2. AIXM Data Types
When included into AIXM-Update.xsd, tThe AIXM-Features sub-schema uses data types declared in
the AIXM-Update-DataTypes.xsd. For example, the <geoLat> child element of the VorUidType is declared
as having the type geoLat, which is defined in AIXM-Update-DataTypes.xsd file.
The data type geoLat is defined differently in AIXM-Update-DataTypes.xsd and AIXM-Snapshot-
　
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