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a new route. For example, a new type of fuel available at the Brussels airport could be encoded as follows:
…
<New>
<Ful>
<FulUid>
<AhpUid>
<codeId>EBBR</codeId>
</AhpUid>
<codeCat>AVGAS-LL</codeCat>
</FulUid>
<txtDescr>Octane 100 Low Lead aviation fuel</txtDescr>
<txtRmk>Indicative price is 99.99 EUR / tone</txtRmk>
</Ful>
</New>
…
The unique identifier (natural key) of the new feature instance is made of the child elements of <FulUid>:
airport identifier ('EBBR') and fuel category ('AVGAS-LL'). If the receiving system already has a feature
with the same natural key, it is expected to trigger an error message. How such error messages are
handled and what is their content is not part of AIXM. This is considered to be implementation specific.
5.1.4. Changed Feature
A <Changed> element contains data about the modified attributes and relationships of one previously
sent feature. As feature attributes and relationships are used in the composition of the unique identifier
(natural key), a mechanism to indicate a change of the unique identifier has been included in the Schema.
2For details about EAD slots, please see the "EAD Slot Management" document available at "http://www.eurocontrol.int/ead/downloads.
html "
16 Edition Number: 4.5
AIXM PRIMER
5.1.4.1. Unchanged Natural Key
Changes that do not affect the natural key of a feature are encoded as in the example below. The Changed
element contains first the natural key of the feature instance (<VorUid> in this example) and then the
rest of its attributes and relationships.
…
<Changed>
<Vor>
<VorUid mid='187652432'>
<codeId>BXL</codeId>
<geoLat>514326.67N</geoLat>
<geoLong>0032345.37E</geoLong>
</VorUid>
<OrgUid>
<txtName>BELGIUM</txtName>
</OrgUid>
<txtName>BRUSSELS VOR</txtName>
<codeType>DVOR</codeType>
<valFreq chg="1">119.00</valFreq>
<uomFreq>MHZ</uomFreq>
<codeTypeNorth>MAG</codeTypeNorth>
<valDeclination>1.3</valDeclination>
<codeDatum>WGE</codeDatum>
<Vtt>
<codeWorkHr>H24</codeWorkHr>
</Vtt>
<txtRmk>This is just an example</txtRmk>
</Vor>
</Changed>
…
The rule is that all feature attributes and relationships that have a value need to be included, both changed
and unchanged ones. In the example above only the frequency of the VOR, encoded in the element
<valFreq> has changed to 119.00 (MHz). However, all the other feature attributes and relationship that
are not null must be included (note: as for a <New> feature). This is to avoid uncertainty and look-up
overheads in determining the current value of a feature attribute or relationship.
The current version of AIXM Schema does not support NULL values. In an Update or Snapshot message,
all feature attributes and relationships that are NULL (do not have a value) are simply not included. For
example, if the name of a VOR becomes NULL (for whatever reason, such as because the VOR does
no longer have a name or it is unknown), then the corresponding <txtName> field would be absent in
the <Changed> message exemplified
In future versions, if it becomes necessary to allow NULL values, AIXM will need to have a specific
schema construct in order for such messages to be valid. Also, a flag would be necessary in the AIXMUpdate
message in order to indicate whether or not an AIXM source is using NULL values.
5.1.4.2. 'chg' attribute
The attribute chg='1' of the <valFreq> element indicates that this is a value which has actually changed.
However, the use of 'chg' attributes is optional. The 'use_chg' attribute of the root <AIXM-Update>
element shall indicate if the changes are explicitly marked by a 'chg='1' attribute or not.
5.1.4.3. 'mid' attribute
As discussed at 3.3 above, the possibility to use artificial unique identifiers is foreseen in the Schema.
In this example, the mid='187652432' attribute of the <VorUid> element contains an example of such
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AIXM PRIMER
an artificial unique identifier. The 'mid' attribute is optional. The decision to use/not use artificial identifiers
in AIXM-Update messages must be made by the implementers of systems which use AIXM as
exchange format.
When data is uploaded into the EAD, the system will use natural keys in order to identify the records
　
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