The ICAO PANS-AIM requires in relation with the AIS data set that: “5.3.3.1.2 When a property is not defined for a particular occurrence of the subjects listed in 5.3.3.1.1, the AIS data sub-set shall include an explicit ‘not applicable’ indication.”

Using AIXM, it is possible to indicate a reason, such as ‘not applicable’, ‘unknown’, etc. for each property (attribute or association) that has no assigned value in the Data Set. This is done by assigning a suitable value to the nilReason attribute. 

The AIXM XML Schema allows a nilReason attribute to be used even when an element has a value. This could lead to confusions. Therefore, a additional rules of technical nature is specified.

A nilReason attribute can only be used for elements that have xsi:nil='True'


There exists a predefined list of values for this attribute, coming from the GML standard. In order to ensure that these values are unambiguously interpreted, their meaning and the use of this attribute are further detailed here.

The following values shall be used for the nilReason attribute:

  • inapplicable - there is no value. It is normal that a value is not assigned because the corresponding real world artifact does not have this kind of property);
  • unknown - the correct value is not known to, and not computable by, the sender of this data;
  • withheld - the value is not divulged. A value exists, but it cannot be provided because it is subject to restricted use (for example, it is a military restricted data, it is commercially sensitive, etc.). Access to this data might be possible through direct contact with the data provider, in special circumstances.
  • other:NOTAM - the value is not provided as part of the BASELINE. A value is allocated and provided through NOTAM, when operationally necessary;

  • other:invalidValueCopiedAsNote - the actual value is in a Note because it does not fit in the constraints for the element type in the current AIXM version. Typically, this is used when the original data comes from a future AIXM version, which is backwards mapped.


Note: the list of values for the nilReason attribute, inherited from the Geographical Markup Language (GML) Schema, also contains two values ("missing" and "template") for which no operational usage was identified in relation with aeronautical data.  Therefore, these shall not be used in digital AIS data sets.

The following values shall not be used for the nilReason attribute:

  • missing - the correct value is not readily available to the sender of this data. Furthermore, a correct value may not exist
  • template - the value will be available later