The ECHO model includes two mechanisms for describing attributes that are not included in the base model: characteristics and additional attributes. Characteristics are used to provide specific information about platforms, instruments and sensors. Characteristics include four attributes (column 1 in Table 1). Additional attributes are unclassified metadata elements that include eight attributes (column 2 in Table 1) which are a superset of the characteristic properties.

Table 1. ECHO Characteristic and Additional Attribute Properties

Current ECHO collection metadata includes a rather small number of unique characteristics (Table 2) and over 300 unique additional attributes.

Table 2. Existing Platform, Instrument, and Sensor Characteristics

Given the overlap between characteristic and additional attribute properties, and the small number of unique platform, instrument, and sensor characteristics, it may make sense to convert the characteristics to additional attributes and to add a type property to the additional attributes in order to allow classification of the additional attributes and translation into appropriate ISO classes. Possible types and appropriate ISO classes are shown in Table 3.

Table 3. Additional Attribute Types and Appropriate ISO Classes

ISO Extensions

Acquisition Information

Several of the ISO classes listed in Table 3 require extensions in order to be able to hold lists of arbitrary attributes. This is done in the ISO standards using a combination of roles with type = RecordType and Record. The RecordType is usually implemented as an xlink to a fragment of an XML schema that defines an object. The Record is an instance of that object with values. The Figures show a first draft idea of how this might look in UML.