<xs:simpleType name="ReferenceFeatureAssociationSpecificationElementEnumType">
  <xs:annotation>
    <xs:documentation>The ReferenceFeatureAssociationSpecificationElementEnumType enumerates values that describe ISO specific non-circled substitute feature characteristic modifiers. C = minimax (Chebyshev) CE = minimax (Chebyshev) external CI = minimax (Chebyshev) internal G = least squares (Gaussian) GE = least squares (Gaussian) external GI = least squares (Gaussian) internal N = maximum inscribed X = minimum circumscribed</xs:documentation>
  </xs:annotation>
  <xs:restriction base="xs:NMTOKEN">
    <xs:enumeration value="C"/>
    <xs:enumeration value="CE"/>
    <xs:enumeration value="CI"/>
    <xs:enumeration value="G"/>
    <xs:enumeration value="GE"/>
    <xs:enumeration value="GI"/>
    <xs:enumeration value="N"/>
    <xs:enumeration value="X"/>
  </xs:restriction>
</xs:simpleType>