<?xml version="1.0" encoding="UTF-8" ?>

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <!-- observation elements -->
  <xs:element name="definition">
    <xs:complexType mixed="true" />
  </xs:element>

  <xs:element name="standard_name">
    <xs:complexType mixed="true" />
  </xs:element>

  <xs:element name="units">
    <xs:complexType mixed="true" />
  </xs:element>

  <!-- observation declaration -->
  <xs:element name="observation">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="standard_name" />
        <xs:element ref="definition" />
        <xs:element ref="units" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>


  <!-- dictionary is a list of observation types -->
  <xs:element name="data_dictionary">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="observation" maxOccurs="unbounded" />
      </xs:sequence>
      <xs:attribute name="id" type="xs:NMTOKEN" use="required" />
    </xs:complexType>
  </xs:element>

</xs:schema>