<element name="factor" type="double"/>

<gml:formula>
  <gml:a>{0,1}</gml:a>
  <gml:b>{1,1}</gml:b>
  <gml:c>{1,1}</gml:c>
  <gml:d>{0,1}</gml:d>
</gml:formula>

<element name="formula" type="gml:FormulaType"/>

<complexType name="ConversionToPreferredUnitType">
  <annotation>
    <documentation>The inherited attribute uom references the preferred unit that this conversion applies to. The conversion of a unit to the preferred unit for this physical quantity type is specified by an arithmetic conversion (scaling and/or offset). The content model extends gml:UnitOfMeasureType, which has a mandatory attribute uom which identifies the preferred unit for the physical quantity type that this conversion applies to. The conversion is specified by a choice of - gml:factor, which defines the scale factor, or - gml:formula, which defines a formula by which a value using the conventional unit of measure can be converted to obtain the corresponding value using the preferred unit of measure. The formula defines the parameters of a simple formula by which a value using the conventional unit of measure can be converted to the corresponding value using the preferred unit of measure. The formula element contains elements a, b, c and d, whose values use the XML Schema type double. These values are used in the formula y = (a + bx) / (c + dx), where x is a value using this unit, and y is the corresponding value using the base unit. The elements a and d are optional, and if values are not provided, those parameters are considered to be zero. If values are not provided for both a and d, the formula is equivalent to a fraction with numerator and denominator parameters.</documentation>
  </annotation>
  <complexContent>
    <extension base="gml:UnitOfMeasureType">
      <choice>
        <element name="factor" type="double"/>
        <element name="formula" type="gml:FormulaType"/>
      </choice>
    </extension>
  </complexContent>
</complexType>