<simpleType name="ReferentTypeEnum">
  <annotation>
    <documentation>Referent types for geotechnical, geoenvironmental and geophysical linear features: common reference points in drilling, sampling and survey operations.</documentation>
  </annotation>
  <restriction base="string">
    <enumeration value="ground surface">
      <annotation>
        <documentation>The natural or existing ground surface at the location of the feature. The usual reference for boreholes, CPT soundings, test pits and other subsurface investigations.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="kelly bushing">
      <annotation>
        <documentation>The kelly bushing on a drilling rig, at the top of the kelly drive system. Typically the reference for measured depth in rotary drilling. Rotary bushing is the preferred term.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="rotary bushing">
      <annotation>
        <documentation>The bushing seated in the rotary table through which the kelly or drill string passes. Its elevation (RKB, rotary kelly bushing) is a common datum for measuring drilled depths and is typically a few feet above the drill floor elevation. Preferred over kelly bushing.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="rotary table">
      <annotation>
        <documentation>The rotating platform of a drilling rig through which the drill string passes. An alternative reference for measured depth.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="crown valve">
      <annotation>
        <documentation>The valve assembly at the crown of the derrick or mast, at or near the crown block. A depth datum in drilling operations, at one of the highest fixed points of the rig.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="derrick floor">
      <annotation>
        <documentation>The derrick floor or rig floor, the main working platform of the drilling rig. Sometimes used instead of the kelly bushing or rotary table.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="mudline">
      <annotation>
        <documentation>The river or lakebed surface in over-water drilling: the sediment-water interface in non-marine aquatic environments.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="sea floor">
      <annotation>
        <documentation>The sea floor surface, used in marine geophysical and geotechnical surveys. Typically the reference for measured depth in subsea wells and offshore boreholes.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="mean sea level">
      <annotation>
        <documentation>Mean sea level, a vertical datum in marine and coastal environments: the arithmetic mean of hourly water elevations observed over a National Tidal Datum Epoch (19 years).</documentation>
      </annotation>
    </enumeration>
    <enumeration value="lowest astronomical tide">
      <annotation>
        <documentation>The lowest tide level over a National Tidal Datum Epoch (19 years). A low-water datum in some coastal applications.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="mean higher high water">
      <annotation>
        <documentation>A tidal datum: the average of the higher high water height of each tidal day observed over a National Tidal Datum Epoch (19 years). Applicable in areas with mixed tidal regimes.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="mean high water">
      <annotation>
        <documentation>A tidal datum: the average of all the high water heights observed over a National Tidal Datum Epoch (19 years).</documentation>
      </annotation>
    </enumeration>
    <enumeration value="mean lower low water">
      <annotation>
        <documentation>A tidal datum: the average of the lower low water height of each tidal day observed over a National Tidal Datum Epoch (19 years). The common datum for nautical charts in the United States.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="mean low water">
      <annotation>
        <documentation>A tidal datum: the average of all the low water heights observed over a National Tidal Datum Epoch (19 years).</documentation>
      </annotation>
    </enumeration>
    <enumeration value="mean tide level">
      <annotation>
        <documentation>A tidal datum: the arithmetic mean of mean high water and mean low water, the same as half-tide level.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="top of casing">
      <annotation>
        <documentation>The top of the protective casing of a borehole or well. A reference for depth measurements in monitoring wells and environmental boreholes.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="bottom of casing">
      <annotation>
        <documentation>The base of the protective casing of a borehole or well, where cased hole changes to open hole.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="casing flange">
      <annotation>
        <documentation>A flange affixed to the top of the casing string, used to attach production equipment. A common reference in oil and gas wells.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="kickoff point">
      <annotation>
        <documentation>The point at which a borehole deviates from vertical in directional drilling. An important reference for deviated well paths and horizontal drilling.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="top of hole">
      <annotation>
        <documentation>The top of a drilled hole; generally the same as the ground surface unless the hole is drilled within an excavation or from a platform.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="bottom of hole">
      <annotation>
        <documentation>The terminal end of a borehole or well at maximum penetration: the total depth (TD) of the hole.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="top of core">
      <annotation>
        <documentation>The defined top of a core sample. Used for positions within core samples that are handled separately from the parent borehole, and a common anchor point for linear referencing within a core.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="bottom of core">
      <annotation>
        <documentation>The defined base of a core sample. Used for core recovery calculations and sample referencing; often better defined than the top of core when slough is present at the top of the cored interval. A common anchor point for bottom-up core logging.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="line start">
      <annotation>
        <documentation>The starting point of a geophysical trackline or survey transect, corresponding to the first coordinate of the linear extent. A common reference for distance along survey lines.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="line end">
      <annotation>
        <documentation>The ending point of a geophysical trackline or survey transect, corresponding to the last coordinate of the linear extent.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="intersection">
      <annotation>
        <documentation>The point where this linear feature crosses another linear feature. Common in grid-based geophysical surveys, where perpendicular survey lines create a network of tie points.</documentation>
      </annotation>
    </enumeration>
    <enumeration value="benchmark">
      <annotation>
        <documentation>A surveyed benchmark used as the elevation reference for the feature: a monumented point with known coordinates in a geodetic reference system.</documentation>
      </annotation>
    </enumeration>
  </restriction>
</simpleType>