Identification_Information:
  Citation:
    Citation_Information:
          Originator: V.E. Langenheim
          Originator: J.T. Watt
          Originator: K.M. Denton
         Publication_Date: 2012
          Title: Magnetic map of the Irish Hills and surrounding areas, central California
          Geospatial_Data_Presentation_Form: point digital data
          Series_Information:
            Series_Name: Open-File Report
            Issue_Identification: 2012-1080
          Publication_Information:
            Publication_Place: Menlo Park, CA
            Publisher: U.S. Geological Survey
            Online Linkage: http://pubs.usgs.gov/of/2012/1080
  Description:
    Abstract: Helicopter magnetic data and physical properties of Irish Hills and surrounding areas, central California
      Digital Files:  The database includes an ASCII file containing point magnetic data from the helicopter survey, two ASCII files with point locations of the local 
maximum horizontal gradient derived from the magnetic data, and one of physical property data.  This metadata file describes the gradient data.   
    Purpose: Aeromagnetic map identifies magnetic features as a basis for geophysical interpretations; the gradients help define the edges of magnetic sources.
    Supplemental_Information: This database updates geophysical information originally presented in smaller scale formats and includes detailed aeromagnetic data collected by New-Sense Geophysics, Ltd.
  Time_Period_of_Content:
    Time_Period_Information:
      Range_of_Dates/Times:
        Beginning_Date:  2009
        Ending_Date:  2009
    Currentness_Reference:  publication date
   Status:
    Progress: Complete
    Maintenance_and_Update_Frequency: None planned
  Spatial_Domain:
    Bounding_Coordinates:
      West_Bounding_Coordinate: -120.955000
      East_Bounding_Coordinate: -120.720000
      North_Bounding_Coordinate: 35.2872000
      South_Bounding_Coordinate: 35.1061000
  Keywords:
    Theme:
      Theme_Keyword: magnetics
      Theme_Keyword: magnetic anomaly
    Place:
      Place_Keyword: northern California
      Place_Keyword: Coast Ranges
  Access_Constraints: none
  Use_Constraints: Uses of this digital data should not violate the spatial or attribute resolution of the data.  Although the digital form of the data stores location and attribute values to several decimals of 
precision, the data is subject to the accuracy and precision inherent in the data due to data collection and processing limitations.  Where this data is used in combination with other data, the precision and accuracy of 
the resulting dataset is limited by the precision and accuracy of the input datasets.  Acknowledgement of the U.S. Geological Survey is appreciated for products derived from these data.
  Point_of_Contact:
    Contact_Information:
      Contact_Person_Primary:
        Contact_Person: Victoria Langenheim
        Contact_Organization: U.S. Geological Survey
      Contact_Position: geophysicist
      Contact_Address:
        Address_Type: physical address
        Address: 345 Middlefield Rd.
        City: Menlo Park
        State_or_Province: CA
        Postal_Code: 94025
        Country: USA
      Contact_Voice_Telephone: 650-329-5313
      Contact_Electronic_Mail_Address: zulanger@usgs.gov
  Native_Data_Set_Environment: Geosoft Oasis Montaj
Native_Data_Set_Environment:
    ARC/INFO versions 9.0., 9.1, 9.2, and 9.3
    Geophysical data was stored and analyzed on a
    SUN Ultra4000 UNIX platform

      Geospatial_Data_Presentation_Form:  ascii digital data (horizontal magnetic gradients)
      Series_Information:
        Series_Name:  U.S. Geological Survey (USGS) Open-File Report
        Issue_Identification:  2012-1080
      Publication_Information:
        Publication_Place:  Menlo Park, California
        Publisher:  U.S. Geological Survey
      Online_Linkage:  <URL:http://pubs.usgs.gov/of/2012/1080>

Data_Quality_Information:
  Attribute_Accuracy:
    Attribute_Accuracy_Report: maximum horizontal gradient values are reported to 0.000001 nT/km
  Positional_Accuracy:
    Horizontal_Positional_Accuracy:
      Horizontal_Positional_Accuracy_Report: accuracy should be within 50 meters (grid size that gradient was calculated from)
  Lineage:
        Process_Step:
      Process_Description: Maximum horizontal gradient is calculated from the magnetic potential grid derived from the magnetic map/grid       
  Process_Date: 2012
Spatial_Data_Organization_Information:
  Direct_Spatial_Reference_Method: Point
  Point_and_Vector_Object_Information:
    SDTS_Terms_Description:
      SDTS_Point_and_Vector_Object_Type: Entity point
      Point_and_Vector_Object_Count: 3084
Spatial_Reference_Information:
  Horizontal_Coordinate_System_Definition:
    Geographic:
      Latitude_Resolution: 0.0001
      Longitude_Resolution: 0.001
      Geographic_Coordinate_Units: Decimal degrees
    Geodetic_Model:
      Horizontal_Datum_Name: North American Datum of 1929
      Ellipsoid_Name: Clarke 1866
   Entity_and_Attribute_Information:
  Detailed_Description:
    Entity_Type:
      Entity_Type_Label: grad_big.xyz
    Attribute:
      Attribute_Label: maximum horizontal gradient
      Attribute_Definition: location of local maximum horizontal gradient from magnetic potential of aeromagnetic grid (larger than the mean of the entire 
area)
    Attribute:
      Attribute_Label: lon_nad27
      Attribute_Definition: longitude coordinate
      Attribute_Value_Accuracy_Information:
        Attribute_Value_Accuracy: 50 m
    Attribute:
      Attribute_Label: lat_nad27
      Attribute_Definition: latitude coordinate
      Attribute_Value_Accuracy_Information:
        Attribute_Value_Accuracy: 50 m
   Entity_Type:
      Entity_Type_Label: grad_small.xyz
    Attribute:
      Attribute_Label: maximum horizontal gradient
      Attribute_Definition: location of local maximum horizontal gradient from magnetic potential of aeromagnetic grid (less than the mean of the entire area)
    Attribute:
      Attribute_Label: lon_nad27
      Attribute_Definition: longitude coordinate
      Attribute_Value_Accuracy_Information:
        Attribute_Value_Accuracy: 50 m
    Attribute:
      Attribute_Label: lat_nad27
      Attribute_Definition: latitude coordinate
      Attribute_Value_Accuracy_Information:
        Attribute_Value_Accuracy: 50m

Distribution_Information:
  Distributor:  U.S. Geological Survey
  Resource_Description: Downloadable Data
  Standard_Order_Process:
    Digital_Form:
      Digital_Transfer_Information:
        Format_Name: ASCII
        Transfer_Size: 483988
        Network_Resource_Name:  http://pubs.usgs.gov/of/XXXX
  Process_Date:  2008-2011
    Process_Step:
      Process_Description:
        Geophysical mapping
        The locations of local maximum horizontal gradient of the magnetic field were derived using a compilation  
        of the newly flown aeromagnetic data (this report) and previously published magnetic data.  The 
        magnetic data were compiled from existing publications
        (Langenheim and others, 2009; Sliter and others, 2009, revised 2010) and 
        the data were interpolated to a regular grid (Briggs, 1974) with cell dimensions of 50 m in both the X 
        and Y directions.  The magnetic base levels were then adjusted to a common 
        magnetic datum used in previous magnetic surveys, with a 1-km gap separating the various surveys.  The interpolated data were 
        mathematically transformed to pseudogravity anomalies (Baranov, 1957).
        Horizontal maximum gradients were then calculated from the pseudogravity field (Blakely and 
        Simpson, 1986).  The two files are grad_big.axyz and grad_small.axyz. 

        Baranov, V.I., 1957, A new method for interpretation of aeromagnetic maps--pseudo-gravimetric anomalies:  
        Geophysics, v. 22, p. 359-383.

        Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or gravity 
        anomalies:  Geophysics, v. 51, p. 1494-1498.

        Briggs, I., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, no. 1, p. 39-48.

        Langenheim, V.E., Jachens, R.C., and Moussaoui, K., 2009, Aeromagnetic Survey Map of the Central California Coast Ranges: U.S. Geological Survey Open-File Report 2009-1044, http://pubs.usgs.gov/of/2009/1044/.

        Sliter, R.W., Triezenberg, P.J., Hart, P.E., Watt, J.T., Johnson, S.Y., and Scheirer, D.S., 2009, revised 2010, High-resolution seismic reflection and marine magnetic data along the Hosgri Fault Zone, Central California:  U.S. Geological Survey Open-File Report 2009-1100, version 1.1, http://pubs.usgs.gov/of/2009/1100/.


Metadata_Reference_Information:
  Metadata_Date: 20120201
  Metadata_Contact:
    Contact_Information:
      Contact_Person_Primary:
        Contact_Person: Victoria Langenheim
        Contact_Organization: U.S. Geological Survey
      Contact_Position: geophysicist
      Contact_Address:
        Address_Type: mailing and physical address
        Address: 345 Middlefield Rd.
        City: Menlo Park
        State_or_Province: CA
        Postal_Code: 94025
      Contact_Voice_Telephone: 650-329-5313
  Metadata_Standard_Name: FGDC Content Standards for Digital Geospatial Metadata
  Metadata_Standard_Version: FGDC-STD-001-1998
  Metadata_Time_Convention: local time
  Metadata_Extensions:
    Online_Linkage: http://www.esri.com/metadata/esriprof80.html
    Profile_Name: ESRI Metadata Profile