Airborne geophysical survey: Valdez

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Frequently anticipated questions:


What does this data set describe?

Title: Airborne geophysical survey: Valdez
Abstract:
Aeromagnetic data were collected along flight lines by instruments in an aircraft that recorded magnetic-field intensity values and location. In surveys such as this one where the data were originally collected in digital form and not digitized from contour maps, the information we provide typically includes latitude, longitude, magnetic anomaly in nanoTeslas, and intermediate values used to derive the magnetic anomaly such as total magnetic field.
  1. How might this data set be cited?
    U.S. Geological Survey, 2014, Airborne geophysical survey: Valdez:.

    Online Links:

    This is part of the following larger work.

    Connard, G.G., Saltus, R.W., Hill, P.L., Carlson, L., and Milicevic, B., 1999, Alaska digital aeromagnetic database description: U.S. Geological Survey Open-File Report 99-503.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -147.02600097656
    East_Bounding_Coordinate: -143.96598815918
    North_Bounding_Coordinate: 62.022003173828
    South_Bounding_Coordinate: 60.957996368408
  3. What does it look like?
    http://mrdata.usgs.gov/geophysics/surveys/geophysics2/AK/AK_3122.jpg (JPEG)
    Reduced-size image depicting the data, 1104 x 868 pixels, 161,222 bytes
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: Jul-1978
    Ending_Date: Aug-1978
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a Point data set. It contains the following vector data types (SDTS terminology):
      • Entity point
    2. What coordinate system is used to represent geographic features?
      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.0001. Longitudes are given to the nearest 0.0001. Latitude and longitude values are specified in decimal degrees. The horizontal datum used is North American Datum of 1927.
      The ellipsoid used is Clarke 1866.
      The semi-major axis of the ellipsoid used is 6378206.4.
      The flattening of the ellipsoid used is 1/294.98.
  7. How does the data set describe geographic features?
    Entity_and_Attribute_Overview:
    The majority of these surveys are available as digital flightline data (data for which digital records of original flightline data exist). We attempted to recover or construct the standard channels (fields) to be included in the final database for all digital flightline data. For some surveys, some of the standard channels (e.g., MAG_BASE ) were not available and were removed from the individual final database file. The standard channels are: Standard Database Channels For Digital Flightline Surveys
    Channel Name  Contents
    Line          Line number
    FID           Fiducial
    LAT           Latitude
    LON           Longitude
    X             X-coordinate (meters) in an Alaska Albers projection
    Y             Y-coordinate (meters) in an Alaska Albers projection
    HEIGHT        Elevation above ground (meters) if available
    FLTSURF       Flight surface (meters above sea level) consistent with AKC
    FLTSURF_M     Flight surface (meters above sea level) consistent with AKM
    MAG_RAW       Raw magnetics (if available)
    MAG_BASE      Basemag-corrected magnetics (if available)
    MAG_DGRF      DGRF-corrected magnetics (if available)
    MAG_COMP      Composite magnetic value (consistent with AKC)
    MAG_MERGE     Merged magnetic value (consistent with AKM)
    USE_FLAG      Used/not-used flag to signal use in AKC
    
    Two additional channels were included in the database files for a few of the INTAK surveys:
    MAG_IGRF      IGRF-corrected magnetics where DGRF-corrected magnetic were not available
    MAG_DRAPE     1,000 ft. draped version of MAG_DGRF or MAG_IGRF
    
    Entity_and_Attribute_Detail_Citation: http://pubs.usgs.gov/of/1999/ofr-99-0503/DBDESC.HTM

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
    • U.S. Geological Survey
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?

Why was the data set created?

Aeromagnetic surveys are used for geophysical prospecting. Some variations in magnetic measurements are caused by rocks that contain significant amounts of magnetic minerals (magnetite being the most common). These anomalies reflect variations in the amount and type of magnetic material and the shape and depth of the body of rock. The features and patterns of aeromagnetic anomalies can also be used to delineate details of subsurface geology including the locations of buried faults and the thickness of surficial sedimentary rocks.

How was the data set created?

  1. From what previous works were the data drawn?
    paper (source 1 of 1)
    U.S. Geological Survey, 1979, Aeromagnetic map of part of the Valdez 1 degree by 3 degrees Quadrangle, Alaska: Open-File Report 79-381.

    Type_of_Source_Media: paper
    Source_Contribution:
    Digital information were drawn from (or are presented in) this report
  2. How were the data generated, processed, and modified?
    Date: Jul-1978 to 197808 (process 1 of 3)
    Aeromagnetic survey flown by LKB in the time period 07/78-08/78 direction N-S at altitude 1000 AG with spacing 1 mile for a total of 6465 line miles
    Date: 1996 (process 2 of 3)
    Northwest Geophysical Associates, Inc. (NGA) was contracted by the U.S. Geological Survey (USGS) to construct a database containing original aeromagnetic data (in digital form) from surveys, maps and grids for the State of Alaska from existing public-domain magnetic data. This database facilitates the detailed study and interpretation of aeromagnetic data along flightline profiles and allows construction of custom grids for selected regions of Alaska. The database is linked to and reflects the work from the statewide gridded compilation completed under a prior contract. The statewide gridded compilation is also described in Saltus and Simmons (1997) and in Saltus and others (1999a, 1999b). NGA subcontracted a significant portion of the work described in this report to Paterson, Grant, and Watson Limited (PGW). Processed data values were tied to the two statewide aeromagnetic grids reported in Saltus and Simmons (1997). The two grids are the Alaska Composite Grid (AKC) and the Alaska Merged Grid (AKM). The AKC is a one-kilometer grid consisting of all the individual surveys at original altitudes with datum shifts to minimize differences at the boundaries. For the AKM, all of the individual surveys were draped at a level of 1,000 feet above the terrain and fit together with seamless joins along the survey boundaries. Saltus and others (1999) presented updated versions of the AKC and the AKM with additional long-wave components removed. For this project, we used the updated versions. In addition, a new Digital Elevation Model (DEM) for the State of Alaska with a 300-meter cell spacing (Riehle and others, 1997) was used to calculate terrain separation or construct a plausible flight surface when measured values for terrain separation or flight surface were not available in the original survey data. Person who carried out this activity:
    U.S. Geological Survey, Mineral Resources Program
    Attn: Richard W. Saltus
    Mail Stop 964 Building 20 Denver Federal Center, West 6th Avenue and Kipling Streets
    Lakewood, Colorado

    303-236-1375 (voice)
    303-236-1425 (FAX)
    saltus@usgs.gov
    Date: Aug-1978 (process 3 of 3)
    USGS rearranged contractor-supplied digital data to ASCII format points. USGS personnel used the software package Oasis Montaj by Geosoft, Inc., to read in the data and create raster JPEG images.
  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?
    Recent aeromagnetic surveys such as this one have generally been carried out on contract by private-sector companies that specialize in these surveys. In some cases we will retain a report from the contractor detailing methods and survey parameters, which could be consulted in order to understand the likely accuracy of the measurements.
  2. How accurate are the geographic locations?
    Detailed positional information is determined by the contractor using navigational controls to specifications given in the contractor reports. Generally these are more precise and accurate for newer surveys than for older ones.
  3. How accurate are the heights or depths?
    The aircraft vertical position was determined using the navigational positioning equipment on the aircraft, which were radar altimeter and barometric altimeter.
  4. Where are the gaps in the data? What is missing?
    Observations were made along flight lines, so these data may not extend into all parts of the geographic bounding extents.
  5. How consistent are the relationships among the observations, including topology?
    In general each original survey was collected by a single contractor, who would be responsible for processing the field observations, and reflect industry standard practices and controls. Variation among surveys is possible, especially if they were carried out during very different time periods.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints: none
Use_Constraints: none
  1. Who distributes the data set? (Distributor 1 of 1)
    U.S. Geological Survey, Mineral Resources Program
    Attn: Peter N. Schweitzer
    Mail Stop 954 USGS National Center, 12201 Sunrise Valley Drive
    Reston, Virginia

    703-648-6533 (voice)
    703-648-6252 (FAX)
    pschweitzer@usgs.gov
  2. What's the catalog number I need to order this data set? AK_3122.zip
  3. What legal disclaimers am I supposed to read?
    Reference herein to any specific commercial product, process, or service by trade name,trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 05-Dec-2014
Metadata author:
U.S. Geological Survey, Mineral Resources Program
Attn: Peter N. Schweitzer
Mail Stop 954 USGS National Center, 12201 Sunrise Valley Drive
Reston, Virginia

703-648-6533 (voice)
703-648-6252 (FAX)
pschweitzer@usgs.gov
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

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