Airborne geophysical survey: Pilot Knob, California

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Frequently anticipated questions:

What does this data set describe?

Title: Airborne geophysical survey: Pilot Knob, California
Aeromagnetic data were collected along flight lines by instruments in an aircraft that recorded magnetic-field values and locations. This dataset presents latitude, longitude, altitude, and magnetic-field values.
  1. How might this data set be cited?
    U.S. Geological Survey, Department of Interior and the National Geophysical Data Center, NOAA, 2012, Airborne geophysical survey: Pilot Knob, California.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -117.66
    East_Bounding_Coordinate: -116.71
    North_Bounding_Coordinate: 35.87
    South_Bounding_Coordinate: 34.88
  3. What does it look like? (JPEG)
    Reduced-size image depicting the data, 591 x 682 pixels, 173,154 bytes
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: Jan-1996
    Ending_Date: Mar-1996
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: tabular digital data
  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.
    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 6,378,206.4.
      The flattening of the ellipsoid used is 1/294.98.
      Altitude_Datum_Name: National Geodetic Vertical Datum of 1929
      Altitude_Resolution: 1
      Altitude_Distance_Units: feet
      Explicit elevation coordinate included with horizontal coordinates
  7. How does the data set describe geographic features?
    Airborne survey specifications These items are constant for the entire survey Project number: 4177 Project name: Pilot Knob, California Survey flown by: Airmag Surveys Survey compiled by: Pearson, deRidder, and Johnson (PRJ) Survey flown for: U.S. Geological Survey Approx. no. of line miles: 5880 Survey height: 820 ft Altitude method: Draped over terrain Flight-line spacing: 0.33 mi Flight-line direction: N45E Aircraft used: unknown Airport - arrival: unknown Airport - departure: unknown Sensor tow distance: unknown Magnetometer used: unknown Data recording interval: unknown Regional field removed: IGRF 1995
    Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1. Dobrin, M.B., 1976, Introduction to Geophysical Prospecting: New York, McGraw-Hill Book Company, Sheriff, R.E., 1984, Encyclopedic dictionary of exploration geophysics: Tulsa, OK, Society of Exploration Geophysicists.

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, Department of Interior and the National Geophysical Data Center, NOAA
  2. Who also contributed to the data set?
    These USGS employees contributed to reformatting and archiving these data: Viki Bankey, Carol Finn, Pat Hill, Holly Hindle, Bob Kucks, Carter Roberts, Vicki Rystrom, Sarah Shearer Cooperating contributors from the National Geophysical Data Center are: Ronald Buhmann, David Dater, Susan McLean, Stewart Racey
  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?
  2. How were the data generated, processed, and modified?
    Date: Mar-1996 (process 1 of 3)
    Conversion of measured values to geographic position and magnetic values was performed by the contractor using industry standard practices. Details are found under Horizontal_Position_Accuracy_Report, and Vertical_Position_Accuracy_Report Unless noted, conversion processes were not reported to the USGS. Unpublished products generated by the contractor included magnetic tapes, a map of residual or total field magnetics, and perhaps some written documentation.
    Date: Mar-2010 (process 2 of 3)
    USGS reformatting of contractor data to standard format. USGS personnel used the software package Oasis Montaj by Geosoft, Inc., to read in the original contractor's data. Latitude, longitude, altitude, and magnetic values were checked for obvious errors or spikes and values of -9999.9 were given where the value could not be reasonably corrected. Other errors in the data were not corrected. Horizontal positions were converted to latitude and longitude if the original values were UTM meters. Elevations were converted from meters to feet (1 m = 3.2808 ft). Often the digital data did not contain date information and only the month and year flown or just a starting date were stated in the written documentation. In these cases, the first day of the month or the date the survey began was used as the Julian date and added to the digital data. Other information was added that was missing from the data file but was recorded elsewhere. The reformatted data were written in the format described in the section on Digital_Transfer_Information.
    Date: 10-Nov-2020 (process 3 of 3)
    Added keywords section with USGS persistent identifier as theme keyword. Person who carried out this activity:
    U.S. Geological Survey
    Attn: VeeAnn A. Cross
    Marine Geologist
    384 Woods Hole Road
    Woods Hole, MA

    508-548-8700 x2251 (voice)
    508-457-2310 (FAX)
  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?
    The data in this file have been processed using various formulas and methods that are not usually documented but that represent industry standard practices for airborne data reduction. For example, position is listed as latitude and longitude, but these values were derived from the raw navigation data depending on the system used. (see notes under horizontal accuracy). Line numbers were added to records, and unusable data at flight-line ends were discarded (as aircraft slowed and turned around). Separate recordings were correlated by time and assigned to the correct location. The exact accuracies of these processing steps may not be known. They are discussed in the sections on attribute or positional accuracies.
  2. How accurate are the geographic locations?
    Flight Path Recovery Horizontal positions of the survey aircraft used to collect data were determined using GPS satellite navigation.
  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. Radar altimeters are estimated to have an error of 2-5% of the altitude (Richard Hansen, PRJ, Inc., written communication). Barometric altimeters are quite accurate, but are typically operated in an uncorrected mode. The diurnal variation in air pressure over the course of a flight can produce a 50-100 ft error in the barometric altimeter reading. In addition, pressure microcells create short- period air pressure changes equivalent to about 10 ft. under typical conditions (Richard Hansen, PRJ, Inc., written communication) This data set was collected as a draped survey having an average terrain clearance of 820 ft. Because aircraft, especially airplanes, cannot safely maintain a constant terrain clearance, error in vertical position is introduced.
  4. Where are the gaps in the data? What is missing?
    Loss of data due to poor transmission, channel dropout, obvious spiking, missing channels, and other obvious errors were replaced with the value -9999.9, such as in the radar and barom channels. Sometimes channels were added and filled with dummy values (-9999.9) to comply with the established format. Some records that had no usable data were deleted.
  5. How consistent are the relationships among the observations, including topology?
    The data in this file were collected by a single contractor or group who were responsible for collecting and processing the data. The data from this survey were collected using the same instruments (magnetometers, altimeters, navigational systems) throughout the survey and were collected in a normal length of time with no long delays between survey beginning and end. Survey contracts specified the conditions and specifications under which these data were collected. Standard industry practices of the time were followed in data collection and processing.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints: none
none. Acknowledgement of the U.S. Geological Survey and the National Geophysical Data Center would be appreciated in products derived from these data.
  1. Who distributes the data set? (Distributor 1 of 2)
    Web site administration: U.S Geological Survey Publishing Service Center
    Publishing Service Center
    USGS MS 902, Box 25046 DFC
    Denver, CO

    303-236-5486 (voice)
  2. What's the catalog number I need to order this data set?
  3. What legal disclaimers am I supposed to read?
    Although all data published on this web site have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
  4. How can I download or order the data?
    • Availability in digital form:
      Data format: ASCII (version 4.1) Each line contains data in the following format, beginning with line 1 (no header included): line_no I5 flight line number directn I4 flight line direction, azimuth degrees from north longitud F11.4 longitude (decimal degrees) latitude F9.4 latitude (decimal degrees) year I5 year flown jul_day I4 Julian day flown fiducial I7 fiducial number radar F8.1 radar altitude above ground (feet) barom F8.1 barometric altitude above mean sea level (feet) totmag F9.2 corrected total field magnetic value (nT) resmag F9.2, 1X residual magnetic value (nT)
      Network links: for link to report
    • Cost to order the data: none

  1. Who distributes the data set? (Distributor 2 of 2)
    Peter N Schweitzer
    USGS Eastern Mineral and Environmental Resources Science Center
    12201 Sunrise Valley Drive
    Reston, VA

    703-648-6533 (voice)
    703-648-6252 (FAX)
  2. What's the catalog number I need to order this data set? CA_4177_meta.txt
  3. What legal disclaimers am I supposed to read?
    This dataset was prepared by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. Any views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
  4. How can I download or order the data?

Who wrote the metadata?

Last modified: 10-Nov-2020
Metadata author:
USGS Crustal Geophysics and Geochemistry Science Center
Magnetic and Gravity Information
U.S. Geological Survey
Box 25046 Mail Stop 964
Denver Federal Center
Denver, CO

303-236-5652 (voice)
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

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