Airborne geophysical survey: Northwest Georgia West, Georgia Survey Part 2 of 2

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Frequently anticipated questions:


What does this data set describe?

Title:
Airborne geophysical survey: Northwest Georgia West, Georgia Survey Part 2 of 2
Abstract:
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.
Supplemental_Information:
The U.S. Geological Survey has contracted or flown numerous airborne surveys over a long period (1950's to present). Not all flight-line data have been released to the public because of lack of personnel to reformat data and service the requests. With the improvement in digital communication and the ability to store and transmit large data sets, the USGS is now able to release the flight-line data in a common format. A companion CD-ROM/web site has been released containing magnetic data that were generated by digitizing analog maps. Digital flight-line data used to create the analog maps are unavailable. Reference is in the Cross-Reference section of this metadata file.
  1. How might this data set be cited?
    U.S. Geological Survey, Department of Interior and the National Geophysical Data Center, NOAA, 2001, Airborne geophysical survey: Northwest Georgia West, Georgia Survey Part 2 of 2:.

    Online Links:

    This is part of the following larger work.

    U.S. Geological Survey, Department of the Interior, and the National Geophysical Data Center, 2001, Digital flight-line aeromagnetic data sets of the Conterminous United States, Hawaii, and Puerto Rico: U.S. Geological Survey Open-File Report OFR 02-0361, U.S. Geological Survey, Denver, CO.

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -85.58
    East_Bounding_Coordinate: -84.75
    North_Bounding_Coordinate: 35.00
    South_Bounding_Coordinate: 34.05
  3. What does it look like?
    http://mrdata.usgs.gov/geophysics/surveys/geophysics2/GA/GA_3145B.jpg (JPEG)
    Reduced-size image depicting the data, 544 x 620 pixels, 42,164 bytes
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 1978
    Ending_Date: 1978
    Currentness_Reference:
    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 .0001. Longitudes are given to the nearest .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.
      Vertical_Coordinate_System_Definition:
      Altitude_System_Definition:
      Altitude_Datum_Name: National Geodetic Vertical Datum of 1929
      Altitude_Resolution: 1
      Altitude_Distance_Units: feet
      Altitude_Encoding_Method:
      Explicit elevation coordinate included with horizontal coordinates
  7. How does the data set describe geographic features?
    Entity_and_Attribute_Overview:
    Airborne survey specifications These items are constant for the entire survey Project number: 3145B Project name: Northwest Georgia West, Georgia Survey flown by: Geodata International, Inc. Survey flown for: U.S. Geological Survey Approx. no. of line miles: 820 Survey height: 500 ft Altitude method: Draped over terrain Flight-line spacing: 2 mi Flight-line direction: E-W Aircraft used: Cessna 182 Airport - arrival: unknown Airport - departure: unknown Magnetometer used: Geometrics G-803 proton Sensor tow distance: unknown
    Entity_and_Attribute_Detail_Citation:
    Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1, p. 83-87. Dobrin, M.B., 1976, Introduction to Geophysical Prospecting: New York, McGraw-Hill Book Company, p. 505-517.
    Entity_and_Attribute_Overview:
    Each record contains the following 11 attributes: No abbrev.name contents 1 line_no flight line number 2 directn flight line direction, azimuth degrees from north (integer) 3 longitud longitude (decimal degrees) 4 latitude latitude (decimal degrees) 5 year year flown (integer) 6 jul_day Julian day flown (integer) 7 fiducial fiducial number (integer) 8 radar radar altimeter reading above ground (feet) 9 barom altitude above mean sea level (feet) 10 totmag corrected magnetic value (nT) 11 resmag residual magnetic value (nT)
    Entity_and_Attribute_Detail_Citation:
    Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1, p. 83-87. Dobrin, M.B., 1976, Introduction to Geophysical Prospecting: New York, McGraw-Hill Book Company, p. 505-517.
    record for one magnetic data point
    The set of all measurements reported for a magnetic data point having the same spatial location. (Source: Standard aeromagnetic data collection procedures as written in the contract for survey collection.)
    line_no
    flight line number (Source: often assigned by airborne survey crew)
    Range of values
    Minimum:1
    Maximum:33
    Units:alphanumeric value
    directn
    direction of flight line, azimuth degrees clockwise from north (Source: self evident)
    Range of values
    Minimum:90
    Maximum:270
    Units:degrees
    longitud
    longitude - geographic coordinate (Source: self evident)
    Range of values
    Minimum:-85.6148
    Maximum:-84.7904
    Units:decimal degrees
    latitude
    latitude - geographic coordinate (Source: self evident)
    Range of values
    Minimum:34.0482
    Maximum:34.9883
    Units:decimal degrees
    year
    year of data point collection (Source: self evident)
    Range of values
    Minimum:1978
    Maximum:1978
    jul_day
    Julian day, where Jan. 1 = 1 and Dec. 31 = 365 or 366 (leap year) (Source: self evident)
    Range of values
    Minimum:305
    Maximum:305
    fiducial
    A fiducial number is a mark which indicates points of simultaneity. It is a user-defined integer used during airborne operations to correlate recording devices (magnetometers) with navigational records (altimeters, camera film, strip charts) that were recorded at the same time. (Source: Sheriff, R.E., 1984, Encyclopedic dictionary of exploration geophysics: Tulsa, OK, Society of Exploration Geophysicists, p. 89.)
    Range of values
    Minimum:81
    Maximum:7470
    radar
    radar altimetry reading, in feet An aircraft navigational system in which short electromagnetic waves are transmitted, and the energy scattered back by reflection is detected. From this measurement, the distance between the aircraft and the ground is calculated and recorded as radar altimetry. Values of -9999.9 indicate missing data. (Source: Sheriff, R.E., 1984, Encyclopedic dictionary of exploration geophysics: Tulsa, OK, Society of Exploration Geophysicists, p. 89.)
    Range of values
    Minimum:255.91
    Maximum:2293.31
    Units:feet
    barom
    altitude above mean sea level, in feet, measured using the barometer that is part of the aircraft navigation system Values of -9999.9 indicate missing data. (Source: self evident)
    Range of values
    Minimum:830.05
    Maximum:4757.22
    Units:feet
    totmag
    Raw magnetic value with base magnetometer corrections applied. Because the magnetic field of the earth varies diurnally, a stationary base magnetometer is maintained on the ground during airborne surveying. The base magnetometer records changes in the magnetic field (in nanoTeslas) as a function of time. The magnetic changes may have an amplitude of 20 to 50 nanoTeslas. If changes are more severe, as would occur from a magnetic storm, surveying is discontinued or the data recorded are not used. Diurnal variations are then removed from the airborne magnetic data based on the common time. Values of -9999.9 indicate missing data. (Source: see: Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1, p. 83-87. A description of magnetometers and how they measure the total magnetic field can be found in: Dobrin, M.B., 1976, Introduction to Geophysical Prospecting: New York, McGraw-Hill Book Company, p. 505-517.)Frequency of measurement: The magnetometer has a recharging interval of 0.5 seconds. The data were recorded at 1 second intervals.
    Range of values
    Minimum:53370.00
    Maximum:54878.60
    Units:nanoTeslas
    resmag
    residual magnetic value The total magnetic value minus a geomagnetic reference field (GRF), which is a long-wavelength regional magnetic field. The most commonly used reference field is determined from a model developed by the International Association of Geomagnetism and Aeronomy (IAGA). The International Geomagnetic Reference Field (IGRF), is a predictive model adopted at the beginning of a model period (e.g. in 1989 for 1990-1995). After the model period, a revised definitive model is adopted, the DGRF. This is the preferred model to use for removing regional magnetic fields. For this survey, the field removed was the IGRF 1975. Values of -9999.9 indicate missing data. (Source: Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1, p. 83-87.)
    Range of values
    Minimum:-929.4
    Maximum:56.6
    Units:nanoTeslas

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, David Daniels, Carol Finn, Pat Hill, Holly Hindle, Bob Kucks, 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?
    USGS Gravity and Magnetics Contact
    U.S. Geological Survey
    Box 25046 Mail Stop 964
    Denver Federal Center
    Denver, CO

    303-236-1343 (voice)
    grav_mag@usgs.gov

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. Aeromagnetic anomaly maps are important tools in mapping surficial and buried igneous rocks. 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: 1978 (process 1 of 2)
    Conversion of measured values to geographic position and magnetic values was performed by the contractor using industry standard practices. Details are found under Attribute Accuracy Report, 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: 25-Oct-2001 (process 2 of 2)
    USGS reformatting of contractor data to standard format. USGS personnel used the software package Oasis Montaj version 4.3 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). Information was added that was missing from the data file but was recorded elsewhere. The reformatted data file was written in the format described in the section on Entity_and_Attribute_Overview.
  3. What similar or related data should the user be aware of?
    U.S. Geological Survey, 1999, Digitized Aeromagnetic Datasets of the Conterminous United States, Hawaii, and Puerto Rico: U.S. Geological Survey Open-File Report OFR 99-557, U.S. Geological Survey, Denver CO.

    Online Links:

    Other_Citation_Details:
    CD-ROM or online files Complements this publication with analog data
    U.S. Geological Survey, 1979, Aeromagnetic map of northwest Georgia: U.S. Geological Survey Open-File Report OFR 79-1369, U.S. Geological Survey, Denver, CO.

    Other_Citation_Details:
    map scale 1:250,000 The OFR map has a note that states that the map was "prepared in cooperation with the state of Georgia, Department of Natural Resources."

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 position of the survey aircraft used to collect data were determined by reconciling down-looking photographs (recorded on continuous-strip film) with topographic maps and orthophotoquadrangle maps. Fiducial numbers and marks, impressed on any paper strips that were recording data or added to magnetic tape records, were included as a function of time to further reconcile location with instrumentation.
  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 at a draped survey having a average terrain clearance of 500 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?
    This is survey part 2 of 2. The first part of this survey can be found under Northwest Georgia East, Georgia, Project #3145A. The survey was separated based on flight spacing (#3145A was flown at 1 mile spacing, and #3145B was flown at 2 mile spacing). Loss of data due to poor transmission, channel dropout, obvious spiking, missing channels, and other obvious errors were replaced with the value -9999.9. There are no tie lines. The values in the barometric altimeter (barom) channel are inconsistent.
  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. There is a discrepancy in the radar altimeter (radar) channels. The USGS contract specifies that the survey was to be flown at 500 feet, while the contractor database has an average radar value of 1129 feet.

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. 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 3)
    CD-ROM distributor: USGS Information Services
    Box 25286, Building 810
    Denver Federal Center
    Denver, CO

    303-202-4700 or 1-888-ASK-USGS (voice)
    www.usgs.gov/pubprod
  2. What's the catalog number I need to order this data set? USGS Open-File Report OFR 02-0361
  3. What legal disclaimers am I supposed to read?
    Although all data published on this CD-ROM 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: Each line contains data in the following format, beginning with line 1(no header included): line_no I5 directn I4 longitud F11.4 latitude F9.4 year I5 jul_day I4 fiducial I7 radar F8.1 barom F8.1 totmag F9.2 resmag F9.2, 1X in format ASCII
      Media you can order: CD-ROM (format ISO 9660)
    • Cost to order the data:

      CD-ROM prices are subject to change. Please call or see http://mapping.usgs.gov/esic/prices/

    • Special instructions:
      Detailed instructions can be found at www.usgs.gov/pubprod

  1. Who distributes the data set? (Distributor 2 of 3)
    Web site administration: U.S Geological Survey Central Publications Group
    Central Publications Group
    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? USGS Open-File Report OFR 02-0361
  3. What legal disclaimers am I supposed to read?
    Although all data published on this CD-ROM 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 1.1) Each line contains data in the following format, beginning with line 1 (no header included): line_no I5 directn I4 longitud F11.4 latitude F9.4 year I5 jul_day I4 fiducial I7 radar F8.1 barom F8.1 totmag F9.2 resmag F9.2, 1X
      Network links: http://pubs.usgs.gov/of/2002/ofr-02-361/
    • Cost to order the data: none


  1. Who distributes the data set? (Distributor 3 of 3)
    Peter N Schweitzer
    USGS Eastern Mineral and Environmental Resources Science Center
    Geologist
    12201 Sunrise Valley Drive
    Reston, VA
    USA

    703-648-6533 (voice)
    703-648-6252 (FAX)
    pschweitzer@usgs.gov
  2. What's the catalog number I need to order this data set? GA_3145B_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?

Dates:
Last modified: 05-Dec-2014
Metadata author:
USGS Gravity and Magnetics contact
U.S. Geological Survey
Box 25046 Mail Stop 964
Denver Federal Center
Denver, CO

303-236-1343 (voice)
grav_mag@usgs.gov
Metadata standard:
Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

This page is <https://mrdata.usgs.gov/geophysics/surveys/waf/GA_3145B_META.faq.html>
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