Airborne geophysical survey: Southern Georgia, Georgia

Frequently anticipated questions:

• What does this data set describe?
  1. How might this data set be cited?
  2. What geographic area does the data set cover?
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
  7. How does the data set describe geographic features?
• Who produced the data set?
  1. Who are the originators of the data set?
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
• Why was the data set created?
• How was the data set created?
  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
  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?
  2. How accurate are the geographic locations?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
  5. How consistent are the relationships among the data, including topology?
• How can someone get a copy of the data set?
  1. Are there legal restrictions on access or use of the data?
  2. Who distributes the data?
  3. What's the catalog number I need to order this data set?
  4. What legal disclaimers am I supposed to read?
  5. How can I download or order the data?
• Who wrote the metadata?

What does this data set describe?

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: Southern Georgia, Georgia:.

   Online Links:

   • https://mrdata.usgs.gov/magnetic/show-survey.php?id=GA_3097

   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: -85.13
   East_Bounding_Coordinate: -81.60
   North_Bounding_Coordinate: 32.88
   South_Bounding_Coordinate: 30.33
   

3. What does it look like?

   http://mrdata.usgs.gov/geophysics/surveys/geophysics2/GA/GA_3097.jpg (JPEG)

   Reduced-size image depicting the data, 645 x 531 pixels, 41,661 bytes

4. Does the data set describe conditions during a particular time period?

   Beginning_Date: Apr-1977

   Ending_Date: Jul-1977

   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: 3097 Project name: Southern Georgia, Georgia Survey flown by: Geodata International, Inc. Survey flown for: U.S. Geological Survey Approx. no. of line miles: 21,700 Survey height: 500 ft Altitude method: Draped over terrain Flight-line spacing: 1 mi Flight-line direction: NW-SE Aircraft used: PA-31 Navajo Airport - arrival: unknown Airport - departure: unknown Magnetometer used: Geometrics G-803 proton precession Sensor tow distance: Stinger length 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:	148
   Units:	alphanumeric value

   directn

   direction of flight line, azimuth degrees clockwise from north (Source: self evident)

   Range of values
   Minimum:	-1
   Maximum:	-1
   Units:	degrees

   longitud

   longitude - geographic coordinate (Source: self evident)

   Range of values
   Minimum:	-85.1721
   Maximum:	-81.5782
   Units:	decimal degrees

   latitude

   latitude - geographic coordinate (Source: self evident)

   Range of values
   Minimum:	30.2802
   Maximum:	32.912
   Units:	decimal degrees

   year

   year of data point collection (Source: self evident)

   Range of values
   Minimum:	1977
   Maximum:	1977

   jul_day

   Julian day, where Jan. 1 = 1 and Dec. 31 = 365 or 366 (leap year) (Source: self evident)

   Range of values
   Minimum:	97
   Maximum:	190

   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:	0
   Maximum:	6126

   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:	309.7
   Maximum:	2173.7
   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:	-9999.9
   Maximum:	-9999.9
   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:	51293.40
   Maximum:	54254.40
   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. The temporal variations cannot be completely predicted, thus error arises in the application of the IGRF reductions. These errors cannot be evaluated until the release of the DGRF, a minimum of five years after the IGRF release. This field is issued to best describe the configuration of the earth's past magnetic field. Accuracy of the DGRF calculations is dependent on the quality of the survey data and sensitive to changes in air pressure. Through processing experimentation and experience, USGS personnel found that significant flight altitude changes produced small changes (0.01-0.03 nT) in DGRF values regardless of the altitude source used (e.g. radar altimeter, barometric altimeter). If radar altimeter data are complete, a terrain channel was calculated using Oasis Montaj DAP (Data Access Protocol), USGS 1:250,000 terrain data, or USGS 15 second terrain data. DAP is the easiest to use but suffers from inconvenient dummy values at sea level. In such cases, 15 second terrain data was used. 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:	-1573.90
   Maximum:	755.20
   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: 1977 (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 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: Dec-2001 (process 2 of 3)

   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.

   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)

   vatnipp@usgs.gov

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:

   • http://pubs.usgs.gov/of/1999/ofr-99-0557/

   Other_Citation_Details:

   CD-ROM or online files Complements this publication with analog data

   U.S. Geological Survey, 1979, Aeromagnetic map of southwest Georgia, OFR 79-756: U.S. Geological Survey Open-File Report OFR 79-756, U.S. Geological Survey, Denver, CO.

   Other_Citation_Details:

   map scale 1:250,000 The OFR map includes a note that the report was "prepared in cooperation with Coastal Plains Regional Commission."

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 using aircraft navigational aids such as line-of-sight electronic systems that measure the distances from each of two ground stations to the aircraft using microwave or radio transmitters. The contractor report states that a GNS-500A Precision Navigation System, an electronic navigation system was used.
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) The magnetometer was carried on a "stinger" that was attached to the aircraft. 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?
   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 barom channel. The original contractor database did not have flight direction (directn), fiducial (fiducial), or barometric altimeter (barom) channels. A dummy value of -1 was used in the directn channel in order to conform to the established template. The values in the fiducial channel were generated based on latitude, using Oasis montaj. Overall, the data is of poor quality due to poor navigation, uneven flight line spacing, poor magnetic leveling, and numerous magnetic dropouts. Dropouts and many of the broad spikes have been replaced with dummy values (-9999.9). Heavy filtering and grid decorrugation is necessary. There are no tie lines. Only half of line 62 contains good data.
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. Data were taken from a UNIX file in a format developed by the U.S. Geological Survey to conserve processing resources. Post file format contains four total-field values correlated with a pair of latitude and longitude coordinates, year, and day (YYYY.DDD). Extent of typical editing (i.e. drop-outs, spikes) performed before the creation of the .pos file is unknown and varied by individual.

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

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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 3.1) 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

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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_3097_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?
   • Availability in digital form:

     Data format:	Flight-line airborne magnetic anomaly measurements in format CSV Size: 8.4003582000732
     Network links:	http://mrdata.usgs.gov/geophysics/surveys/geophysics2/GA/GA_3097.ZIP

   • Cost to order the data: none

Who wrote the metadata?

Dates:

Last modified: 10-Nov-2020

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)