Digital data grids for the magnetic anomaly map of North America

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Frequently anticipated questions:


What does this data set describe?

Title:
Digital data grids for the magnetic anomaly map of North America
Abstract:
A digital magnetic anomaly database and map for the North American continent is the result of a joint effort by the Geological Survey of Canada (GSC), U. S. Geological Survey (USGS), and Consejo de Recursos Minerales of Mexico (CRM). The database and map represent a substantial upgrade from the previous compilation of magnetic anomaly data for North America, now over a decade old.
This report presents three unique, gridded data sets used to make the magnetic anomaly map of North America. All three grids have 1-km spacing and are projected to the DNAG projection. These grids are provided in Geosoft binary grid format, with two files describing each of the grids (suffixes .grd and .gi).
The first grids (NAmag_origmrg.grd and USmag_origmrg.grd) show the magnetic field at 1,000 m. above terrain.
For the second grids (NAmag_hp500.grd and USmag_hp500.grd) we removed long-wavelength anomalies (500 km and greater) from the first grid. This grid was used for the published map. Although the North American merged grid represents a significant upgrade to older compilations, the existing patchwork of surveys is inherently unable to accurately represent anomalies with long (greater than roughly 150 km) wavelengths, particularly in the US and Canada (U.S. Magnetic- Anomaly Data Set Task Group, 1994). The lack of information about long wavelength anomalies is primarily related to datum shifts between merged surveys, caused by data acquisition at widely different times and by differences in merging procedures. Therefore, we removed anomalies with wavelengths greater than 500 km from the merged grid to reduce the effects caused by the spurious long wavelengths but still maintain the continuity of anomalies. The correction was accomplished by transforming the merged grid to the frequency domain, filtering the transformed data with a long-wavelength cutoff at 500 km, and subtracting the long-wavelength data grid from the merged grid.
In addition to the 500-km high pass filter, an equivalent source method, based on long-wavelength characterization using satellite data (CHAMP satellite anomalies, Maus and others, 2002), was also used to correct for spurious shifts in the original magnetic anomaly grid (Ravat and others, 2002). These results are presented in the third grids (NAmag_CM.grd and USmag_CM.grd), in which the wavelengths longer than 500 km have been replaced by downward-continued satellite data.
Supplemental_Information:
Publicly available airborne and marine magnetic data have been collected in North America primarily by the governments of Canada, the U.S., and Mexico.
  1. How might this data set be cited?
    U.S. Geological Survey, 2002, Digital data grids for the magnetic anomaly map of North America: U.S. Geological Survey Open-File Report 02-414, U.S. Geological Survey, Denver, Colorado, USA.

    Online Links:

  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: 170.5
    East_Bounding_Coordinate: -10.0
    North_Bounding_Coordinate: 87.0
    South_Bounding_Coordinate: 7.35
  3. What does it look like?
    ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414/NAmag.jpg (JPEG)
    Reduced-size image of 1000-meter merged gridded data, 1146x1053 pixels, 592k bytes
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 1945
    Ending_Date: 2001
    Currentness_Reference:
    data collection period
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: raster digital data
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a raster data set. It contains the following raster data types:
      • Dimensions 8511 x 8901, type grid cell
    2. What coordinate system is used to represent geographic features?
      The map projection used is Transverse Mercator.
      Projection parameters:
      Scale_Factor_at_Central_Meridian: 0.926
      Longitude_of_Central_Meridian: -100.0
      Latitude_of_Projection_Origin: 0.0
      False_Easting: 0
      False_Northing: 0
      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 1000
      Ordinates (y-coordinates) are specified to the nearest 1000
      Planar coordinates are specified in meters
      The horizontal datum used is North American Datum of 1927.
      The ellipsoid used is Sphere.
      The semi-major axis of the ellipsoid used is 6371204.
      The flattening of the ellipsoid used is 1/1.0.
  7. How does the data set describe geographic features?
    Entity_and_Attribute_Overview:
    This metadata file describes three individual but related grids. The first grid contains the residual total intensity of the Earth's magnetic field. The second grid was made by removing a 500-km high-pass filtered from the total intensity data. The third grid is the result of removing a magnetic field that was calculated from satellite magnetic data by Tiku Ravat of Southern Illinois University.
    Each grid has an accompanying file with the suffix .gi that gives projection information. The file names are:
    NAmag_origmrg.grd = 232,371 KB
    NAmag_origmag.grd.gi = 21 KB
    
    NAmag_hp500.grd = 232,753 KB
    NAmag_hp500.grd.gi = 22 KB
    
    NAmag_CM.grd = 232,557 KB
    NAmag_CM.grd.gi = 22 KB
    
    Entity_and_Attribute_Detail_Citation: http://pubs.usgs.gov/sm/mag_map/mag_book_s.pdf
    NAmag_origmrg.grd
    Residual total intensity of Earth's magnetic field
    origmrg grid cell 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 (Source: 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.)
    ValueDefinition
    -1.0E+32missing data
    Range of values
    Minimum:-22711.6
    Maximum:27553.6
    Units:nanoteslas
    Resolution:1-10 nanoteslas
    NAmag_hp500.grd
    500-km high-pass filtered grid calculated from residual magnetic grid
    hp500 grid cell value
    Because wavelengths greater than roughly 150 km are unreliable in the compilation, applying a high-pass wavelength filter would appear to be a viable solution to remove these unreliable wavelengths. However, removing wavelengths less than 500 km from the merged grid creates artifacts, such as spurious separation of continuous anomalies. Therefore, we removed anomalies with wavelengths greater than 500 km from the merged grid to reduce the effects caused by the erroneous long wavelengths but maintaining continuity of anomalies. The correction was accomplished by transforming the merged grid to the frequency domain, filtering the transformed data with a long-wavelength cutoff at 500 km, and subtracting the long-wavelength data grid from the merged grid.
    ValueDefinition
    -1.0E+32missing data
    Range of values
    Minimum:-22729.3
    Maximum:27565.8
    NAmag_CM.grd
    Equivalent-source corrected grid
    CM grid cell value
    An equivalent source method based on long-wavelength characterization using satellite data was used to correct for long-wavelength shifts.
    We produced an aeromagnetic grid in which the wavelengths longer than 500 km have been replaced by downward-continued CHAMP satellite data. Steps 0 and 6 were performed by Bob Kucks. Steps 1-4 were performed by Tiku Ravat. Step 5 was performed by Jeff Phillips.
    0. The North American 1 km merged grid was decimated to 5 km.
    1. This 5 km grid was converted to a 0.05 degree grid. This grid was low-pass filtered using a Gaussian filter with a 500 km cutoff, then decimated to 1 degree.
    2. A joint inversion of this 1 degree low-pass aeromagnetic grid and CHAMP satellite data, with the aeromagnetic data weighted very low, produced a stabilized downward continuation of the CHAMP data.
    3. The inverted data were interpolated to 0.05 degree and again low-pass filtered using the same Gaussian 500 km filter to remove
    4. The low-pass grid from step 1 was subtracted from the original 0.05 degree aeromagnetic grid to create a 500 km high-pass aeromagnetic grid. This grid was added to the low-pass inverted grid from step 3 to get a corrected 0.05 degree aeromagnetic grid.
    5. The corrected 0.05 aeromagnetic degree grid was projected to the DNAG projection and regridded to 5 km. This was subtracted from the decimated 5 km aeromagnetic grid to generate a 5 km correction grid. A matched filter was used to remove short-wavelength artifacts resulting from the projection and regridding process.
    6. The resulting 5 km correction grid was regridded to the original 1 km grid and subtracted from the original 1 km aeromagnetic grid to generate the final 1 km corrected aeromagnetic grid. (Source: Ravat, D., Whaler, K.A., Pilkington, M., Sabaka, T., and Purucker, M., 2002, Compatibility of high-altitude aeromagnetic and satellite-altitude magnetic anomalies over Canada: Geophysics, v. 67, p. 546-554.)
    ValueDefinition
    -1.0E+32missing data
    Range of values
    Minimum:-22724
    Maximum:27541

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?
    North American Magnetic Anomaly Group (NAMAG)
    Viki Bankey (USGS)
    Alejandro Cuevas (CRM)
    David Daniels (USGS)
    Carol A. Finn (USGS)
    Israel Hernandez (CRM)
    Patricia Hill (USGS)
    Robert Kucks (USGS)
    Warner Miles (GSC)
    Mark Pilkington (GSC)
    Carter Roberts (USGS)
    Walter Roest (GSC)
    Victoria Rystrom (USGS)
    Sarah Shearer (USGS)
    Stephen Snyder (USGS)
    Ronald Sweeney (USGS)
    Julio Velez (CRM)
    Jeffrey Phillips (USGS)
    D.K.A. Ravat (Southern Illinois University)
    
  3. To whom should users address questions about the data?
    Robert P. Kucks
    U.S. Geological Survey
    Box 25046, MS964
    Denver Federal Center
    Denver, CO
    USA

    303-236-1405 (voice)
    rkucks@usgs.gov

Why was the data set created?

This integrated, readily accessible, modern digital database of magnetic anomaly data will be a powerful tool for further evaluation of the structure, geologic processes, and tectonic evolution of the continent and may also be used to help resolve societal and scientific issues that span national boundaries. The North American magnetic anomaly map derived from the digital database provides a comprehensive magnetic view of continental-scale trends not available in individual data sets, helps link widely separated areas of outcrop, and unifies disparate geologic studies.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    (process 1 of 7)
    We obtained all available aeromagnetic surveys. Some data were digitized from analog maps if digital data were unavailable. The remainder were processed using digital flight-line data.
    Details of data acquisition are described in the booklet that accompanies the printed map.
    The marine data were obtained from the National Geophysical Data Center of the National Oceanic and Atmospheric Administration spanned the years 1958 through 1997.
    (process 2 of 7)
    Aeromagnetic data were processed so that residual magnetic values were calculated in a consistent manner:
    - The DGRF, updated to survey date, was removed
    - The flight elevation was used to mathematically calculate the equivalent magnetic field at 1,000 ft. above terrain
    - obvious errors were corrected
    - x-y locations were calculated for the described DNAG projection
    - data were gridded at 1/3 - 1/4 of the flight-line spacing, then regridded to 1 km.
    (process 3 of 7)
    Using various USGS in-house programs (before 1999) or Geosoft/Oasis Montaj (after 1999), gridded data were mathematically merged together. For the U.S. part, scientists merged groups of surveys by state or groups of states. States were merged into regions, and regions into countries. For progress on state compilations, go to http://crustal.usgs.gov/projects/namad/
    Procedures and software used to compile the 1-km grid of magnetic data for Canada are detailed on the Geophysical Data Centre website
    (process 4 of 7)
    Creation of a wavelength filtered grid.
    Because wavelengths greater than roughly 150 km are unreliable in the compilation, applying a high-pass wavelength filter would appear to be a viable solution to remove these unreliable wavelengths. However, removing wavelengths less than 500 km from the merged grid creates artifacts, such as spurious separation of continuous anomalies. Therefore, we removed anomalies with wavelengths greater than 500 km from the merged grid to reduce the effects caused by the erroneous long wavelengths but maintaining continuity of anomalies. The correction was accomplished by transforming the merged grid to the frequency domain, filtering the transformed data with a long-wavelength cutoff at 500 km, and subtracting the long-wavelength data grid from the merged grid.
    (process 5 of 7)
    An equivalent source method based on long-wavelength characterization using satellite data was used to correct for long-wavelength shifts.
    We produced an aeromagnetic grid in which the wavelengths longer than 500 km have been replaced by downward-continued CHAMP satellite data. Steps 0 and 6 were performed by Bob Kucks. Steps 1-4 were performed by Tiku Ravat. Step 5 was performed by Jeff Phillips.
    0. The North American 1 km merged grid was decimated to 5 km.
    1. This 5 km grid was converted to a 0.05 degree grid. This grid was low-pass filtered using a Gaussian filter with a 500 km cutoff, then decimated to 1 degree.
    2. A joint inversion of this 1 degree low-pass aeromagnetic grid and CHAMP satellite data, with the aeromagnetic data weighted very low, produced a stabilized downward continuation of the CHAMP data.
    3. The inverted data were interpolated to 0.05 degree and again low-pass filtered using the same Gaussian 500 km filter to remove
    4. The low-pass grid from step 1 was subtracted from the original 0.05 degree aeromagnetic grid to create a 500 km high-pass aeromagnetic grid. This grid was added to the low-pass inverted grid from step 3 to get a corrected 0.05 degree aeromagnetic grid.
    5. The corrected 0.05 aeromagnetic degree grid was projected to the DNAG projection and regridded to 5 km. This was subtracted from the decimated 5 km aeromagnetic grid to generate a 5 km correction grid. A matched filter was used to remove short-wavelength artifacts resulting from the projection and regridding process.
    6. The resulting 5 km correction grid was regridded to the original 1 km grid and subtracted from the original 1 km aeromagnetic grid to generate the final 1 km corrected aeromagnetic grid.
    Date: 16-Jul-2012 (process 6 of 7)
    Modified metadata to describe projection as Spherical Transverse Mercator with radius 6371204 meters, NAD27. Person who carried out this activity:
    Peter N Schweitzer
    USGS ER GD
    Geologist
    12201 Sunrise Valley Drive
    Reston, VA
    USA

    703-648-6533 (voice)
    703-648-6252 (FAX)
    pschweitzer@usgs.gov
    Date: 16-Jul-2012 (process 7 of 7)
    Creation of original metadata record Person who carried out this activity:
    Viki Bankey
    U.S. Geological Survey
    Box 25046 Mail Stop 964
    Denver Federal Center
    Denver, CO
    USA

    303-236-1348 (voice)
    viki@usgs.gov
  3. What similar or related data should the user be aware of?
    U.S. Geological Survey, Consejo de Recursos Minerales, Geological Survey of Canada, and North American Magnetic Anomaly Group, 2002, Magnetic Map of North America: U.S.Geological Survey Special Map Magnetic Anomaly Map of North America, U.S. Geological Survey, Denver, Colorado, USA.

    Online Links:

    Other_Citation_Details:
    This citation refers to the printed map and booklet of the North American magnetic anomaly map.
    U.S. Geological Survey, and National Geophysical Data Center, 2002, Digital aeromagnetic data sets of the Conterminous United States and Hawaii: U.S. Geological Survey Open-File Report 02-361, U.S. Geological Survey, Denver, CO, USA.

    Online Links:

    Other_Citation_Details:
    Report contains digital aeromagnetic profile data from airborne surveys flown in the United States for the USGS. These data were processed from the original digital data provided by the contractor or the USGS.
    Data are available either on a DVD or online at the address below.
    U.S. Geological Survey, 1999, Digitized Aeromagnetic datasets for the conterminous United States, Hawaii, and Puerto Rico.: U.S. Geological Survey Open-File Report 99-557, U.S. Geological Survey, Denver, CO.

    Online Links:

    Other_Citation_Details:
    The data available in this report are a compilation of digitally converted analog magnetic field intensity maps from the USGS or other public sources.

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

  1. How well have the observations been checked?
    Grid values represent the total intensity of the Earth's magnetic field after removal of the International Geomagnetic Reference Field (IGRF). Measurements were made using a variety of magnetometer systems with typical accuracies of 1 to 10 nanotesla (nT).
    The grids presented in this report were made from numerous individual grids that were mathematically merged together using standard techniques. Individual metadata files exist for USGS gridded data created from digital flight-line data (USGS Open-File Report 02-0361).
    The data in the original grids have been processed using formulas and methods that are not usually documented but that represent industry standard practices for airborne data reduction.
  2. How accurate are the geographic locations?
    Flight Path Recovery - before about 1975 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.
    Flight Path Recovery - before about 1990 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.
    Flight Path Recovery - after about 1990 Horizontal position 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)
    The magnetometer was carried on a "stinger" that was attached to the aircraft or was carried in a bird towed on a line that was below the aircraft. The bird as it is towed is slightly behind the aircraft and therefore the vertical distance between the magnetometer and the aircraft is slightly less than the length of the line but remains constant for the survey.
    Data were either collected at a fixed barometric altitude or were collected as a draped survey having an average terrain clearance above the ground. 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?
    The North American Magnetic Anomaly grids were compiled from the best available aermomagnetic surveys publicly available in 2002. Best available would be data having the highest resolution, closest line-spacing, most accurate instrumentation, most accurate flight-elevation surface, and similar specifications commonly used in the airborne surveying industry. In many areas, only one survey was available. In many offshore areas and some onshore areas, no data were available at all, and these areas were left blank.
  5. How consistent are the relationships among the observations, including topology?
    The data in this file were collected by various contractors or groups who were responsible for collecting and processing the data.
    The data used to make these grids were collected using different instruments (magnetometers, altimeters, navigational systems) throughout the survey, but were usually consistent within each survey.
    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
Use_Constraints: none
  1. Who distributes the data set? (Distributor 1 of 1)
    DVD distributor: USGS Information Services
    Box 25286, Building 810
    Denver Federal Center
    Denver, CO
    USA

    303-202-4700 (voice)
    ask@usgs.gov
  2. What's the catalog number I need to order this data set? USGS Open-File Report OFR 02-0414
  3. What legal disclaimers am I supposed to read?
    Although all data published in these grids 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?

Who wrote the metadata?

Dates:
Last modified: 13-Jun-2016
Metadata author:
Peter N Schweitzer
USGS Midwest Area
Collection manager, USGS Geoscience Data Clearinghouse, http://geo-nsdi.er.usgs.gov/
Mail Stop 954
12201 Sunrise Valley Dr
Reston, VA
USA

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

This page is <https://geo-nsdi.er.usgs.gov/metadata/open-file/02-414/metadata.faq.html>
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