U.S. Geological Survey

South  Dakota Aeromagnetic Data Processing

The assembly of 15 individual aeromagnetic surveys and grids to create the South Dakota state-wide compilation was done in several steps using the commercial software package Geosoft / Oasis montaj. References listed are those known to be used as part of the package.


  1. Grids were constructed from the original aeromagnetic survey data with a cell size of between 1/3 and 1/5 of the flightline spacing of the survey using a minimum curvature gridding algorithm (Webring, 1982). For digitized contour line data, the initial grid was constructed using a minimum curvature algorithm and a spacing appropriate for the scale of the digitized map.
  2. The Definitive Geomagnetic Reference Field (DGRF) calculated for the date of the original survey was removed (in some cases this required the determination and addition of the original reference field removed before removal of the DGRF).
  3. Much of the data required microlevelling, using OASIS montaj, before continuation to our reference surface. This procedure uses decorrugation, and attempts to remove any artificial trends or anomalies due to residual flight line noise remaining in the data. The grids resultant from this step (whether or not microlevelling was performed) whose grid spacing is less than the final grid cell size of 1000 meters for the composite grid are available for downloading as indicated on the "Detailed Aeromag Maps" page.
  4. The original survey grids were continued to 304.8 meters (1000 feet) above ground by converting to draped as necessary. Upward continuation of the NURE and digitized draped surveys was by standard 2D FFT filtering techniques. Level-to-drape continuation and surface-to-drape continuation where true measurement elevations were known, were performed using the chessboard method (Cordell and others,1992).
  5. The survey grids were regridded, as necessary, to the final grid cell size of 1000 meters using a minimum curvature algorithm.
  6. The datum levels of the converted grids were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries.
  7. These adjusted grids were combined into a single merged grid that shows the magnetic field measured or calculated at 304.8 meters (1000 feet) above ground. This level was selected in order to be compatible with recent USGS state compilations including adjacent state compilations in Montana (McCafferty and others, 1998) and Wyoming (Kucks and others, 2000).


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