South Dakota Aeromagnetic Data
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.
DATA PROCESSING STEPS
- 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.
- 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).
- 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.
- 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).
- The survey grids were regridded, as necessary, to the final grid cell size
of 1000 meters using a minimum curvature algorithm.
- The datum levels of the converted grids were adjusted (by addition or
subtraction of a constant value) to minimize differences at the boundaries.
- 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,
GRID PROJECTION SPECIFICATIONS
- Projection = Lambert conformal conic
- Central meridian = 100 degrees W
- Base latitude = 43 degrees N
- Standard parallels = 33 and 45 degrees N
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
- Horizontal Datum = NAD27
- Ellipsoid = Clarke 1866
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