The assembly of the 26 individual aeromagnetic surveys and
grids to create the Wisconsin state-wide compilation was
done in several steps.
DATA PROCESSING STEPS
- Grids were constructed from the original aeromagnetic
survey data, projected to the specifications given below, with a cell size of between 1/3 and 1/5 of
the flight-line spacing of the survey, using a minimum
curvature gridding algorithm. 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.
- Where feasible, the Definitive Geomagnetic Reference Field (DGRF) was calculated for the date
and altitude of the original survey. Where only residual field field data are available, the original reference field
may be added back if the field specifics are known; then the DGRF is
- Flight lines were adjusted and spurious anomalies
(generated by equipment malfunction) were removed as
- Strike filtering in the direction of the flight lines was
applied as necessary.
- The survey grids were regridded, as necessary, to the
final grid cell size of 250 m using a minimum curvature
- The original survey grids were continued to 1000 feet
above ground and converted from level to drape as
necessary. Upward continuation of surveys flown at elevations less than
1000 ft above ground was by
standard 2D Fast Fourier Transform (FFT) filtering techniques. Surveys flown at altitudes
greater than 1000 ft were continued downward by means of the "Compudrape"
program in the Oasis Montaj package by Geosoft.
- The datum levels of the converted grids were adjusted (by
addition or subtraction of a constant value) to minimize
differences at the boundaries.
- These grids were combined into a single merged
grid that shows the magnetic field measured or calculated
at 1000 feet above ground. This grid is
available for downloading in several formats.
GRID PROJECTION SPECIFICATIONS
- Projection = Lambert conformal conic
- Central meridian = 90 degrees W
- Base latitude = 0 degrees N
- Standard parallels = 33 and 45 N
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
|| Wisconsin Mag Page || USGS
- Eastern Minerals Team || USGS - Geology
|| USGS - Home Page || References
Contact: David L. Daniels
[an error occurred while processing this directive]