Illinois, Indiana, and Ohio Magnetic Data Processing
The assembly of 19 individual magnetic surveys and grids to create the
Illinois, Indiana, and Ohio State-wide compilation was accomplished 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
The following steps were executed using commercial software package Geosoft Oasis montaj:
- Grids were constructed from the original magnetic survey data 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 (Webring,
1981). 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 that was removed before
final removal of the DGRF).
- Some of the data required microlevelling 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 these three steps
(whether or not microlevelling was performed) whose grid spacing is less than or equal to a grid cell
size of 100 m (~330 ft) are available for downloading as indicated on the "Higher Resolution Aeromagnetic Maps" page.
- The original survey grids were continued to 305 m (~1,000 ft) above
ground as necessary. Upward continuation of the NURE and digitized
draped surveys was by standard 2D FFT (two-dimensional Fast Fourier Transform) 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, 1982).
- The survey grids were regridded, as necessary, to the final grid cell size
of 500 m (~1,640 ft) using a minimum curvature algorithm.
- These adjusted grids were levelled and merged into a single grid that depicts
the magnetic field measured or calculated at 305 m (~1,000 ft) above
ground. This level was selected in order to be compatible with recent USGS
GRID PROJECTION SPECIFICATIONS
- Projection = Lambert conformal conic
- Central meridian = 83 degrees W. longitude
- Base latitude = 0 degrees N. latitude
- Standard parallels = 33 and 45 degrees N. latitudes
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
- Horizontal datum = NAD27
- Ellipsoid = Clarke 1866
- Local Datum Transform = USA NADCON Geoid99