The assembly of 25 individual magnetic surveys and grids to create the Michigan State-wide compilation was accomplished in several steps using Geosoft's commercial software package Oasis montaj. References listed are those known to be used as part of the packages.

DATA PROCESSING STEPS

1. 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.
2. The Definitive Geomagnetic Reference Field (DGRF) calculated for the date of the original survey was removed. In some cases, this removal required the determination and addition of the original reference field that was removed before final removal of the DGRF.
3. Some of the data required micro-leveling before continuation to the reference surface. This procedure used de-corrugation and attempted to remove any artificial trends or anomalies due to residual flight-line noise remaining in the data.
4. 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 performed by standard two-dimensional Fast Fourier Transform (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 400 m (~1,312 ft) 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 grid/survey boundaries.
7. The adjusted grids were combined into a single merged grid that shows the magnetic field strength measured or calculated at 1,000 ft (305 m) above ground.

MAGNETIC GRID PROJECTION SPECIFICATIONS

• Projection = Lambert conformal conic
• Central meridian = 84 degrees W
• Base latitude = 0 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