The assembly of 25 individual magnetic surveys and grids to create the
Michigan Statewide 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
 Grids were constructed from the original magnetic 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, 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 removal required the determination and addition of the original reference field that was removed before final removal of the DGRF.
 Some of the data required microleveling before continuation to
the reference surface. This procedure used decorrugation
and attempted to remove any artificial trends or anomalies due to
residual flightline noise remaining in the data.
 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
twodimensional Fast Fourier Transform (2D FFT) filtering techniques. Leveltodrape continuation and surfacetodrape 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
400 m (~1,312 ft) 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 grid/survey boundaries.
 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
 Semimajor ellipsoid axis = 6378206.4 m
 Eccentricity squared = 0.0067686579973
 Horizontal datum = NAD27
 Ellipsoid = Clarke 1866
