The assembly of 65 individual aeromagnetic surveys and grids to build the Montana state-wide compilation was done in several steps.

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 when necessary due to wide flightline spacing. 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.
- Data quality problems were addressed.
- The Definitive Geomagnetic Reference Field (DGRF) was applied for the date of the original survey (in some cases this required the determination and removal of the original reference field applied).
- The survey grids were regridded, as necessary, to the final grid cell size of 500 m using a minimum curvature algorithm.
- The datum levels of adjacent surveys were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries.
- The original survey grids were upward or downward continued and converted from level to drape as necessary to produce a consistent survey specification of 1000 ft above ground. Upward continuation of the NURE surveys was by standard 2D FFT filtering techniques. Downward continuation and level-to-drape was performed (Cordell and others, 1992).
- The datum levels of the converted grids were then adjusted to minimize differences at the boundaries.
- These adjusted grids were combined into a single merged grid.

GRID PROJECTION SPECIFICATIONS

- Projection = Lambert conformal conic
- Central meridian = 110 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

U.S. Department of the Interior

U.S. Geological Survey

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Last updated 07 Dec 2016