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

DATA PROCESSING STEPS

1. 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.
2. Data quality problems were addressed.
3. 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).
4. The survey grids were regridded, as necessary, to the final grid cell size of 500 m using a minimum curvature algorithm.
5. The datum levels of adjacent surveys were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries.
6. 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).
7. The datum levels of the converted grids were then adjusted to minimize differences at the boundaries.
8. These adjusted grids were combined into a single merged grid.