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Data Series 135: Iowa Magnetic Data Processing

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The assembly of nine individual magnetic surveys and grids to create the Iowa State-wide compilation was done in several steps using in-house USGS software.

DATA PROCESSING STEPS

1. Grids were constructed from the digitized magnetic survey data with a cell size of between 1/3 and 1/5 of the original flight-line spacing of the survey using a minimum curvature gridding algorithm (Webring, 1982).

2. The Definitive Geomagnetic Reference Field (DGRF) calculated for the date of the original survey was removed (in some cases this required the determination, calculation, and addition of the original reference field that was subtracted before removal of the DGRF).

3. The original survey grids were continued to 305 m (1,000 ft) above ground as necessary. Level-to-drape continuation was performed using the chessboard method (Cordell and others,1992).

4. The survey grids were regridded, as necessary, to the final grid cell size of 1,000 m (3,280 ft) using a minimum curvature algorithm.

5. The datum levels of the converted grids were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries.

6. These adjusted grids were combined into a single merged 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 State compilations.

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