Washington Aeromagnetic Maps and Data

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 bidirectional 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), in conjunction with the International Geomagnetic Reference Field (IGRF) 1990, was applied for the date of the original survey (in some cases this required the determination and correction for 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 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).
6. The datum levels of the converted grids were then adjusted to minimize differences at the boundaries.
7. These adjusted grids were combined into a single merged grid.

GRID PROJECTION SPECIFICATIONS

• Projection = Lambert conformal conic
• Central meridian = 122 W
• Base latitude = 45 N
• Standard parallels = 33 and 45 N
• Semi-major ellipsoid axis = 6378206.4 m
• Eccentricity squared = 0.0067686579973