USGS Data Series 352: Arkansas and Louisiana Aeromagnetic and Gravity Maps and Data

Arkansas and Louisiana Aeromagnetic Data Processing

The assembly of individual aeromagnetic surveys and grids to create the Arkansas and Louisiana statewide compilation was accomplished in several steps using the commercial software package Geosoft Oasis montaj. References listed are those known to be used as part of the package.

GENERAL DATA PROCESSING STEPS

1. Using the original aeromagnetic survey data, grids were constructed with a cell size of 1/3 to 1/5 of the flight-line spacing by 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.

2. The definitive geomagnetic reference field (DGRF) calculated for the date and elevation of the original survey was subtracted (and in some cases required determining and restoring the original reference field before DGRF subtraction).

3. Flight lines were adjusted, and spurious anomalies (perhaps generated by equipment malfunction) were removed as necessary.

4. The original survey grids were continued to 1,000 ft (305 m) above ground, if necessary, and converted from a level to a draped surface as needed. Upward continuation of the NURE surveys was by standard two-dimensional fast Fourier transform filtering techniques. Level-to-drape or surface-to-drape continuation, where true measurement elevations were known, were performed using the chessboard method (Cordell and others, 1992).

5. The survey grids were regridded as necessary to the final grid-cell size of 1,000 m (3,281 ft) using a minimum curvature algorithm.

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

7. These adjusted grids were combined into a single merged grid that shows the magnetic field measured or calculated at 1,000 ft (305 m) above ground. Weighted averaging of the data was applied in the overlap areas.

GRID PROJECTION SPECIFICATIONS

• Projection = Lambert conformal conic

• Central meridian = 92° W. longitude

• Base latitude = 0° N. latitude

• Standard parallels = 33° and 45° N. latitudes

• Semi-major ellipsoid axis = 6378206.4 m

• Eccentricity squared = 0.0067686579973

• Horizontal datum = NAD27

• Local datum transform = USA NADCON Geoid99

• Ellipsoid = Clarke 1866