The assembly of 82 individual magnetic surveys and grids to create the Nevada State-wide compilation was accomplished in several steps using the commercial software packages Geosoft Oasis montaj and Intrepid Gridmerge. References listed are those known to be used as part of the packages.
DATA PROCESSING STEPS
The following steps were executed using commercial software package Geosoft Oasis montaj:
- Grids were constructed from the original magnetic survey data with a cell size of between 1/3 and 1/5 of the flight-line spacing of the survey 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.
- The Definitive Geomagnetic Reference Field (DGRF) calculated for the date of the original survey was removed (in some cases this required the determination and addition of the original reference field that was removed before final removal of the DGRF).
- Some of the data required microlevelling before continuation to our reference surface. This procedure uses decorrugation and attempts to remove any artificial trends or anomalies due to residual flight-line noise remaining in the data. The grids resultant from these three steps (whether or not microlevelling was performed) whose grid spacing is less than or equal to a grid cell size of 100 m (~330 ft) are available for downloading as indicated on the "Higher Resolution Aeromag Maps" page.
- The original survey grids were continued to 305 m (~1,000 ft) above ground as necessary. Upward continuation of the NURE and digitized draped surveys was by standard 2D FFT (two-dimensional Fast Fourier Transform) filtering techniques. Level-to-drape continuation and surface-to-drape continuation, where true measurement elevations were known, were performed using the chessboard method (Cordell and others, 1992).
- The survey grids were regridded, as necessary, to the final grid cell size of 500 m (~1,640 ft) using a minimum curvature algorithm.
The following steps were executed using commercial software package Intrepid Gridmerge:
- The datum levels of the converted grids were adjusted (by addition or subtraction of a constant value) by fitting the surface to a model of the lithospheric magnetic field generated by Lesur and Maus (website). This constrained the long-wavelength anomalies to agree with the CHAMP (Challenging Minisatellite Payload) satellite model and also minimize the magnetic anomaly value differences at the grid/survey boundaries.
- These adjusted grids were feathered and merged into a single 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.
GRID PROJECTION SPECIFICATIONS
- Projection = Lambert conformal conic
- Central meridian = 117 degrees W.
- Base latitude = 35 degrees N.
- Standard parallels = 33 and 45 degrees N.
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
- Horizontal datum = NAD27
- Ellipsoid = Clarke 1866