Virginia Aeromagnetic and Gravity Maps and Data: A Web Site for Distribution of Data

The Virginia aeromagnetic grid is constructed from grids that combine information collected in 23 separate aeromagnetic surveys conducted between 1948 and 1994. The data from these surveys are of varying quality. The design and specifications (terrain clearance, sampling rates, line spacing, and reduction procedures) varied from survey to survey depending on the purpose of the project and the technology of that time. Every attempt was made to acquire the data in digital form. Most of the available digital data were obtained from aeromagnetic surveys flown by the U.S. Geological Survey (USGS), flown on contract with the USGS, or were obtained from other federal and state agencies. All of the pre-1975 data are available only on hand-contoured maps and had to be digitized. These maps were digitized along flight-line/contour-line intersections, which is considered to be the most accurate method of recovering the original data. Digitized data are available as USGS Open File Report 99-557 (URL: http://pubs.usgs.gov/of/1999/ofr-99-0557/html/mag_home.htm). The surveys were flown mostly in east-west flight lines, spaced from 0.125 to 3.0 miles apart, with elevations ranging from 400 ft to 1000 ft above ground in the drape mode and 5000 ft above sea level in the constant barometric elevation mode. Surveys flown at elevations other than the target elevation were mathematically continued to the target elevation. About half of the state was flown at a line spacing of 0.5 mile or smaller. All surveys have been continued to 152.4 meters (500 feet) above ground and then blended or merged together. The entire state grid was then continued upward to 304.8 meters (1000 feet) above ground. Grids at both elevations are available in several formats. This Virginia aeromagnetic compilation is one part of a national digital compilation by the U.S. Geological Survey, where the standard selected for this national compilation is a survey elevation of 304.8 m (1000 feet) above mean terrain. Other state compilations can be found online at: http://crustal.usgs.gov/geophysics/state.html