FS–237–95 OCTOBER 1997

Compiling Magnetic Databases

Introduction

Aeromagnetic anomaly maps show variations 

in the Earth’s magnetic field that are caused 

mainly by magnetic minerals in crustal rocks. 

These anomalies vary with amount and type of 

magnetic material and the geometry and depth of 

the magnetic body. Igneous and metamorphic 

rocks frequently are magnetic enough to cause 

anomalies, whereas sedimentary rocks commonly 

are nonmagnetic. Aeromagnetic anomaly maps 

are important tools for mapping surficial and 

buried rocks, for determining geologic structure, 

and for discovering some types of mineral deposits. Regional geologic features may become evident after individual aeromagnetic surveys are 

compiled and plotted at the same scale in a consistent way.

History of aeromagnetic surveying


	 Magnetic surveying from an aircraft 

became possible with the development of 

stable airborne magnetometers used to detect 

submarines during World War II. The first airborne magnetic survey for geologic purposes 

over land was completed in 1944. Shipborne 

magnetic surveys of the ocean floor emerged 

in the 1950’s, and satellite magnetic surveys 

over much of the Earth’s surface became available in the 1960’s.

	 In the ensuing years, numerous aeromagnetic surveys were conducted for geologic 

mapping and mineral exploration by both Government agencies and private companies. Most 

of these surveys were small, and no attempt 

was made to tie them to neighboring surveys 

by using the same flight elevation or spacing.

	 In the 1970’s, the U.S. Government completed a national airborne survey for uranium 

resources (the National Uranium Resource 

Evaluation, or NURE Program). Although the 

main purpose of this survey was radioelement 

measurement, a second purpose was to acquire 

aeromagnetic data, which were compiled for 

almost all 2-degree quadrangles in the United 

States. These data were collected 400 ft above 

the land surface and usually had flight-line 

spacings 3 or 6 miles apart. These data are useful for giving the geoscientist some indication 

of the regional magnetic field for an area of 

interest anywhere in the United States.

Combining magnetic data

	 The optimum aeromagnetic map of the 

United States would be one created from a new 

survey flown with closely spaced lines at a 

uniform elevation. In the absence of such an 

ideal survey, geoscientists have been creating 

large magnetic data sets by “stitching” together 

existing surveys with their many differences 

and inconsistencies. Differences in magnetic 

data include

•	 whether the survey was flown at a constant

	 barometric elevation (e.g., 9,000 ft above	

Figure showing


22 aeromagnetic


surveys covering 


Utah; each has 


different flight 


specifications.
	 sea level) or was flown a constant distance 

	 above terrain (e.g., 400 ft above the ground,

	 also called “draped”)

•	 original flight elevation

•	 whether original data were obtained from

	 digital tapes or had to be digitized from maps

•	 original flight-line spacing

The first two differences can be minimized using 

mathematical techniques to predict magnetic 

fields at different elevations.

Composite and merged magnetic maps and 

databases


	 Surveys may be plotted as a composite map, 

where each piece is juxtaposed against the others 

without regard to differences in flight elevation 

or spacing. A composite map retains the original 

data resolution, but computer methods to further 

enhance or model regional features crossing survey boundaries cannot be applied. All USGS 

magnetic maps published before 1980 are composite magnetic maps. The first-generation composite magnetic map of the United States was 

created by the Committee for the Magnetic 

Anomaly Map of North America as part of the 

Geological Society of America’s Decade of 

North American Geology (DNAG) project.

	 Computer methods have been developed to 

transform magnetic data into a merged map, 

wherein all survey data are analytically reduced 

to a common flight elevation and then digitally 

merged at the survey boundaries. In theory, a 

merged data set is homogeneous—the data represent the magnetic field at a uniform flight elevation, whether at a constant barometric elevation 

or draped over topography. In practice, homogeneity is only partially achieved due to differences 

in the resolution and quality of the original data 

sets. Poor measurements caused by wide flightline spacing, surveying too far above the magnetic source, and insufficiently sensitive magnetometers cannot be improved.

	 Databases and maps of merged magnetic data 

covering large areas are listed in a bibliography 

(USGS Open-File Report 95–77), also found at 

the web site listed under Additional Information.

How are aeromagnetic data merged?


	 If original flight-line data are available, these 

are gridded at an interval 1⁄3 to 1⁄4 of the flight-line 

spacing and are then regridded to the final 

merged grid interval. This two-step gridding process preserves more detail than does gridding at 

the coarser interval first.

	 Original flight-line data from surveys 

flown before the 1970’s are no longer available, and only the published maps remain. To 

recapture the data, most of these maps have 

been digitized, primarily along flight lines 

where actual data were recorded, but 

sometimes between flight lines wherever necessary. Digitizing reproduces the map, but 

reproducing the measured magnetic field 

depends on the carefulness of the mapmaker. 

The randomly digitized points are gridded, 

converting them to a representation of the magnetic field at equally spaced locations, which 

further modifies the original data.

	 The International Geomagnetic Reference 

Field (IGRF), a model of the Earth’s main 

magnetic field, is calculated for the time and 

flight elevation of the survey and then removed 

from the gridded data; sometimes this step has 

already been accomplished. If a regional field 

other than the IGRF has been previously 

removed, it is restored, and the IGRF is 

removed subsequently. The gridded data are 

mathematically transformed to a new flightelevation surface. The adjacent surveys are 

merged by blending overlapping data or by 

butting surveys together and smoothing the 

seams. 

Additional information

	 USGS Open-File Report 95–77 lists many 

USGS computer programs and databases used 

to create magnetic maps and lists some published magnetic maps.

	 Information on the availability of magnetic 

maps and data in specific areas and other general information on USGS airborne coverage 

can be obtained from:

Pat Hill

U.S. Geological Survey

Box 25046, MS 964

Denver Federal Center

Denver, CO 80225

(303) 236-1343

pathill@musette.cr.usgs.gov

(136.177.80.14)

Viki Bankey (same address)

(303) 236-1348

viki@musette.cr.usgs.gov

web site:

http://minerals.er.usgs.gov