Aeromagnetic data were collected along flight lines by instruments in an aircraft that recorded magnetic-field values and locations. In the earlier days of surveying, the only way to represent this data was to generate an analog map with contour lines. This dataset is a representation of the digitized contour lines either by following the lines or by choosing the intersection of the contour and flight-line to create a value of the magnetic field. The values presented are latitude, longitude, and map magnetic-field values.
What coordinate system is used to represent geographic features?
Horizontal positions are specified in geographic coordinates, that is, latitude and longitude.
Latitudes are given to the nearest 0.0001.
Longitudes are given to the nearest 0.0001.
Latitude and longitude values are specified in decimal degrees.
The horizontal datum used is North American Datum of 1927.
The ellipsoid used is Clarke 1866.
The semi-major axis of the ellipsoid used is 6378206.4.
The flattening of the ellipsoid used is 1/294.98.
Generally these data files contain columnar text consisting of a longitude, latitude, and magnetic anomaly value in nanoTeslas. Where the data files have more columns the package should include a text file explaining the additional columns; that arrangement applies to data recorded originally in digital format, and includes some of the intermediate measurements and calculated parameters such as the total magnetic field.
Aeromagnetic surveys are used for geophysical prospecting. Some variations in magnetic measurements are caused by rocks that contain significant amounts of magnetic minerals (magnetite being the most common). These anomalies reflect variations in the amount and type of magnetic material and the shape and depth of the body of rock. The features and patterns of aeromagnetic anomalies can also be used to delineate details of subsurface geology including the locations of buried faults and the thickness of surficial sedimentary rocks.
How well have the observations been checked?
Because these data were digitized from printed contour maps that were drawn originally by researchers who produced the source materials, the values presented here should be regarded as approximate and faithful to the published interpretation (contouring) of the original instrument data. Factors affecting accuracy include the accuracy and precision of the original aircraft flight line navigation and elevation as well as the calibration of the instruments used to measure the magnetic field.
How accurate are the geographic locations?
Horizontal position of the survey aircraft used to collect data were determined by reconciling down-looking photographs (recorded on continuous-strip film) with topographic maps and orthophotoquadrangle maps. Fiducial numbers and marks, impressed on any paper strips that were recording data or added to magnetic tape records, were included as a function of time to further reconcile location with instrumentation
How accurate are the heights or depths?
The aircraft vertical position was determined using the navigational positioning equipment on the aircraft, which were radar altimeter and barometric altimeter.
How consistent are the relationships among the observations, including topology?
In general each original map shows data collected by a single contractor or group responsible for collecting and processing the field observations, and reflect industry standard practices and controls as of the time period when the maps were published. Variation among surveys is possible, especially if they were carried out during very different time periods.
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