Aeromagnetic data were collected along flight lines by instruments in an
aircraft that recorded magnetic-field values and locations. This dataset
presents latitude, longitude, altitude, and 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 6,378,206.4.
The flattening of the ellipsoid used is 1/294.98.
Altitude_Datum_Name:National Geodetic Vertical Datum of 1929 Altitude_Resolution:1 Altitude_Distance_Units:feet Altitude_Encoding_Method:
Explicit elevation coordinate included with horizontal coordinates
Airborne survey specifications
These items are constant for the entire survey
Project number: WI12
Project name: Southeast Wisconsin
Survey flown by: Mobil
Survey flown for: Mobil
Approx. no. of line miles: 1230
Survey height: 2000 ft
Altitude method: Constant barometric elevation
Flight-line spacing: 0.5 mi
Flight-line direction: E-W
Aircraft used: unknown
Airport - arrival: unknown
Airport - departure: unknown
Sensor tow distance: unknown
Magnetometer used: unknown
Data recording interval: unknown
Regional field removed: unknown
Nettleton, L.L., 1971, Elementary Gravity and Magnetics for
Geologists and Seismologists: Society of Exploration Geophysicists
Monograph Series No. 1.
Dobrin, M.B., 1976, Introduction to Geophysical Prospecting:
New York, McGraw-Hill Book Company,
Sheriff, R.E., 1984, Encyclopedic dictionary of exploration
geophysics: Tulsa, OK, Society of Exploration Geophysicists.
These USGS employees contributed to reformatting and archiving these data:
Viki Bankey, David Daniels, Carol Finn, Pat Hill, Holly Hindle,
Bob Kucks, Vicki Rystrom, Sarah Shearer
Cooperating contributors from the National Geophysical Data Center are:
Ronald Buhmann, David Dater, Susan McLean, Stewart Racey
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.
Conversion of measured values to geographic position and magnetic
values was performed by the contractor using industry standard
Details are found under Horizontal_Position_Accuracy_Report, and
Unless noted, conversion processes were not reported to the USGS.
Unpublished products generated by the contractor included magnetic
tapes, a map of residual or total field magnetics, and perhaps some
Date: Dec-2002 (process 2 of 2)
USGS reformatting of contractor data to standard format.
USGS personnel used the software package Oasis Montaj by
Geosoft, Inc., to read in the original contractor's data.
Latitude, longitude, altitude, and magnetic values were checked
for obvious errors or spikes and values of -9999.9 were given
where the value could not be reasonably corrected. Other errors
in the data were not corrected. Horizontal positions were
converted to latitude and longitude if the original values were
UTM meters. Elevations were converted from meters to feet
(1 m = 3.2808 ft).
Often the digital data did not contain date information and only the
month and year flown or just a starting date were stated in the
written documentation. In these cases, the first day of the month
or the date the survey began was used as the Julian date and added
to the digital data.
Other information was added that was missing from the data file
but was recorded elsewhere. The reformatted data were written in
the format described in the section on Digital_Transfer_Information.
Wisconsin Geological and Natural History Survey, 1998, Flight-line gridded aeromagnetic data for Kenosha, Racine, and Walworth Counties, southeastern Wisconsin: Wisconsin Geological and Natural History Survey Open-File Report OFR 1998-6, Wisconsin Geological and Natural History Survey, Madison, Wisconsin.
How well have the observations been checked?
The data in this file have been processed using various formulas and
methods that are not usually documented but that represent industry
standard practices for airborne data reduction. For example, position
is listed as latitude and longitude, but these values were derived
from the raw navigation data depending on the system used. (see notes
under horizontal accuracy). Line numbers were added to records, and
unusable data at flight-line ends were discarded (as aircraft slowed
and turned around). Separate recordings were correlated by time and
assigned to the correct location.
How accurate are the geographic locations?
Flight Path Recovery
Horizontal positions of the survey aircraft used to collect data
were determined using aircraft navigational aids such as line-of-sight
electronic systems that measure the distances from each of two ground
stations to the aircraft.
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.
Radar altimeters are estimated to have an error of 2-5% of the
altitude (Richard Hansen, PRJ, Inc., written communication).
Barometric altimeters are quite accurate, but are typically operated
in an uncorrected mode. The diurnal variation in air pressure over
the course of a flight can produce a 50-100 ft error in the barometric
altimeter reading. In addition, pressure microcells create short-
period air pressure changes equivalent to about 10 ft. under typical
conditions (Richard Hansen, PRJ, Inc., written communication)
This data set was collected at a fixed barometric altitude of 2000 ft.
Where are the gaps in the data? What is missing?
Loss of data due to poor transmission, channel dropout, obvious spiking,
missing channels, and other obvious errors were replaced with the value
-9999.9, such as in the radar and barom channels. Sometimes channels were
added and filled with dummy values (-9999.9) to comply with the
established format. Some records that had no usable data were deleted.
How consistent are the relationships among the observations, including topology?
The data in this file were collected by a single contractor or group
who were responsible for collecting and processing the data.
The data from this survey were collected using the same instruments
(magnetometers, altimeters, navigational systems) throughout the survey
and were collected in a normal length of time with no long delays
between survey beginning and end.
Survey contracts specified the conditions and specifications under which
these data were collected. Standard industry practices of the time
were followed in data collection and processing.
Although all data published on this web site have been used by the USGS,
no warranty, expressed or implied, is made by the USGS as to the
accuracy of the data and related materials. The act of distribution
shall not constitute any such warranty, and no responsibility is
assumed by the USGS in the use of these data or related materials.
Any use of trade, product, or firm names is for descriptive purposes
only and does not imply endorsement by the U.S. Government.
Each line contains data in the following format,
beginning with line 1 (no header included):
line_no I5 flight line number
directn I4 flight line direction, azimuth degrees
longitud F11.4 longitude (decimal degrees)
latitude F9.4 latitude (decimal degrees)
year I5 year flown
jul_day I4 Julian day flown
fiducial I7 fiducial number
radar F8.1 radar altitude above ground (feet)
barom F8.1 barometric altitude above mean sea
totmag F9.2 corrected total field magnetic value (nT)
resmag F9.2, 1X residual magnetic value (nT)
This dataset was prepared by an agency of the United States
Government. Neither the United States Government nor any agency
thereof, nor any of their employees, make any warranty, expressed or
implied, or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information, apparatus,
product, or process disclosed in this report, or represents that its
use would not infringe privately owned rights. Reference therein to
any specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by the United
States Government or any agency thereof. Any views and opinions of
authors expressed herein do not necessarily state or reflect those of
the United States Government or any agency thereof.