sim3188_readme.txt

The digital data, maps and pamphlet of this scientific investigations map 
are available from the following U.S. Geological Survey URL: 
http://pubs.usgs.gov/sim/3188

Data Contents:

The database includes an ASCII file containing new aeromagnetic data and 
two ASCII files with point locations of the local maximum horizontal 
gradient derived from the aeromagnetic data. 

The digital dataset consists of one file (ukiah_mag.xyz) containing new 
aeromagnetic data.  Approximately 1,650,000 point data were gridded, 
merged with earlier survey data, and contoured to produce aeromagnetic 
contours on the map.  The file contains the principal data fields of the 
aeromagnetic measurements, with the first line (marked by a  "/") that has 
an abbreviated description for each measurement.

The format of the data is described below.  Each measurement has a line 
number, latitude, longitude, date and time information, radar, barometric 
and GPS altitude and various levels of magnetic field measurements.

Measurement Description:

Line           *                  Line number (edited)

lonnad27       decimal degrees    DGPS longitude (NAD-27)

latnad27       decimal degrees    DGPS latitude  (NAD-27)

xwgs84utm10    meters             DGPS UTME (WGS-84,Z10N)

ywgs84utm10    meters             DGPS UTMN (WGS-84,Z10N)

xnad27utm10    meters             DGPS UTME (NAD-27 mean CONUS, Z10N)

ynad27utm10    meters             DGPS UTMN (NAD-27 mean CONUS, Z10N)

gtime          sec                GPS time, seconds past midnight

date           *                  Acquisition year, month, and day 
                                  YYYY/MM/DD

radarm         meters             Radar altitude above ground level (AGL)

barom          meters             Barometric altitude relative to mean sea 
                                  level (MSL)

galt           meters             GPS height above WGS84 geoid

diurnal        nT                 Mag base station.  Base located at Ukiah 
                                  airport, California

rawmag         nT                 TMF (Total Magnetic Field), 
                                  Uncompensated TAIL, Tail sensor, raw

cmag           nT                 Compensated rawmag 

magdc          nT                 Diurnally corrected compensated magnetic 
                                  field

maglag         nT                 Lag corrected magdc

igrf           nT                 IGRF (International Geomagnetic 
                                  Reference Field 2005) value calculated 
                                  with actual time and elevation of data 
                                  acquisition

magigrf        nT                 maglag minus igrf correction plus 54425 
                                  nT

tielev         nT                 Preliminary leveled magnetic field (tie 
                                  lines only)

maglev         nT                 Preliminary leveled traverse lines 
                                  (using tielev)

magfinal       nT                 Final tieline-leveled, decorrugated and 
                                  microleveled magnetic field

Two additional files are also included that contain the horizontal maximum 
gradients calculated using the method described in Blakely and Simpson 
(1988).  grad_big.xyz contains those gradients larger than the mean 
gradient; grad_small.xyz contains those gradients that are smaller than 
the mean gradient. See report for details of the measurements. The format
of the file is longitude, latitude, and amplitude of horizontal maximum 
gradient. Locations are on the NAD27 datum.  

sheet1a_utm.e00    an arc grid of the mosaic aeromagnetic map shown in 
                   Sheet 1A.

sheet1b_utm.e00    an arc grid of the merged aeromagnetic map shown in 
                   Sheet 1B.

sheet2_utm.e00     an arc grid of the residual aeromagnetic map shown in 
                   Sheet 2 and 3.


Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source 
bodies from magnetic or gravity anomalies: Geophysics, v. 51, p. 1494-
1498.