Johnston's Atoll, Central Pacific Ocean

File Structure

I.  Each of the three files corresponds to a flight.  The sample 
interval is one per every five seconds.  Each flight is made up 
of survey track lines and perpendicular cross tracks.  Survey track 
lines are one nautical mile apart with perpendicular cross tracks 
at 10 nautical mile spacings.  Within a file, data from individual 
track lines are separated by a track number in format I4 at the 
beginning, and a line of zeroes in format 6F12.4 at the end.  
There is also an initial header record in each flight.  The format 
for each is as follows:

     Header Record

     Columns        Description

     1-15           Project Identification
     16             Blank
     17-27          Flight Number
     28             Blank
     29-35          Inclusive Julian day(s)
     36             Blank
     37-40          Year of Survey
     all others          Not Used

     Data Records

     Each logical data record includes the following information 
     in format 6F12.4.

Columns

 1-12     Greenwich Mean Time in seconds from midnight of survey date.
13-24     Latitude in degrees decimal; positive sign denotes north
          latitude; negative sign denotes south latitude.
25-36     Longitude in degrees decimal; positive sign denotes east
          longitude; negative sign denotes west longitude.
37-48     Observed total intensity in nanoTeslas (nT).
49-60     Observed total intensity (nT) corrected for temporal
          variation determined from ground station data.
61 -72    Computed residual intensity (nT), (total intensity,
          corrected for temporal variation, minus modeled Earth's
          field- WC-85).

II.  The last two files are gridded files. The first of these
     contains latitude, longitude and magnetic total intensity
     corrected for temporal variation.  The second of the gridded
     files contains latitude, longitude, and magnetic residual
     intensity corrected for temporal variation.  The residuals
     were computed using World Chart 1985 (WC-85).  Both files
     contain an initial header which describes the geographic
     limits of the entire file.  The gridded data files are
     sequential by latitude band starting with the southwest most
     grid point.  A change in latitude indicates a change to the
     next latitude band.  Longitude also changes back to the
     west-most value when the latitude changes.  The end grid
     point is the northeast-most grid value. A value of
     1.0 x 10**5 is supplied for grid values outside the survey area.

     Header Record
 
     Columns      Description                                  Format
     1-12         West Longitude (decimal degrees)             F12.4
    13-24         East Longitude (decimal degrees)             F12.4
    25-36         South Latitude (decimal degrees)             F12.4
    37-48         North Latitude (decimal degrees)             F12.4
    49-53         Grid size in minutes                         F5.2
    54-59         Number of grid columns                       I6
    60-65         Number of grid rows                          I6
    65-80         Blanks                                       13X

                   Data Records

     Columns       Description                                  Format

     1-36          Latitude; longitude, magnetic                3F12.4
                   intensity value (nT)

         For technical assistance, please contact:

National Geophysical Data Center   or   Commanding Officer
Attn: Potential Fields   (E/GC1)        Naval Oceanographic Office
325 Broadway                            Stennis Space Center, MS 39522-5001
Boulder, CO 80303                       ATTN:  Geomagnetic Data Library
(303) 497-6128; FAX: (303) 497-6513     AUTOVON 485-5832
                                        Commercial (601) 688-5832
          __________                         __________

The Johnston's Atoll digital data file was submitted to NGDC in 1992 by:

               Allen H. Cogbill
               Geophysics Group
               Los Alamos National Laboratory
               Los Alamos, NM 87545

The original survey was conducted by the Defense Mapping Agency
(USNAVO) per a request from the Los Alamos National Laboratory.
The survey was completed in early December, 1987.  The data are
unclassified and can be made available in the public domain.
