MGD77 Data File Format

August 1998 (Revised)
Source:
KEY TO GEOPHYSICAL RECORDS DOCUMENTATION NO. 10 (REVISED)
NATIONAL GEOPHYSICAL DATA CENTER, NESDIS, NOAA, US

URL : ftp://ftp.ngdc.noaa.gov/MGG/geodas/docs/mgd77.txt

The Header Record:

The purpose of the Header Record is to document both the content and structure of the geophysical data contained within subsequent data records. In general, documentation that is constant throughout the survey will be in the Header Record, while documentation that is variable will be in the Data Records.

For sequential files, the MGD77 Header consists of 24 80-character sequential records. For 9 track magnetic tapes the MGD77 Header is a physical record (block) consisting of 1,920 characters. In both cases the data records follow immediately with no intervening end-of-file marks.

The Header Record contains fields which are both fixed and freely formatted. All field lengths within the Header that have not been coded with information should be blank-filled, and all plain language statements should be left-justified. The Header consists of a "sequence" of twenty-four 80-character images. The field lengths within the Header Records are designed to allow one to read the information (from magnetic tapes) as a series of 120-character logical records -- the same logical record length as the data records.

To help the marine geophysical community prepare this documentation, a coding pad is available free of charge from the National Geophysical Data Center.

Format Conventions for the Header Record:
  1. All decimal points are implied.
  2. Leading zeros and blanks are equivalent.
  3. Unknown or unused fields are to be blank filled.
  4. All "corrections", such as time zone, diurnal magnetics, and Eotvos, are understood to be added (e.g., time-zone correction is the number of hours which must be added to the recorded time to determine GMT).

Header Record (Sequence No. : 1)

Field Type Name Descriptions
01 int Record Type Set to "4"
02-09 char Survey Identifier Identifier supplied by the contributing organization, else given by NGDC in a manner which represents the data. Identical to that in data record.
10-14 char Format Acronym Set to "MGD77"
15-22 int Data Center File Number Survey identifier bestowed by the data center. First 2 chars indicate the source, first 4 indicate platform.
23-26   Filler Blank
27-31   Parameters Surveyed Code Status of geophysical parameters for this survey.

(Code for columns 27 - 31)
0 or Blank : Unspecified
1 : Parameter NOT surveyed
3 : Parameter surveyed, not contained in file
5 : Parameter surveyed, contained in file
  27   int   Bathymetry e.g., 12 kHz or 3.5 kHz used for digitized bathymetry
  28   int   Magnetics  
  29   int   Gravity  
  30   int   High Resolution Seismic e.g., 3.5kHz
  31   int   Deep Penetration Seismic e.g., large airgun
32-39 int File Creation Date Date data records were last altered (including century).
(YYYYMMDD)
YYYY : Year, MM : Month ,DD : Day
40-78 char Source Institution Organization which collected the data. Include contributor if different from collector.
79-80 int Sequence Number Set to "01"

Header Record (Sequence No. : 2)

Field Type Name Descriptions
01-18 char Country  
19-39 char Platform Name  
40 int Platform Type Code 0 : Unspecified
1 : Surface ship
2 : Submersible ship
3 : Aircraft
4 : Buoy
5 : Mobile land
6 : Fixed land
7 : Deep tow
8 : Anchored seafloor instrument
9 : Other, specify
41-46 char Platform Type e.g., "SHIP", "PLANE", "SUB", etc.
47-78 char Chief Scientist(s)  
79-80 int Sequence Number Set to "02"

Header Record (Sequence No. : 3)

Field Type Name Descriptions
01-58 char Project e.g., "SURVOPS 6-69", "INDOPAC, Leg 3"
59-78 char Fundingi.e., agency or institution
79-80 int Sequence Number Set to "03"

Header Record (Sequence No. : 4)

Field Type Name Descriptions
01-08 int Survey Departure Date YYYYMMDD
YYYY : Year , MM : Month , DD : Day
09-40 char Port of Departure i.e., city, country
41-48 int Survey Arrival Date YYYYMMDD
YYYY : Year , MM : Month , DD : Day
49-78 char Port of Arrival i.e., city, country
79-80 int Sequence Number Set to "04"

Header Record (Sequence No. : 5)

Field Type Name Descriptions
01-40 char Navigation Instrumentation e.g., "SAT/LORAN A/SEXTANT"
41-78 char Geodetic Datum/Position Determination Method e.g., "WGS84/PRIM - SATELLITE, SEC-LORAN A"
79-80 int Sequence Number Set to "05"

Header Record (Sequence No. : 6)

Field Type Name Descriptions
01-40 char Bathymetry Instrumentation Include information such as frequency, beam width, and sweep speed of recorder.
41-78 char Additional Forms of Bathymetric Data e.g., "MICROFILM", "ANALOG RECORDS"
79-80 int Sequence Number Set to "06"

Header Record (Sequence No. : 7)

Field Type Name Descriptions
01-40 char Magnetics Instrumentation e.g., "PROTON PRECESSION MAG-GEOMETRICS G-801"
41-78 char Additional Forms of Magnetics Data e.g., "ANALOG RECORDS"
79-80 int Sequence Number Set to "07"

Header Record (Sequence No. : 8)

Field Type Name Descriptions
01-40 char Gravity Instrumentation e.g., "L and R S-26"
41-78 char Additional Forms of Gravity Data e.g., "MICROFILM", "ANALOG RECORDS"
79-80 int Sequence Number Set to "08"

Header Record (Sequence No. : 9)

Field Type Name Descriptions
01-40 char Seismic Instrumentation Include the size of the sound source, the recording frequency filters, and the number of channels (e.g., "1700 cu. in., AIRGUN, 8-62 Hz, 36 CHANNELS")
41-78 char Formats of Seismic Data e.g., "DIGITAL", "MICROFILM", "NEGATIVES", etc.
79-80 int Sequence Number Set to "09"

Header Record (Sequence No. : 10)

Field Type Name Descriptions
01 char Format Type Set to "A", which means format contains integers, floating points, and alphanumerics
02-75 char Format Description This is one method of reading (not writing) the data in FORTRAN.
Set to the following:
"(I1,A8,F5.2,4I2,F5.3,F8.5,F9.5,I1,F6.4,F6.1,I2,I1,3F6.1,I1,F5.1,F6.0,F7.1,"
(NOTE: continued in sequence number 11)
76-78   Filler Blank
79-80 int Sequence Number Set to "10"

Header Record (Sequence No. : 11)

Field Type Name Descriptions
01-19 char Format Description Continued, set to the following:
"F6.1,F5.1,A5,A6,I1)"
20-40   Filler Blank
41-43 int Topmost Latitude of Survey (to next whole degree) **
44-46 int Bottommost Latitude **
47-50 int Leftmost Longitude **
51-54 int Rightmost Longitude **
55-78   Filler Blank
79-80 int Sequence Number Set to "11"

Header Record (Sequence No. : 12)

Field Type Name Descriptions
01-03 real General Digitizing Rate of Bathymetry In tenths of minutes.
The rate which is present within the data records (e.g., if values were coded every 5 minutes, set to "050")
04-15 char General Sampling Rate of Bathymetry This rate is instrumentation dependent (e.g., "1/SECOND")
16-20 real Assumed Sound Velocity In tenths of meters per second.
Historically, in the U.S., this speed has been 800 fathoms/sec, which equals 1463.0 meters/sec.; however, some recorders have a calibration of 1500 meters/sec (e.g., "14630")
21-22 int Bathymetry Datum Code 00 : No correction applied (sea level)
01 : Lowest normal low water
02 : Mean lower low water
03 : Lowest low water
04 : Mean lower low water spring
05 : Indian spring low water
06 : Mean low water spring
07 : Mean sea level
08 : Mean low water
09 : Equatorial spring low water
10 : Tropic lower low water
11 : Lowest astronomical tide
88 : Other, specify in additional documentation
23-78 char Interpolation Scheme This field allows for a description of the interpolation scheme used, should some of the data records contain interpolated values (e.g., "5-MINUTE INTERVALS AND PEAKS AND TROUGHS").
79-80 int Sequence Number Set to "12"

Header Record (Sequence No. : 13)

Field Type Name Descriptions
01-03 real General Digitizing Rate of Magnetics In tenth of minutes.
The rate which is present within the data records.
04-05 int General Sampling Rate of Magnetics In seconds.
This rate is instrumentation dependent (e.g., if the pulse rate is every 3 sec, set to "03")
06-09 int Magnetic Sensor Tow Distance In meters.
The distance from the navigation reference to the leading sensor.
10-14 real Sensor Depth In tenths of meters.
This is the estimated depth of the lead magnetic sensor.
15-17 int Horizontal Sensor Separation In meters.
If two sensors are used.
18-19 int Reference Field Code This is the reference field used to determine the residual magnetics:

00 : Unused
01 : AWC 70
02 : AWC 75
03 : IGRF-65
04 : IGRF-75
05 : GSFC-1266
06 : GSFC (POGO) 0674
07 : UK 75
08 : POGO 0368
09 : POGO 1068
10 : POGO 0869
11 : IGRF-80
12 : IGRF-85
13 : IGRF-90
88 : Other, specify
20-31 char Reference Field e.g., "IGRF-85"
32-78 char Method of Applying Residual Field The procedure used in applying this reduction to the data (e.g. "LINEAR INTERP. in 60-mile SQUARE")
79-80 int Sequence Number Set to "13"

Header Record (Sequence No. : 14)

Field Type Name Descriptions
01-03 real General Digitizing Rate of Gravity In tenths of minutes.
The rate present within the data records
04-05 int General Sampling Rate of Gravity In seconds.
This rate is instrumentation dependent. If recording is continuous, set to "00"
06 int Theoretical Gravity Formula Code 1 : Heiskanen 1924
2 : International 1930
3 : IAG System 1967
4 : IAG System 1980
8 : Other, specify
07-23 char Theoretical Gravity Formula e.g., "INTERNATIONAL '30", "IAG SYSTEM (1967)", etc.
24 int Reference System Code Identifies the reference field:

1 : Local system, specify
2 : Potsdam system
3 : System IGSN 71
9 : Other, specify
25-40 char Reference System e.g., "POTSDAM SYSTEM","SYSTEM IGSN 71", etc.
41-78 char Corrections Applied Drift, tare and bias corrections applied. (e.g., "+0.075 MGAL PER DAY")
79-80 int Sequence Number Set to "14"

Header Record (Sequence No. : 15)

Field Type Name Descriptions
01-07 real Departure Base Station Gravity In tenths of milligals.
At sea level (Network value preferred.)
08-40 char Departure Base Station Description Indicates name and number of station
41-47 real Arrival Base Station Gravity In tenths of milligals.
At sea level (Network value preferred.)
48-78 char Arrival Base Station Description Indicates name and number of station
79-80 int Sequence Number Set to "15"

Header Record (Sequence No. : 16)

Field Type Name Descriptions
01-02 int Number of 10-Degree Identifiers ** This is the number of 4-digit 10-degree identifiers, excluding the "9999" flag, which will follow this field. (see Appendix A)
03   Filler Blank
04-78 int 10-Degree Identifiers ** This is a series of 4-digit codes,separated by commas, which identify the 10-degree squares through which the survey collected data (see Appendix A). Code "9999" after last identifier.
79-80 int Sequence Number Set to "16"

Header Record (Sequence No. : 17)

Field Type Name Descriptions
01-75 int 10-Degree Identifiers ** Continued
76-78   Filler Blank
79-80 int Sequence Number Set to "17"

Header Record (Sequence No. : 18 - 24)

Field Type Name Descriptions
01-78 char Additional Documentation Information concerning this survey not contained in header fields.
79-80 int Sequence Number "18" thru "24"

** Fields 41-54 in sequence Number 11 and Fields 1-78 in sequence numbers 16 and 17 may be blank filled by the contributing institution. The data center can determine these numbers by a computer search of the latitudes and longitudes within the MGD77 file.

Data Record:

The data record presents underway marine geophysical data in a correlative manner. Geophysical data (bathymetry, magnetics, and gravity) and seismic identification (shot-point identification) are presented with a corresponding time and position. Documentation that is variable throughout the survey also is included within each data record. If primary navigation exists at a juncture where no geophysical data are present, this record should be included with the data parameter fields left unused (9s filled).

The logical record length is 120 characters and the blocking factor is 16 logical records per physical record (i.e., 1,920 characters).

Format Conventions:
  1. All decimal points are implied.
  2. Leading zeros and blanks are equivalent.
  3. Unknown or unused fields are to be filled with 9s (DO NOT BLANK FILL).
  4. All "corrections", such as time zone, diurnal magnetics, and Eotvos, are understood to be added (e.g., time-zone correction is the number of hours which must be added to the recorded time to determine GMT).

Field Type Name Descriptions
001 int Data Record Type Set to "5" for data record.
002-009 char Survey Identifier Identifier supplied by the contributing organization, else given by NGDC in a manner which represents the data. Identical to that in header record.
010-012 int Time Zone Correction Corrects time (in characters 13-27) to GMT when added: equals zero when time is GMT. Timezone normally falls between -13 and +12 inclusively.
013-016 int Year including century (e.g. 1972)
017-018 int Month e.g. May is represented as 05
019-020 int Day Day of month
021-022 int Hour Hour of day
023-027 real Minutes X 1000  
028-035 real Latitude X 100000 + : North; - : South
Between -9000000 and 9000000
036-044 real Longitude X 100000 + : East; - : West
Between -18000000 and 18000000
045 int Position Type Code Indicates how lat/lon was obtained:

1 : Observed fix
2 : Interpolated
9 : Unspecified
046-051 real Bathymetry, 2-Way Traveltime In ten-thousandths of seconds.
Corrected for transducer depth and other such corrections, especially in shallow water
052-057 real Bathymetry, Corrected Depth In tenths of meters.
058-059 int Bathymetric Correction Code This code details the procedure used for determining the sound velocity correction to depth:

01-55 : Matthews' Zones with zone
59 : Matthews' Zones, no zone
60 : S. Kuwahara Formula
61 : Wilson Formula
62 : Del Grosso Formula
63 : Carter's Tables
88 : Other (see Add. Doc.)
99 : Unspecified
060 int Bathymetry Type Code Indicates how the data record's bathymetric value was obtained:

1 : Observed
2 : Interpolated (Header Seq. 12)
9 : Unspecified
061-066 real Magnetics Total Field, 1st Sensor In tenths of nanoteslas (gammas).
For leading sensor. Use this field for single sensor.
067-072 real Magnetics Total Field, 2nd sensor In tenths of nanoteslas (gammas).
For trailing sensor.
073-078 real Magnetics Residual Field In tenths of nanoteslas (gammas).
The reference field used is in Header Seq. 13.
079 int Sensor for Residual Field 1 : 1st or leading sensor
2 : 2nd or trailing sensor
9 : Unspecified
080-084 real Magnetics Diurnal Correction In tenths of nanoteslas (gammas).
(In nanoteslas) if 9-filled (i.e., set to "+9999"), total and residual fields are assumed to be uncorrected; if used, total and residuals are assumed to have been already corrected.
085-090 real Depth or Altitude of Magnetics Sensor In meters.
+ : Below sealevel
- : Above sealevel
091-097 real Observed Gravity In tenths of milligals.
Corrected for Eotvos, drift, and tares
098-103 real Eotvos Correction In tenths of milligals.
E = 7.5 V cos phi sin alpha + 0.0042 V*V
104-108 real Free-Air Anomaly In tenths of milligals.
Free-air Anomaly = G(observed) - G(theoretical)
109-113 char Seismic Line Number Used for cross referencing with seismic data.
114-119 char Seismic Shot-Point Number  
120 int Quality Code for Navigation
 5 : Suspected, by the originating institution
6 : Suspected, by the data center
9 : No identifiable problem found

(NOTE - Institution will most frequently 9-fill this field; however, should they wish to code a "5", the data center will not contradict. The data center's quality control program, which performs (among other checks) a vectorial analysis of the navigation, is available in a printout form upon request.)

Appendix A

10-degree-square identifier code

A 10-degree-square area can be easily identified by constructing a four-digit number. The components of this number, in order of their construction are described as follows:

Quadrant -
A one-digit number identifies the quadrant of the world with the following significance to each digit:
1st digit = Quadrant number

Qc Code Latitude Longitude
1 North East
3 South East
5 South West
7 North West

10-Degree Square -
The next three digits identify a unique 10-degree square;
thus, the significant digits consist of:
2nd digit = Tens digit of degrees latitude
3rd digit = Hundreds digit of degrees longitude
4th digit = Tens digit of degrees longitude

Examples:
(Qc, Lat., Long., Long. : 10-degree sq. ident. code)

  Qc Lat. Long. Long.
(1) 37 degrees 48' S, 4 degrees 13' E 3 3 0 0
(2) 21.6 degrees S, 14.3 degrees W 5 2 0 1
(3) 34 degrees 28' N, 143 degrees 27' W 7 3 1 4
(4) 75 degrees N, 43 degrees E 1 7 0 4
Last update : September 1, 1999