THE MARINE GEOPHYSICAL DATA EXCHANGE FORMAT - "MGD77"
(Bathymetry, Magnetics, and Gravity)
NATIONAL GEOPHYSICAL DATA CENTER
NATIONAL ENVIRONMENTAL SATELLITE, DATA, AND INFORMATION SERVICE
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
U.S. DEPARTMENT OF COMMERCE
KEY TO GEOPHYSICAL RECORDS DOCUMENTATION NO. 10 (REVISED)
COMPILED BY THE MGD77 TASK GROUP
Allen M. Hittelman, Chairman
Robert C. Groman
Richard T. Haworth
Troy L. Holcombe
Graig McHendrie
Stuart M. Smith
National Geophysical Data Center
Boulder, Colorado
September 1977
December 1981 (Revised by Dan Metzger)
February 1989 (Revised by Dan Metzger)
January 1993 (Revised by Dan Metzger)
October 1995 (Revised by Dan Metzger)
August 1998 (Revised by Dan Metzger)
INTRODUCTION...............................................I
GENERAL DESCRIPTION.......................................II
THE HEADER RECORD........................................III
THE DATA RECORD...........................................IV
10-DEGREE IDENTIFIER CODE.........................APPENDIX A
NGDC CONTACTS.....................................APPENDIX B
I. INTRODUCTION
In January of 1977, a group of 24 geophysical data
managers from academia, government, industry and foreign
countries participated in a workshop at the National
Geophysical Data Center (NGDC) in Boulder, Colorado. The
"Workshop for Marine Geophysical Data Formats" established
the basic outline of a new format for the exchange of
digital underway geophysics data. A six member task force
was formed to work with NGDC in implementing the decisions
of the workshop into the new format. By the end of 1977 the
"MGD77" format was being disseminated by NGDC as its
standard exchange format.
The "MGD77" format has experienced much success over
the past 15 years. It has been sanctioned by the Intergov-
ernmental Oceanographic Commission (IOC) as an accepted
standard for international data exchange, and it has been
translated into French, Japanese, and Russian. Most con-
tributors of data to NGDC now send transfer data over the
internet in the "MGD77" format.
This newest revision makes the MGD77 format "Year 2000
Compliant", ensuring it's success into 21st Century.
II. GENERAL DESCRIPTION
The digital format presented, and referred to as
"MGD77", is an exchange format for marine geophysical data
(bathymetry, magnetics, and gravity) It is intended to be
used for the transmission of data to and from a data center
and may be useful for the exchange of data to and from a
data center and may be useful for the exchange of data
between marine institutions. Data is to be exchanged in
files, one file per survey operation. Generally each survey
operation is a port-to-port operation of a survey vessel,
but in some cases several port-to-port operations of the
same vessel are combined in single survey operation,
especially if this is the manner of organizing the data at
the contributing institution. Data may be exchanged on via
the Internet or on various mass storage devices such as 8mm
or 9 track tapes, removable disks. The National Geophysical
Data Center uses CD-ROM disks as its chief method of distribution
of these data.
Data Exchange
1. For exchange of MGD77 data via mass storage files on
magnetic or optical media participants shall establish type
and format of the media to be exchanged.
2. Each survey operation shall be contained in one file
with Header and Data Records, or in 2 files, one Header
file and one Data Record file. If the media is too small
to contain one survey the data may be continued on a second
media.
3. Each survey operation shall have one MGD77 Header
consisting of 24 80-character logical records.
4. The MGD77 data records are sequentially and chronologically
organized until the end of the file. The data records are 120
logical characters.
5. A survey is defined as all observations that conveniently
constitute a survey operation (e.g., a port-to-port survey or
in some cases several surveys). A survey file(s) ideally
should not span two media.
6. For sequential files, the MGD77 Header shall consist of
24 sequential records of 80 logical characters each separated
by an end-of-record character(s) and, if the data records are
in the same file, the Header shall be at the beginning of the
file. The MGD77 Data Records shall be 120 logical characters
each.
7. 9 Track tapes should be recorded as ASCI 1600 or 6250 BPI.
The tape structure consists of physical records of 1920
characters each, a header followed by data records, separated
by inter-record gaps (IRG) and organized into files. The files
are separated by end-of-file (EOF) marks (sometimes called
tape marks).
III. 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 (e.g. 1234 in 10ths of units means 123.4)
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).
The following is a detailed description of the Header Record.
Fields can be of type integer, real or character. Fields that
represent whole numbers are integers Fields that contain a
decimal component are real, and fields that are alphanumeric
are character.
Length
Character of
Nos. Field Type Description
_____________________________________________________
Sequence No. 1
1 1 int RECORD TYPE - Set to "4" (Header)
2-9 8 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 5 char FORMAT ACRONYM - Set to"MGD77"
15-22 8 int DATA CENTER FILE NUMBER
Survey identifier bestowed by the data
center. First 2 chars indicate the
source, first 4 indicate platform.
27-31 5 int PARAMETERS SURVEYED CODE
Status of geophysical parameters for
this survey.
COLUMN PARAMETER SURVEYED
27 bathymetry (e.g., 12
kHz or 3.5 kHz used
for bathymetry)
28 magnetics
29 gravity
30 high-resolution seismics
(e.g., 3.5 kHz)
31 deep penetration seismics
(e.g., large airgun)
_________________________________
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
32-39 8 int FILE CREATION DATE (YYYYMMDD)
Date data records were last
altered (including century).
40-78 39 char SOURCE INSTITUTION
Organization which collected the
data. Include contributor if different
from collector.
79-80 2 int SEQUENCE NUMBER - Set to "01"
Sequence No. 2
1-18 18 char COUNTRY
19-39 21 char PLATFORM NAME
40 1 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 6 char PLATFORM TYPE
(e.g., "SHIP","PLANE", "SUB", etc.)
47-78 32 char CHIEF SCIENTIST(S)
79-80 2 int SEQUENCE NUMBER - Set to "02"
Sequence No. 3
1-58 58 char PROJECT
(e.g., "SURVOPS 6-69",
"INDOPAC, Leg3")
59-78 20 char FUNDING
(i.e. agency or institution)
79-80 2 int SEQUENCE NUMBER - Set to "03"
Sequence No. 4
1-8 8 int SURVEY DEPARTURE DATE (YYYYMMDD)
9-40 32 char PORT OF DEPARTURE
(i.e. city, country)
41-48 8 int SURVEY ARRIVAL DATE (YYYYMMDD)
49-78 30 char PORT OF ARRIVAL
(i.e. city, country)
79-80 2 int SEQUENCE NUMBER - Set to "04"
Sequence No. 5
1-40 40 char NAVIGATION INSTRUMENTATION
(e.g. "SAT/LORAN A/SEXTANT")
41-78 38 char GEODETIC DATUM/POSITION
DETERMINATION METHOD
(e.g. "WGS84/PRIM - SATELLITE,
SEC-LORAN A")
79-80 2 int SEQUENCE NUMBER - Set to "05"
Sequence No. 6
1-40 40 char BATHYMETRY INSTRUMENTATION
Include information such as
frequency, beam width, and sweep
speed of recorder.
41-78 38 char ADDITIONAL FORMS OF BATHYMETRIC DATA
(e.g., "MICROFILM",
"ANALOG RECORDS")
79-80 2 int SEQUENCE NUMBER - Set to "06"
Sequence No. 7
1-40 40 char MAGNETICS INSTRUMENTATION
(e.g., "PROTON PRECESSION
MAG-GEOMETRICS G-801")
41-78 38 char ADDITIONAL FORMS OF MAGNETICS DATA
(e.g., "PUNCH TAPE",
"ANALOG RECORDS")
79-80 2 int SEQUENCE NUMBER - Set to "07"
Sequence No. 8
1-40 40 char GRAVITY INSTRUMENTATION
(e.g., "L and R S-26")
41-78 38 char ADDITIONAL FORMS OF GRAVITY DATA
(e.g., "MICROFILM", "ANALOG
RECORDS")
79-80 2 int SEQUENCE NUMBER - Set to "08"
Sequence No. 9
1-40 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 38 char FORMATS OF SEISMIC DATA
(e.g., "DIGITAL", "MICROFILM",
"NEGATIVES", etc.)
79-80 2 int SEQUENCE NUMBER - Set to "09"
Sequence No. 10
1 1 char FORMAT TYPE
Set to "A", which means format
contains integers, floating points,
and alphanumerics
2-76 75 char FORMAT DESCRIPTION
This is one method of reading
(not writing) the data in FORTRAN.
Set to the following:
"(I1,A8,I3,I4,3I2,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 no. 11)
79-80 2 int SEQUENCE NUMBER - Set to "10"
Sequence No. 11
1-19 19 char FORMAT DESCRIPTION
Continued, set to following:
"F6.1,F5.1,A5,A6,I1)"
41-43 3 int TOPMOST LATITUDE OF SURVEY **
(to next whole degree)
44-46 3 int BOTTOMMOST LATITUDE
47-50 4 int LEFTMOST LONGITUDE
51-54 4 int RIGHTMOST LONGITUDE
79-80 2 int SEQUENCE NUMBER - Set to "11"
Sequence No. 12
1-3 3 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")
4-15 12 char GENERAL SAMPLING RATE OF
BATHYMETRY
This rate is instrumentation dependent
(e.g., "1/SECOND")
16-20 5 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 2 int BATHYMETRIC 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 56 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 2 int SEQUENCE NUMBER - Set to "12"
Sequence No. 13
1-3 3 real GENERAL DIGITIZING RATE OF
MAGNETICS
In tenths of minutes.
The rate which is present
within the data records.
4-5 2 int GENERAL SAMPLING RATE OF
MAGNETICS
In seconds.
This rate isinstrumentation dependent
(e.g., if the pulse rate is every 3 sec,
set to "03")
6-9 4 int MAGNETIC SENSOR TOW DISTANCE
In meters.
The distance from the navigation
reference to the leading sensor.
10-14 5 real SENSOR DEPTH
In tenths of meters.
This is the estimated depth of the
lead magnetic sensor.
15-17 3 int HORIZONTAL SENSOR SEPARATION
In meters.
If two sensors are used.
18-19 2 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
14 - IGRF-95
15 - IGRF-00
88 - Other, specify
20-31 12 char REFERENCE FIELD
(e.g., "IGRF-85")
32-78 47 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 2 int SEQUENCE NUMBER - Set to "13"
Sequence No. 14
1-3 3 real GENERAL DIGITIZING RATE OF GRAVITY
In tenths of minutes.
The rate present within the data records
4-5 2 int GENERAL SAMPLING RATE OF GRAVITY
In seconds.
This rate is instrumentation dependent.
If recordingis continuous, set to "00"
6 1 int THEORETICAL GRAVITY FORMULA CODE
1 - Heiskanen 1924
2 - International 1930
3 - IAG System 1967
4 - IAG System 1980
8 - Other, specify
7-23 17 char THEORETICAL GRAVITY FORMULA
(e.g., "INTERNATIONAL '30", "IAG
SYSTEM (1967)", etc.)
24 1 int REFERENCE SYSTEM CODE
Identifies the reference field:
1 - Local system, specify
2 - Potsdam system
3 - System IGSN 71
9 - Other, specify
25-40 16 char REFERENCE SYSTEM
(e.g., "POTSDAM SYSTEM",
"SYSTEM IGSN 71", etc.)
41-78 38 char CORRECTIONS APPLIED
Drift, tare and bias corrections
applied. (e.g., "+0.075 MGAL PER DAY")
79-80 2 int SEQUENCE NUMBER - Set to "14"
Sequence No. 15
1-7 7 real DEPARTURE BASE STATION GRAVITY
In tenths of milligals.
At sea level (Network value preferred.)
8-40 33 char DEPARTURE BASE STATION DESCRIPTION
Indicates name and number of station
41-47 7 real ARRIVAL BASE STATION GRAVITY
In tenths of milligals.
At sea level (Network value preferred.)
48-78 31 char ARRIVAL BASE STATION DESCRIPTION
Indicates name and number of station
79-80 2 int SEQUENCE NUMBER - Set to "15"
Sequence No. 16
1-2 2 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 B)
4-78 75 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 B).
Code "9999" after last identifier.
79-80 2 int SEQUENCE NUMBER - Set to "16"
Sequence No. 17
1-75 75 int 10-DEGREE IDENTIFIERS
Continued
79-80 2 int SEQUENCE NUMBER - Set to "17"
Sequence Nos. 18-24
1-78 78 char ADDITIONAL DOCUMENTATION
information concerning this survey
not contained in header fields.
79-80 2 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.
IV. THE 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 (e.g. 1234 in 10ths of units means 123.4)
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).
Length
Character of Fortran
Nos. Field code Description
____________________________________________________________
1 1 int DATA RECORD TYPE
Set to "5" for data record.
2-9 8 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.
10-12 3 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.
13-16 4 int YEAR
including century (e.g. 1972)
17-18 2 int MONTH
(e.g. May is represented as 05)
19-20 2 int DAY
Day of month
21-22 2 int HOUR
Hour of day
23-27 5 real MINUTES X 1000
28-35 8 real LATITUDE X 100000
+ = North; - = South
Between -9000000 and 9000000
36-44 9 real LONGITUDE X 100000
+ = East; - = West
Between -18000000 and 18000000
45 1 int POSITION TYPE CODE
Indicates how lat/lon was obtained:
1 = Observed fix
3 = Interpolated
9 = Unspecified
46-51 6 real BATHYMETRY, 2- WAY TRAVELTIME
In ten-thousandths of seconds.
Corrected for transducer depth and
other such corrections, especially in
shallow water
52-57 6 real BATHYMETRY, CORRECTED DEPTH
In tenths of meters.
58-59 2 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.)
97 Computed using 1500 meters/sec
98 Unknown if Corrected
99 Unspecified
60 1 int BATHYMETRIC TYPE CODE
Indicates how the data record's
bathymetric value was obtained:
1 = Observed
3 = Interpolated (Header Seq. 12)
9 = Unspecified
61-66 6 real MAGNETICS TOTAL FIELD, 1ST SENSOR
In tenths of nanoteslas (gammas).
For leading sensor. Use this field
for single sensor.
67-72 6 real MAGNETICS TOTAL FIELD, 2ND SENSOR
In tenths of nanoteslas (gammas).
For trailing sensor.
73-78 6 real MAGNETICS RESIDUAL FIELD
In tenths of nanoteslas (gammas).
The reference field used is in
Header Seq. 13.
79 1 int SENSOR FOR RESIDUAL FIELD
1 = 1st or leading sensor
2 = 2nd or trailing sensor
9 = Unspecified
80-84 5 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.
85-90 6 F6.0 DEPTH OR ALTITUDE OF MAGNETICS SENSOR
In meters.
+ = Below sealevel
- = Above sealevel
91-97 7 real OBSERVED GRAVITY
In tenths of milligals.
Corrected for Eotvos, drift, and
tares
98-103 6 real EOTVOS CORRECTION
In tenths of milligals.
E = 7.5 V cos phi sin alpha +
0.0042 V*V
104-108 5 real FREE-AIR ANOMALY
In tenths of milligals.
Free-air Anomaly = G(observed) -
G(theoretical)
109-113 5 char SEISMIC LINE NUMBER
Used for cross referencing with
seismic data.
114-119 6 char SEISMIC SHOT-POINT NUMBER
120 1 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
10-DEGREE SQ IDENT. CODE
________________________
EXAMPLES: Qc Lat Long Long
(i) 37 degrees 48'S, 4 degrees 13'E 3 3 0 0
(ii) 21.6 degrees S, 14.3 degrees W 5 2 0 1
(iii) 34 degrees 28'N, 143 degrees 27'W 7 3 1 4
(iv) 75 degrees N, 43 degrees E 1 7 0 4
___________________________________________________________
APPENDIX B NGDC CONTACTS
Dan R. Metzger: (303) 497-6542 Dan.R.Metzger@noaa.gov
or
John G. Campagnoli : (303) 497-3158 John.G.Campagnoli@noaa.gov
National Geophysical Data Center
NOAA, E/GC3
325 Broadway
Boulder, CO 80303-3328
TELEX 592811 NOAA MASC BDR
FAX (303) 497-6513
____________________________________________________________