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.
Return to the Top of this Document
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).
Return to the Top of this Document
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.
- 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).
*** CHANGES SINCE LAST REVISION ***
Several changes in the MGD77 Header were required in order to
make the format “Year 2000 Compliant” (Y2K Compliant):
| SEQUENCE |
DESCRIPTOR |
CHANGE |
| 01 |
RECORD TYPE |
Change: from “1” to “4” |
| 01 |
FILE CREATION DATE |
Change: to include century
From 6 digits (col 32-37) to 8 digits (col 32-39) |
| 01 |
SOURCE INSTITUTION |
Change: from 41 chars (col 38-78)to 39 chars (col 40-78) |
| 04 |
SURVEY DEPARTURE DATE |
Change: to include century
From 6 digits (col 1-6) to 8 digits (col 1-8) |
| 04 |
PORT OF DEPARTURE |
Change from 34 chars (col 7-40) to 32 chars (col 9-40) |
| 04 |
SURVEY ARRIVAL DATE |
Change: to include century
From 6 digits (col 41-46) to 8 digits (col 41-48) |
| 04 |
PORT OF ARRIVAL |
Change: from 32 chars (col 47-78) to 30 chars (col 49-78) |
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.
| Character Nos. |
Length of 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 |
| 2-75 |
74 |
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 no. 11) |
| 79-80 |
2 |
int |
SEQUENCE NUMBER - Set to “10” |
| Sequence No. 11 |
| 1-17 |
17 |
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
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.
Return to the Top of this Document
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).
*** CHANGES SINCE LAST REVISION ***
Several changes in the MGD77 Record were required in order to
make the format “Year 2000 Compliant” (Y2K Compliant):
| DESCRIPTOR |
CHANGE |
| DATA-RECORD TYPE |
Change from “3” to “5” (col 1) |
| TIME ZONE CORRECTION |
Change from hundredths of hours to hours
From 5 digits (col 10-14) to 3 digits (col 10-12) |
| YEAR |
Change to include century
From 2 digits (col 15-16) to 4 digits (col 13-16) |
- 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).
| Character Nos. |
Length of Field |
Fortran 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.)
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. |
Return to the Top of this Document
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 Metzger
- (303) 497-6542
- or
- John Campagnoli
- (303) 497-3158
- National Geophysical Data Center
- NOAA, E/GC3
- 325 Broadway
- Boulder, CO 80305-3328
- TELEX 592811 NOAA MASC BDR
- FAX (303) 497-6513
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