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THE MARINE GEOPHYSICAL DATA EXCHANGE FORMAT
“MGD77” - REVISED
(Bathymetry, Magnetics, and Gravity)

This newest revision makes the MGD77 format “Year 2000 Compliant”, ensuring it's success into 21st Century.

The complete description of the MGD77 record format is available.

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 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 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.

SPECIAL VERSION - TRUNCATED MGD77 DATA RECORD GRAVITY DATA ONLY

ASCII FORMAT DESCRIPTION

Character Nos. Length of Field Type Description
1-8 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.
9-16 8 real LATITUDE X 100000
+ = North; - = South
Between -90 and 90 degrees
17-25 9 real LONGITUDE X 100000
+ = East; - = West
Between -180 and 180 degrees
26-31 6 real Bathymetry, Corrected Depth
In tenths of meters
32-38 7 real OBSERVED GRAVITY
In tenths of milligals. Corrected for Eotvos, drift, and tares
39-43 5 real FREE-AIR ANOMALY
In tenths of milligals. Free-air Anomaly = G(observed) minus G(theoretical)

NOTE: Where there are no data values for bathymetry or gravity, those fields are filled with 9's. (e.g. bathymetry corrected depth = “999999”, observed gravity = “999999”, free-air anomaly = “9999”)



BINARY FORMAT DESCRIPTION

Data are stored on the CD-ROM in binary form.

8 byte string 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.
4 byte integer LATITUDE X 100000
+ = North; - = South
Between -90 and 90 degrees
4 byte integer LONGITUDE X 100000
+ = East; - = West
Between -180 and 180 degrees
4 byte integer Bathymetry, Corrected Depth
In tenths of meters
4 byte integer OBSERVED GRAVITY
In tenths of milligals. Corrected for Eotvos, drift, and tares
2 byte integer FREE-AIR ANOMALY
In tenths of milligals. Free-air Anomaly = G(observed) minus G(theoretical)

NOTE: Where there are no data values for bathymetry or gravity, those fields are filled with 9's. (e.g. bathymetry corrected depth = “999999”, observed gravity = “999999”, free-air anomaly = “9999”)



National Geophysical Data Center
NOAA, E/GC3
325 Broadway
Boulder, CO 80303-3328
TELEX: 592811 NOAA MASC BDR
FAX: (303) 497-6513


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