GEODAS Documentation Headers provide documentation for data files in the various GEODAS Data Exchange Formats.
The Record type in GEODAS Headers is always set to 4, meaning Header Record.
Identifier supplied by the contributing organization, else given by NGDC in a manner which represents the survey data. It consists of 1 to 8 Uppercase Alphanumeric characters with - or / allowed.
GEODAS covers several geophysical database types. Each database
has a digital exchange format which includes both Survey Documentation
(the Header) and data records. For Marine Trackline data:
MGD77 - Marine Geophysical Data Format 1977
A unique 8-character identifier assigned by NGDC to each survey in the
database. The scheme for numbering is as follows:
The first 2 characters identify the source (for MGD77 or HYD93)
The first 4 characters identify the platform or project
The last 4 digits are assigned consecutively
The geophysical parameters collected during this survey. The 5
codes for MGD77 Headers are as follows (in order in the field):
bathymetry
magnetics
gravity
high resolution seismics
deep penetration seismics
The codes for each parameter are:
0 - unspecified
1 - Parameter NOT surveyed
3 - Parameter surveyed, not contained in file
5 - Parameter surveyed contained in file
This is the date (YYYYMMDD) when the digital data records were last altered.
Institution, agency or other group which collected the data, or occasionally the group which contributed the data to NGDC.
Each GEODAS Header is made up of 24 80-character records. The last 2 characters of each record is the sequence number, 01 through 24.
The country which originally collected the survey data.
Name(s) of the ship, aircraft, etc. which collected the survey data.
The Platform Type Code, followed by a plain language description,
details the type of platform which collected the data:
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 in platform type field
The Chief Scientist or Hydrogapher is often the final source for information about the survey.
The descriptive name for a survey or set of surveys, as given by the collecting organization. This may contain abbreviations and/or plain language descriptions.
Source of funds for data collection and data reduction.
The starting date of this survey formatted as YYYYMMDD (Year,Month,Day).
Sarting place of survey, including city and country if known.
The ending date of this survey formatted as YYYYMMDD (Year,Month,Day).
Ending place of survey, including city and country if known
Instrumentation used determining geographic (latitude/longitude) positioning of the platform. (e.g. Sattelite/Loran-A/Sextant)
The geographic datum used, if known, and the method of determining geographic positioning.(e.g. WGS84/PRIM:SAT, SEC:LORAN A)
Instrument(s) used in aquisition of geophysical data parameter, including relevant specifications and model if known. (e.g. Proton Mag GEOMETRICS G-801)
Additional forms of data available for this geophysical parameter, not necessarily at NGDC. (e.g. microfilm, analog strip)
All formats which the seismic data was saved in. (e.g. digital SEG-Y, Sections on microfilm)
The format of the MGD77 Standard data records, as described using standard Fortran edit descriptors. (Truncated MGD7records formats use the same descriptors, but only contain those fields chosen by the user.)
The confines of the survey are represented by 4 values: top latitude, bottom latitude, left longitude, and right longitude. In the MGD77 Headers, these boundaries expand to the next whole degree.
For bathymetry, magnetics or gravity, this is the general or averaged rate, in tenths of minutes (implied decimal), that data records appear in the file. (e.g. if values appear in file every 5 minutes, this would be 050)
The general rate the instrument sampled the data at. For bathymetry this is written as number of soundings per second (e.g. 1/second); for magnetics and gravity it is the instrument sampling rate in seconds.
Sound velocity of bathymetric instrument used, in tenths of seconds (implied decimal). (generally 15000 or 14630)
Vertical bathymetric datum codes to which MGD77 data is referenced:
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 add. doc.
Description of interpolation method used in processing the bathymetry, should some of the records be interpolated. (e.g. 5 minute intervals plus peaks and troughs)
The distance aft the platform in meters, at which the magnetics sensor was towed.
Average or suspected depth of the magnetics sensor in tenths of meters (implied decimal). If airborne, use negative values to give altitude of sensor.
When two magnetics sensors are used, the distance in meters between the sensors.
Reference field used to determine residual magnetics value is
preceded by code:
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
88 - Others, specify
Procedure used in applying Magnetics Reference Field to derive Residual Magnetics value. (e.g. Linear interp. in 60 mile square)
The gravity formula used to define this surveys gravity values
is preceded by code:
1 - Heiskanen 1924
2 - International 1930
3 - IAG System 1967
4 - IAG System 1980
8 - Other - specify
Gravity reference system determines base gravity for observed
gravity values. It is preceded by code:
1 - Local system - specify
2 - Potsdam system
3 - System IGSN 71
9 - Other - specify
Drift, tare and bias corrections applied since departure. (e.g. +0.075 mgal per day)
Beginning (network) gravity value at sea level in tenths of mgals (implicit decimal) along with the station description, including station identifier.
Ending (network) gravity value at sea level in tenths of mgals (implicit decimal) along with the station description, including station identifier.
These are a series of area identifiers based on a four-digit number
which translates to a specific 10 degree by 10 degree area on
the globe. MGD77 contains fields for up to 30 identifiers in Header
Sequences 16 and 17. ARO88 has fields for up to 70 identifiers
in Sequences 12 thru 16. The first field in the set is the number
of identifiers for the survey. The identifiers themselves follow,
the last being followed by 9999 unless all 30 or 70 fields are
needed. The construction of the identifiers is as follows:
Quadrant - The first-digit identifies the quadrant of the
world as follows:
Code Latitude Longitude
1 North East
3 South East
5 South West
7 North West
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