Downloading Data: Formats

Formats for Downloaded Data Files

Select a Format (choose one):

MGD77 ASCII Exchange Format:

This is the most common format used for exchange of deep ocean bathymetry, magnetics and gravity data. Standard MGD77 has a data record of 120 ASCII characters and is described in THE MARINE GEOPHYSICAL DATA EXCHANGE FORMAT - "MGD77"
You may choose to download the Truncated MGD77 version of the data records by selecting only those fields you want in your records. The fields will be formatted identically and in the same order as Standard MGD77 Format.

Output filenames will contain extension .a77 unless changed by the user

HYD93 ASCII Exchange Format:

This is the most common format used for exchange of hydrographic survey data. Standard HYD93 has a data record of 37 ASCII characters and is described in THE HYDROGRAPHIC SURVEYS DATA EXCHANGE FORMAT - "HYD93".
You may choose to download the Truncated HYD93 version of the data records by selecting only those fields you want in your records. The fields will be formatted identically and in the same order as Standard HYD93 Format.

Output filenames will contain extension .a93 unless changed by the user

Binary HYD93 CD/DVD Format:

This format is identical to that on GEODAS Hydrographic Surveys CD/DVD sets. The fields included in this version are binary representations of some of the fields in the HYD93 ASCII Format.

The Binary HYD93 record structure follows:

LATITUDE X 1,000,000 4 Byte Integer degrees
LONGITUDE X 1,000,000 4 Byte Integer degrees
DEPTH VALUE X 10 4 Byte Integer meters
TYPE/CARTO CODE 2 Byte Integer type-code * 1000 + carto-code

(type/carto code combines the type code field with the carto code field by mulitplying the type times 1000 and adding the carto code.)

Output filenames will contain extension .b93 unless changed by the user

ASCII or Binary XYZ Format :

This option offers standardized ASCII or Binary XYZ formats which can be easily input into numerous applications. Each data record contains fields for longitude, latitude, and value.

X = longitude
Y = latitude
Z = choose one data parameter from the list

The user must choose from several XYZ Formats. These include ASCII space-delimited xyz format, the GEODAS BINARY INTEGER XYZ FORMAT, Binary 8-byte Floating Point xyz format or Binary 8-byte Floating Point xyz format. All of these xyz formats can be used with the GEODAS Hydro-Plot application. Each data record contains fields for longitude, latitude, and value, as follows:

ASCII delimited XYZ Format: The records are ASCII. Each field is separated by a delimiter (space, comma or tab).
GEODAS Binary Integer XYZ Format: Files are a series of binary records. The 3 fields are 4-byte binary integers. These fields are longitude in millionths of degrees, latitude in millionths of degrees, and value in tenths of units.
Binary 8-byte XYZ Format: Files are a series of binary records. The 3 fields are 8-byte (double precision) binary floating point values.
Binary 4-byte XYZ Format: Files are a series of binary records. The 3 fields are 4-byte (single precision) binary floating point values. Note that this format may not carry enough precision for many applications.
NOTE: User may select specific fields to download from MGD77 or HYD93 formats by clicking on the fields with the mouse.

Possible GEODAS Data Types in these XYZ Formats are:
corrected depth in meters
uncorrected depth in meters
magnetics total field in nanoteslas (gammas)
magnetics residual field in nanoteslas (gammas)
observed gravity in milligals
free air gravity in milligals

If depths are chosen as data type, the user must then determine the units of depth measurements (meters, feet or fathoms), and whether the depths will be negative or positive values in the records (negative depths are standard).

GEODAS output filenames will contain file extension .xyz unless changed by the user.