Data file saved in the Marine Geophysical Data Exchange (MGD77) format; contains geophysical data in ASCII text format; includes the survey ID, date, magnetics data, timezone, observed gravity, latitude and longitude coordinates, and water depth.

The MGD77 data format was first developed in the 1970s by the National Geophysical Data Center (NGDC) to exchange digital underway geophysics data. Today, it is used as an exchange format for geophysical data, which includes magnetics, bathymetry, gravity, and seismic navigation. MGD77 files are created from raw data collected by marine researchers studying underwater depth of lake and ocean floors. The data is stored in InfoBank, a structured information storage scheme of databases and software, and then exported in the MGD77 format to transmit data to and from a data center for analysis. The MGD77T format is a new revision of the MGD77 format. It allows space savings, expanded field lengths, and easier import into other programs. This format uses the .M77T file extension.