The mgd77 supplement to the generic mapping tools

Marine geophysical data collected by government and academic vessels are archived at the US National Geophysical Data Center in Boulder, Colorado. Data exchanges between NGDC and source institutions use an ASCII, punch-card format known as the MGD77 format, reflecting a style of file design common in the 1970s. We have developed a set of new software tools that can convert between this exchange format and a new COARDS-compliant, netCDF-based, architecture-independent file format that we call the MGD77+ format. The new mgd77 tools allow the data to be manipulated in a variety of ways useful for marine research. These tools are written in POSIX-compliant C, are distributed as a supplement to the Generic Mapping Tools, and can be installed on any computer platform. Because the new format is COARDS and CF-1.0 compliant, the files can be read by any general-purpose program capable of decoding these standards; a welcome side effect. One such program is the Java application ncBrowse developed by NOAA. Furthermore, the more compact netCDF files have file sizes that are, on average, only 30% of the original sizes. Because procedural changes at NGDC and source institutions necessarily occur infrequently, it is expected that the MGD77 format will remain the official exchange format for underway geophysical data for some time, whereas the new MGD77+ format offers users much needed flexibility in how they use the data.