***** File TEXTFORM.TXT Text Data Descriptions The International Halley Watch agreed early in the project that all data would be submitted from the individual Disciplines to the Lead Center using the FITS format (Wells et al., 1981). When the decision was made to distribute this information on CD-ROM, it was determined that the data had to have even broader accessibility. For this reason the original FITS files, with contiguous headers and data, were split into separate files distinguishable by their filename extensions (.HDR for headers). The file sizes were preserved as multiples of 2880 bytes, allowing the original FITS byte stream to be recovered by concatenating the appropriate header and datafile. PDS labels were constructed to allow definition of the datafiles for the Planetary Data System. For each datafile there must always be an associated FITS header. The convention for naming files on the IHW CD-ROMs was proposed by the Lead Center and NASA/Goddard Space Flight Center (GSFC) personnel to include a unique data qualifier for the data. Specifically, a set of subnet codes was established to enable identification of the IHW Discipline/subdiscipline from the filename itself. A CD-ROM running number and file extension complete the filename. A short list of this convention for the Astrometry Discipline follows: PDS Object FITS Discipline Subnet File Ext (description) NAXIS = Code ______________________________________________________________________ text 1 Astrometry ASTR .dat ______________________________________________________________________ The file extensions follow suggestions by the Planetary Data System (SPIDS v1.1; Martin et al., 1988) for tabular and image data. In addition, for IHW FITS, the original headers and data were split into separate files, with filename extensions as listed below. .DAT - other non-image or non-table data .HDR - FITS header records .LBL - detached PDS stream format These PDS labels are metadata (as headers describing data submitted to the archive). There has been no effort to duplicate the documentation contained in the full FITS headers because the PDS and FITS headers for a given data file differ only in the filename extension. Instead we have attempted to use the power of the PDS label syntax to fully describe the data structures and thus gain access to software by that group. Standards for the Preparation and Interchange of Data Sets, Martin, T. Z., et al, Document D-4683, Jet Propulsio Laboratory, California Institute of Technology, Pasadena, CA document version 1.1 was the primary reference to the Object Description Language (ODL) necessary to create the PDS labels. (We acknowledge R. Borgen and M. Martin PDS-CN, JPL, for assisting the IHW through version 1.1 of the ODL implementation for SPECTRUM.) The basic PDS descriptors such as SFDU_LABEL, RECORD_TYPE, RECORD_BYTES, and FILE_RECORDS are explained in the SPIDS document. The RECORD_TYPE for all data files is FIXED_LENGTH. The PDS labels have been formed as fixed length (78 byte) plus an embedded CR and LF. The TEXT object (which is used for the Astrometry Discipline's data with extension .DAT) is an 80 byte fixed length record that contains only ASCII values. In the FITS formulation, the 80-byte records are strung together, typically as 4 or 5 "card" images with no delimiters and padded to fill the 2880 byte record structure. It can be recognized in the FITS formulation by the NAXIS=1 statement, which indicates that a byte stream follows usually carrying "text" description. The Astrometry data has been separated into its own subdirectory (ASTROM) throughout the Archive discs. REFERENCES Martin, T.Z., Martin, M.D., Davis, R.L., Mehlman, R., Braun, M., Johnson M.: October 3, 1988, Standards for the Preparation and Interchange of Data Sets, Version 1.1, JPL D-4683. Wells, D.C., Greisen, E.W., and Harten, R.H.: 1981, Astron. Astrophys. Suppl. Ser. 44, 363.