***** 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.