***** File IMAGFORM.TXT                                                       
                                                                              
                                                                              
                         Image 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 (example:  LSPN0059.IBG).  A short list of this convention by        
Discipline and subnet (or experiment) is given below for image data:          
                                                                              
                                                                              
                                                                              
  PDS Object              FITS  Discipline      Subnet  Extensions            
 (description)         NAXIS =                   Code                         
______________________________________________________________________        
                                                                              
image                      2    IR Studies       IRIM      .img               
image (browse)             2          "          LSPN      .ibg               
image                      2    Near Nucleus     NNSN      .img               
image                      2    Radio Studies    RSCN      .img               
image (multiple)           3          "          RSOC      .img               
image (spectrum)           2    Spectroscopy     SPEC      .img               
______________________________________________________________________        
                                                                              
                                                                              
A table linking the subnet codes above and the subdiscipline names is given in
Section 5 ('Filenaming Conventions') of the file HALGUIDE.TXT.  Concerning the
numeric portion of filenames, calibration files for IRIM, LSPN, and SPEC begin
at 4001, whereas the Halley data themselves for all disciplines and           
subdisciplines start at 0001.                                                 
                                                                              
     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.                                     
                                                                              
                                                                              
                 .HDR - FITS header records                                   
                 .IBG - data records for subsampled browse image              
                 .IMG - image data 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          
Propulsion 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 2.0 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. 
                                                                              
     In the case of images, we have included a new keyword describing the byte
ordering of the data (MSB_INTEGER) required by FITS.  In PDS, images (.IMG,   
.IBG) are in terms of LINES (FITS keyword NAXIS2) and SAMPLES (FITS keyword   
NAXIS1), given knowledge of the SAMPLE_BITS (FITS keyword BITPIX), and are    
easy for the split files.  The final form of the label for compressed images  
is still under active discussion.  Unlike previous PDS efforts with compressed
images, we chose not to compress the header (or label) and thus have included 
a keyword to describe the type of compression (ENCODING_TYPE =                
"PREVIOUS_PIXEL") used. The label for compressed images also contains         
information to permit software to skip over the data if the decoding algorithm
is unknown (ITEMS, ITEM_TYPE, and ITEM_BITS).   We use ODL to indicate various
subclass structures for the data objects.  An example of this is the          
DIFFERENCE modifier applied to IMAGE, yielding the keyword DIFFERENCE_IMAGE,  
which indicates that a processing step was applied to the original image.     
                                                                              
                                                                              
                                                                              
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.