***** File IMAGFORM.TXT                                                       
                                                                              
                         Image Data Descriptions                              
                                                                              
                                                                              
     The International Halley Watch agreed early on in the project that       
all data would be submitted from the individual disciplines to the Lead       
Center using the FITS format.  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. The         
original FITS byte stream could then be recovered by concatenating the        
appropriate header and data files.   PDS labels were constructed to allow     
definition of the data files for the Planetary Data System.   For each        
data there must always be a FITS header associated.                           
                                                                              
     The convention for naming files on the CD-ROM was proposed by the        
IHW Lead Center to include a unique data qualifier for the data.   A          
specific set of rules was established to identify the net and subnet for      
each discipline (given by a letter code).  A CD-ROM running number relates    
the information contained in the various indices to the files containing      
the actual data.  A short list of this convention for images follows:         
                                                                              
  PDS Object              FITS      Discipline      Letter    File Ext        
 (description)         NAXIS =                        Code                    
                                                                              
image                      2        IR Studies        IRIM        .img        
image(compressed)          1        Large Scale Phen  LSPN        .imq        
image(browse)              2            "             LSPN        .ibg        
image                      2        Near Nucleus      NNSN        .img        
image(multiple)            3            "             RSOC        .img        
image                      2        Spectroscopy      SPEC        .img        
                                                                              
Note that the Subnet coding (explained in the Filenaming Conventions)         
is one way to distinguish file extension and content.                         
                                                                              
     The file extensions follow suggestions by the Planetary Data System      
(SPIDS v1.1) 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.                                                   
                                                                              
                 .IBG - data records for subsampled browse image              
                 .IMG - image data records                                    
                 .IMQ - data records for compressed image                     
                                                                              
    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 thankfully acknowledge the help of M. Martin,    
JPL, for assisting the IHW through version 1.1 of the ODL that allows         
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) with 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, .IMQ, .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.