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Archive Preparation Guide
Proposerís Archive Guide
Standards Reference
Data Preparerís Workbook
Quick-Start Introduction to
PDS Archiving
Quick-Start Introduction to PDS Archiving

PDS Data Files and Labels


Users of archived data need the data bits themselves, but they also need information describing the format and content of the data files. PDS data are always accompanied by labels which provide this information. In general each data file has its own label; a single label may be used to reference several distinct files in cases where those files are intimately related (chapters of a document, for example). Labels may be attached (stored as part of the data file) or detached (the label stands as a file in its own right).

The label provides a field-by-field description of the contents of the associated file; it may also provide information describing the circumstances of the recorded observation (start time, for example) and may provides links to other related PDS files, through various identification fields. Labels are constructed so they can be read by both people and machines.

PDS labels are written in the Object Description Language (ODL), as are all PDS catalog files. ODL consists of a series of lines of the form "keyword = value", with certain keywords (for example,OBJECT) being used to delimit structures within the label. PDS prefers to use non-cryptic keywords, so that the data labels are more easily interpreted by users browsing through them.

  • More about ODL (ODL is defined and described in full in Chapter 12 of the PDS Standards Reference, which is available for on-line browsing.
  • Label file requirements are described in full in Chapter 5 of the PDS Standards Reference, which is available for on-line browsing.


The keywords in the data label are generally laid out in the following order:

  1. Standards Identifiers: one or two keywords defining which version of PDS standards were observed during creation of the label and data set
  2. File Characteristics: physical attributes of the data file, such as whether the records are fixed-length or variable-length, record length (if applicable), and number of records.
  3. Data Pointers: links to the start of the actual data, either past the end of the label in the same file, or the name of another file where the data reside
  4. Identification Parameters: numbers and codes used to link this data file with other related data and with descriptions in the PDS catalog. These include things like the target being observed, the mission, the instrument recording the data, the identification number for the data set, etc.
  5. Observational Parameters: filter names, pointing information, or any other descriptive keywords relevant to the data
  6. Data Structure Definition: a definition of both the physical organization and logical significance of the bytes in the data file

Comments, beginning with "/*", may be included in the label file as desired for internal documentation and clarification.

Data Structure Definition

The physical and logical data structure are included in the PDS label using the OBJECT and END_OBJECT keywords as delimiters. (The "object" concept anticipates the development of software which can treat a complex data structure as a single basic entity.) OBJECT definitions may be nested where that is appropriate.

To promote uniformity of description and to simplify development of general-purpose tools, PDS has defined several standard OBJECTs which should be used to describe the structure of data files wherever that is possible. It is nearly always preferable to reformat the data to conform with an existing OBJECT definition than to develop a new OBJECT specific to the pathology of a particular data set. The defined PDS OBJECTS are reasonably flexible and generic, however, so any reformatting required tends to be minimal for non-proprietary data formats.

The type of PDS OBJECT indicated by the OBJECT keyword defines the general data structure (for example, data table vs. image). Within that OBJECT definition, keywords are used to provide physical specifications, such as field widths and byte locations, as well as logical specifications, such as units of measure or field names.

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