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#### 3.6.5 PDS4

NASA's Planetary Data System version 4 format is described at https://pds.nasa.gov/datastandards/. This implementation is based on v1.16.0 of PDS4.

PDS4 files consist of an XML Label file which provides detailed metadata, and which may also contain references to external data files stored alongside it. This input handler looks for (binary, character or delimited) tables in the Label; depending on the configuration it may restrict them to those in the `File_Area_Observational` area. The Label is the file which has to be presented to this input handler to read the table data. Because of the relationship between the label and the data files, it is usually necessary to move them around together.

If there are multiple tables in the label, you can refer to an individual one using the "`#`" specifier after the label file name by table `name`, `local_identifier`, or 1-based index (e.g. "`label.xml#1`" refers to the first table).

If there are `Special_Constants` defined in the label, they are in most cases interpreted as blank values in the output table data. At present, the following special values are interpreted as blanks: `saturated_constant`, `missing_constant`, `error_constant`, `invalid_constant`, `unknown_constant`, `not_applicable_constant`, `high_instrument_saturation`, `high_representation_saturation`, `low_instrument_saturation`, `low_representation_saturation` .

Fields within top-level Groups are interpreted as array values. Any fields in nested groups are ignored. For these array values only limited null-value substitution can be done (since array elements are primitives and so cannot take null values).

This input handler is somewhat experimental, and the author is not a PDS expert. If it behaves strangely or you have suggestions for how it could work better, please contact the author.

The handler behaviour may be modified by specifying one or more comma-separated name=value configuration options in parentheses after the handler name, e.g. "`pds4(observational=true,checkmagic=false)`". The following options are available:

`observational = true|false`
  Determines whether only tables within a `<File_Area_Observational>` element of the PDS4 label should be included. If true, only observational tables are found, if false, other tables will be found as well. (Default: `false`)
`checkmagic = true|false`
  Determines whether an initial test is made to see whether the file looks like PDS4 before attempting to read it as one. The tests are ad-hoc and look for certain elements and namespaces that are expected to appear near the start of a table-containing PDS4 file, but it's not bulletproof. Setting this true is generally a good idea to avoid attempting to parse non-PDS4 files, but you can set it false to attempt to read an PDS4 file that starts with the wrong sequence. (Default: `true`)

Files in this format may contain multiple tables; depending on the context, either one or all tables will be read. Where only one table is required, either the first one in the file is used, or the required one can be specified after the "`#`" character at the end of the filename.

This format can be automatically identified by its content so you do not need to specify the format explicitly when reading PDS4 tables, regardless of the filename.

The handler class for files of this format is `Pds4TableBuilder`.

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STIL - Starlink Tables Infrastructure Library
Starlink User Note252
STIL web page: http://www.starlink.ac.uk/stil/
Author email: m.b.taylor@bristol.ac.uk