Line headers for standard seismic formats have many fields. They can be described using DDS notation for binary structures. (See the Fmt Control section.) Each line header field is assigned a symbolic name, type, position, size, alignment, and descriptive text. These are described in the the Formats subsections: usp, segy, and asp.

Input Line Header information is converted into definitions. Each definition name is constructed from the format and Line Header field name. For example, the SEGY field ReelNum becomes "segy_ReelNum= nnn". Precedence is given to control parameters defined in par:. This allows the end user to override an incorrect header value. If not specified, line header information is honored. If not available, the input history can provide a default.

Output Line Headers are constructed from current definitions. If nothing is defined, a default value (zero, or blank) is assumed.

What's the bottom line? Line Header information for a particular format is accurately preserved and reconstructed via Dictionaries. Extraneous Definitions are minimized, through the use of defaults. Values that were "hidden" in a binary field, can be examined in Dictionaries by mortals. Explicit Definitions can supplement or correct erroneous input Line Header values.