The mapping Dictionary may also contain "read:" Definitions which define values to be assigned to Trace Header fields as the binary is being read. These are a little different from the mapping Definitions and are needed to provide some of the built-in functions that are only meaningful in the sequence of reading traces. They take on the form:

read:in.field= expression
    

• "in" can be the format name (eg. "usp" or "segy"), alias names (eg. "in" or "vel"), or "*".
  
  
• "field" can be any Trace Header name in the input dataset.
  
  
• "expression" is an algebraic expression. It may contain constants, field names from the input format, operators, built-in functions, and parentheses.

For example, segy header words LineTrcNum and ReelTrcNum can be assigned new values on input.

read:segy.LineTrcNum= 1 + linetrc()
read:segy.ReelTrcNum= 1 + reeltrc()
    

These examples will assign sequence numbers to these headers using the built-in mapping functions (these functions take no arguments).