Map Definition are used when one data format is mapped to another. Definition names are prefixed by "map:" in the form:

map:in:out.field= expression

• "in" and "out" can be the format name (eg. "usp" or "segy") or alias names (eg. "in" or "vel"); but don't intermix them. It is advisable to use "*" in place of "in" for output mapping because most applications (except "bridge") use an internal intermediate buffer, so the input dataset may not be the actual buffer being used internally by an application.
  
  
• "field" can be any Trace Header name in the output dataset.
  
  
• "expression" is an algebraic expression. It may contain constants, field names from the input format, operators, built-in functions, and parentheses.

Example map definitions:

comment= set a field to a constant
map:*:usp.GrpElv= 0.0

comment= map SEGY header "CdpTrcNum", to USP "TrcNum"
map:segy:usp.TrcNum= CdpTrcNum

comment= calculate a Midpoint for fields
map:*:out.SrRcMX= 0.5 * (SrcX + GrpX)

comment= calculate offset distance
map:segy:usp.DstUsg= sqrt((SrcX-GrpX)*(SrcX-GrpX)
                        + (SrcY-GrpY)*(SrcY-GrpY))
    

NOTE: any Trace Headers that have the same names in the output as the input will get mapped automatically. Mapping Definitions are therefore only needed when the names change. However, "Spare.Hdr.LinInd" is not the same name as "LinInd". Also, there are no default mappings going into a union, because of conflicting datatypes for the same byte locations.