SUBROUTINE SELMOD ( )
Description
THIS ROUTINE SELECTS THE SAMPLE AREA TO BE USED IN RAY PATH 
CALCULATIONS.
Source file:selmod.f
Intrinsic Functions Called
GENERIC            ABS
Local Variables (+ indicates altered content)
REAL              +DELFI
Referenced Common Block Variables (+ indicates altered content)
FRQ                INTEGER           +JMODE
RON                INTEGER            KFX
RON                REAL               FI(3,5)