function fintrp(xtab, ytab, ntab, xx)
      implicit double precision (o-z, a-h)
      dimension xtab(ntab), ytab(ntab)
      i1 = locate(xtab,ntab,xx)
      i2 = i1 + 1
      rr = (xx - xtab(i1)) / (xtab(i2) - xtab(i1))
      fintrp = ytab(i1) + (rr * (ytab(i2) - ytab(i1)))
      return 
      end

      function locate(xtab, ndim, x)
      implicit double precision (o-z, a-h)
      dimension xtab(ndim)
      jl = 0
      ju = ndim + 1
   10 if ((ju - jl) .gt. 1) then
      jm = (ju + jl) / 2
      if ((xtab(ndim) .gt. xtab(1)) .eqv. (x .gt. xtab(jm))) then
      jl = jm
      else
      ju = jm
      end if
      goto 10
      end if
      locate = jl
      return 
      end