Are your z values the same for each x-y-t slice? Would this help? Author/source info are in the header below.
;=========================================================== ====================
;+
; ROUTINE: findex
;
; PURPOSE: Compute "floating point index" into a table using binary
; search. The resulting output may be used with INTERPOLATE.
;
; USEAGE: result = findex(u,v)
;
; INPUT:
; u a monitically increasing or decreasing 1-D grid
; v a scalor, or array of values
;
; OUTPUT:
; result Floating point index. Integer part of RESULT(i) gives
; the index into to U such that V(i) is between
; U(RESULT(i)) and U(RESULT(i)+1). The fractional part
; is the weighting factor
;
; V(i)-U(RESULT(i))
; ---------------------
; U(RESULT(i)+1)-U(RESULT(i))
;
;
; DISCUSSION:
; This routine is used to expedite one dimensional
; interpolation on irregular 1-d grids. Using this routine
; with INTERPOLATE is much faster then IDL's INTERPOL
; procedure because it uses a binary instead of linear
; search algorithm. The speedup is even more dramatic when
; the same independent variable (V) and grid (U) are used
; for several dependent variable interpolations.
;
;
; EXAMPLE:
;
;; In this example I found the FINDEX + INTERPOLATE combination
;; to be about 60 times faster then INTERPOL.
;
; u=randomu(iseed,200000) & u=u(sort(u))
; v=randomu(iseed,10) & v=v(sort(v))
; y=randomu(iseed,200000) & y=y(sort(y))
;
; t=systime(1) & y1=interpolate(y,findex(u,v)) & print,systime(1)-t
; t=systime(1) & y2=interpol(y,u,v) & print,systime(1)-t
; print,f='(3(a,10f7.4/))','findex: ',y1,'interpol: ',y2,'diff: ',y1-y2
;
; AUTHOR: Paul Ricchiazzi 21 Feb 97
; Institute for Computational Earth System Science
; University of California, Santa Barbara
; paul@icess.ucsb.edu
;
; REVISIONS:
;
;-
;
;=========================================================== ====================
function findex,u,v
;=========================================================== ====================
;=========================================================== ====================
nu = n_elements(u)
nv = n_elements(v)
us = u-shift(u,+1)
us = us(1:*)
umx = max(us,min=umn)
if (umx gt 0) and (umn lt 0) then message,'u must be monotonic'
if (umx gt 0) then inc=1 else inc=0
;=========================================================== ====================
; maxcomp = maximum number of binary search iterations
;=========================================================== ====================
maxcomp = fix(alog(float(nu))/alog(2.)+.5)
jlim = lonarr(2,nv)
jlim(0,*) = 0 ; array of lower limits
jlim(1,*) = nu-1 ; array of upper limits
iter = 0
repeat begin
jj = (jlim[0,*]+jlim[1,*])/2
ii = where(v ge u[jj],n) & if (n gt 0) then jlim(1-inc,ii) = jj[ii]
ii = where(v lt u[jj],n) & if (n gt 0) then jlim(inc,ii) = jj[ii]
jdif = max(jlim[1,*]-jlim[0,*])
if iter gt maxcomp then begin
print,maxcomp,iter, jdif
message,'binary search failed'
endif
iter = iter+1
endrep until jdif eq 1
w = v-v
w[*] = (v-u[jlim[0,*]])/(u[jlim[0,*]+1]-u[jlim[0,*]]) + jlim[0,*]
return,w
end
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