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subroutine crosscorr (ishift,rxy,xcross,ycross,npx)
c******************************************************************************
c This routine determines the correlation of array 'x' and array 'y',
c comparing array elements 'i' in 'x' with elements 'i+ishift';
c the number of points in each array is assumed to be equal.
c******************************************************************************
implicit real*8 (a-h,o-z)
real*4 xcross(100000),ycross(100000)
real*8 xsum,ysum,xysum,x2sum,y2sum,covar,varx,vary
real*8 xx,yy
xsum = 0.
ysum = 0.
xysum = 0.
x2sum = 0.
y2sum = 0.
if (ishift .ge. 0) then
minpt = 1
maxpt = npx - ishift
else
minpt = iabs(ishift) + 1
maxpt = npx
endif
do 10 i=minpt,maxpt
xx = xcross(i)
yy = ycross(i+ishift)
xsum = xsum + xx
ysum = ysum + yy
xysum = xysum + xx*yy
x2sum = x2sum + xx*xx
10 y2sum = y2sum + yy*yy
xn = real(maxpt - minpt + 1)
covar = xn*xysum - xsum*ysum
varx = dsqrt(xn*x2sum - xsum*xsum)
vary = dsqrt(xn*y2sum - ysum*ysum)
rxy = sngl(covar/(varx*vary))
return
end
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