spline interpolation for repeated (non periodic) data?

Question

Madan Kumar 2017-8-6

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评论： Madan Kumar 2017-8-6

Hi,

I have a matrix (33175x12) with repeated (non-periodic) values (x below). I tried to interpolate but getting error "The data sites should be distinct.". I think sorting data will not be applicable either because I have sorted year-month-day wise. Program is like,

%a=year,b=month,c=day,x=altitude,y=density
  a  b c  x y
1995 1 1 80 8
1995 1 1 81 9
  .  . .  . .
  .  . .  . .
  .  . .  . .
1995 1 1 90 5
1995 1 2 80 7
1995 1 2 81 6
  .  . .  . .
  .  . .  . .
  .  . .  . .
1995 1 2 100 9
1995 1 2 105 8
1995 1 3 75 9
1995 1 3 76 8
  .  . .  . .
  .  . .  . .
  .  . .  . .
1995 1 3 110 8
  .  . .  . .
  .  . .  . .
  .  . .  . .

I am interpolating data as follows,

xx=80:1:105;
Intdata=spline(y,x,xx); Any help/suggestions. Thank you.

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Answer 1

John D'Errico 2017-8-6

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在 MATLAB Online 中打开

Um, a spline is a tool that allows you to predict a SINGLE valued function, of the form

y = f(x)

You are calling spline by passing in y in place of x, and x in place of y. So you are creating a relation that is NOT single valued, because thereare multiple values of X at the same locations.

Should spline be smart enough to know that you swapped x and y? How should it be that intelligent? Maybe you WANTED to do that?

Read the help for spline. If you do, it suggests the form:

YY = spline(X,Y,XX);

If you swap the first two arguments, expect problems.

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Madan Kumar 2017-8-6

Thank you so much. I also tried it. Problem is that "x" is not monotonic. x repeats but not periodically. Again not all values of x repeat.I am getting error "The data sites should be distinct."

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