Interpolating

[rm2]

Hello,

I have to vectors (flow and pressure).
I am scatter plotting them (flow vs pressure). I did it before, I do not recall how I interpolated around for a certain value of let say pressure and have the flow value on that specific point...
hope anyone can help me, thanks

 

[electricpete]

Have you tried linear interpolation?

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[IRstuff]

Excel has functions for curve fitting, like linest and logest, from which you get the parameters for doing the interpolations.

TTFN

FAQ731-376

 

[rm2]

is it interp2?
I will try tomorrow morning

 

[MikeyP]

Do you want an interpolation or a best fit line?

use "polyfit" for the latter

M

--
Dr Michael F Platten

 

[rm2]

Interpolation

 

[electricpete]

I'm sure there are some built in excel functions that's do what you want.

Personally rather then worrying about that syntax I would rather just write the formula. Then you can do the same thing in Matlab or Maple or whatever. Math works in any language.

Step 1: What is the slope of a line passing through two points: (X1,Y1) and (X2,Y2).
Think about it before you read the answer.
Answer: slope = (Y2-Y1)/(X2-X1)

Step 2: How do I use that slop to find the value of Y at some arbitrary X in the interval (X1,YX)?
Think about it before you read the answer.
Answer:
Y = Y1 + slope * (X-X1)

Sorry I am not trying to be a jerk. It is something you should spend time understanding (rather than memorizing) imo.


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[electricpete]

obvious typo correction in bold:

Step 2: How do I use that slope to find the value of Y at some arbitrary X in the interval (X1,X2)?

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[electricpete]

I apologize if I have underestimated your problem. Maybe you want a general formula to work with two columns of X, Y data. Then you need to use some lookups if you are going to build the arguments for the interpolation formula if you are going to build it yourself.

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[electricpete]

Here is a vba function to return the interpolation from a range of X's and Y's:

Function InterpL(LookupValue, KnownXs, KnownYs)
Dim pointer As Integer
Dim XO As Double, YO As Double, X1 As Double, Y1 As Double
pointer = Application.Match(LookupValue, KnownXs, 1)

XO = KnownXs(pointer)
YO = KnownYs(pointer)
X1 = KnownXs(pointer + 1)
Y1 = KnownYs(pointer + 1)
InterpL = YO + (LookupValue - XO) * (Y1 - YO) / (X1 - XO)
End Function

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[electricpete]

And yes this is linear interpolation. For a higher order interpolation you need a spline technique. Let us know if you are interested in that.

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[electricpete]

Hmmm. I thought I was in the excel forum - hence the vba function. IRStuff sent me off track (or maybe I just wasn't paying attention).

Use interp1 for simple interpolation like you are talking about.

Interp2 is for 2-d interpolation.

There is a lot of help in the help

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[SomptingGuy]

If you are using interp1, be sure that your independent axis (you haven't told us whether it's pressure or flow) is monotonic.

- Steve