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Old   October 25, 2004, 11:31
Default How to digitised a curve
  #1
Anne
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I found very useful experimental curve in literature, and I want to digitize it and feed into my CFD code. Does anybody know how to do it? Please give me some info and hint on it1

Thanks ahead!
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Old   October 25, 2004, 12:04
Default Re: How to digitised a curve
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Tom
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Try to read some (x,y) coordinates manually from the curve. Once you have the coordinates, fit an orthogonal polynomial (i.e. discrete fourier, Chebyshev (rescaled to [-1,1], or other polynomial) using a collocation (gridpoint) or spectral coefficient approach. This is low tech, but may be your most efficient method.
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Old   October 25, 2004, 13:32
Default Re: How to digitised a curve
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Anne
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thanks, Tom.

Can you give me more detail on it. I mean the tools, such as what software (matlab, tecplot, origin) is best to fit the selected data. And what is the collocation (gridpoint) or spectral coefficient approach.

Thanks a lot for your reply1
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Old   October 25, 2004, 15:55
Default Re: How to digitised a curve
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Jonas Larsson
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I always use the Engague Digitizer - a free tool which allows you to digitize a scanned plot. You can download it from here:

http://digitizer.sourceforge.net/

It works really well and has lost of nice features.
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Old   October 25, 2004, 17:11
Default Re: How to digitised a curve
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Anne
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thanks a lot, Jonas, that's really helpful.
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Old   October 26, 2004, 13:10
Default Re: How to digitised a curve
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Amadou Sowe
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I use the same one too.
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Old   October 27, 2004, 07:59
Default Re: How to digitised a curve
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Anne
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Hi, dear all,

After digitizing the curve, a set of data is obtained. If I want to fit them, such as lnear fit, polynomial fit, even a distribution function, what software do you recommend? I used Origin before, and the fitting function is too simple. Can you give me any suggestion?

Thanks !
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Old   October 27, 2004, 08:11
Default Re: How to digitised a curve
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Jörn Beilke
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The best I have used is Xact (www.scilab.de) but it is not really cheap.

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Old   October 27, 2004, 10:49
Default Re: How to digitised a curve
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anindya
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Origin Lab is very Good. There are lots of Curve fitting options from simple to complex. You have to experiment which curve fits best. Dont just try the default linear ones. Try the non-linear ones and there are hundred and hundreds of options there. Also you can use TableCurve 2D.
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Old   October 27, 2004, 12:06
Default Re: How to digitised a curve
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Amadou Sowe
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Matlab is my tool of choice.
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Old   October 27, 2004, 17:32
Default Re: How to digitised a curve
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Anne
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Then, could you please tell me more on how to do it. I only know the polyfit function for polynomial fit. Is there any other functions, in which toolbox? Is it flexible to use?

Thanks again!
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Old   October 28, 2004, 09:31
Default Re: How to digitised a curve
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Amadou Sowe
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This is difficult for me to answer, because it is problem dependent. If it is 2-D data I will first us the plot function to see what it looks like. After this step I will decide whether I need a hyperbolic, exponential, polynomial, or some other fit.

Matlab has a section on regression and curve fitting in addition to their curve fitting, statistic, and optimization tool boxes that are very helpful for curve fitting tasks. Good luck.
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Old   November 18, 2004, 18:40
Default Re: How to digitised a curve
  #13
Mani
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I like PlotDigitizer better than Engauge, but either program will work. PlotDigitizer is a Java program and works on various platforms.

http://plotdigitizer.sourceforge.net/

Nonlinear curve fitting is very powerful, as you can fit your data to any function that you specify (with a number of free coefficients). I like using xmgrace for this purpose. Like most other good programs, it's open source.

http://plasma-gate.weizmann.ac.il/Grace/
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