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October 25, 2004, 11:31 
How to digitised a curve

#1 
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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! 

October 25, 2004, 12:04 
Re: How to digitised a curve

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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.


October 25, 2004, 13:32 
Re: How to digitised a curve

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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 

October 25, 2004, 15:55 
Re: How to digitised a curve

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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. 

October 25, 2004, 17:11 
Re: How to digitised a curve

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thanks a lot, Jonas, that's really helpful.


October 26, 2004, 13:10 
Re: How to digitised a curve

#6 
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I use the same one too.


October 27, 2004, 07:59 
Re: How to digitised a curve

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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 ! 

October 27, 2004, 08:11 
Re: How to digitised a curve

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October 27, 2004, 10:49 
Re: How to digitised a curve

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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 nonlinear ones and there are hundred and hundreds of options there. Also you can use TableCurve 2D.


October 27, 2004, 12:06 
Re: How to digitised a curve

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Matlab is my tool of choice.


October 27, 2004, 17:32 
Re: How to digitised a curve

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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! 

October 28, 2004, 09:31 
Re: How to digitised a curve

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This is difficult for me to answer, because it is problem dependent. If it is 2D 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. 

November 18, 2004, 18:40 
Re: How to digitised a curve

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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://plasmagate.weizmann.ac.il/Grace/ 

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