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jabronyo Newbie
Joined: 16 Apr 2012 Posts: 1

Posted: Mon Apr 16, 2012 4:57 pm Post subject: Deriving Equations Given Graphs 


Hi Guys,
I'm trying to make a database of values for a given graph that seems to be semilinear in nature. (See below)
I'm also trying to do the same for something that seems logarithmic, but not entirely. (See below)
Any ideas on how to pull this off?
Thanks,
JR 

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UmTheMuse General User
Joined: 05 Apr 2012 Posts: 33 Location: United States

Posted: Mon Apr 23, 2012 12:23 pm Post subject: 


You don't have any of the original numbers, right? If you did, you could maybe do a line of best fit for each curve. You could also use a statistical analysis software, such as gretl (freeware for econometrics) with multiple offsets.
Without the original data, you'd probably have to eyeball, guestimate it. Draw a line to more or less fit the existing line. Check the change in height over the change in length to give you the slope of a line and give the lines different (negative) yintercepts for the offset between each line.
The nonlinear equations would be more complicated, but basically the same thing. The hard part would be calculating the slope. You'll need to know calculus to figure out the slope of whatever equation you think will fit best. For example, the derivative of the natural log, ln(x) = 1/x. Draw a curve that would fit your equation and superimpose it over the graph so you can figure out the intercepts. 

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jrkrideau Super User
Joined: 08 Aug 2005 Posts: 6732 Location: Kingston ON Canada

Posted: Thu Apr 26, 2012 4:09 am Post subject: 


I have never used them but I believe that there are several "graph digitizers' available. Have a look at http://digitizer.sourceforge.net/ for one Open Source example. _________________ jrkrideau
Kingston ON Canada
Currently using Windows 7 & OOo 3.4.0 and Ubuntu 12.04 & LibreOffice 3.5.2.2 

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