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Plotting a spiral with SigmaPlot

  1. Mar 18, 2010 #1
    I am trying to use the 'Regression Wizard' in SigmaPlot to obtain a best-fit curve to a data set of ~15 data points that map out a spiral. None of the equation categories given in the regression wizard seem to work (i.e. polynomial, exponential, logarithm, hyperbola etc. etc.) and I am unsure how to set a user defined equation to the data set.

    It's a logarithmic spiral so I'm assuming the eqn in polar co-ords should be;

    r(theta)=a*exp(b*theta)

    parametrically it should be;

    x=r*cos(theta)=a*cos(theta)*exp(b*theta)
    y=r*sin(theta)=a*sin(theta)*exp(b*theta)

    and in cartesian co-ords;

    x^2+y^2=exp((theta)*cot(a))

    Any advice on SigmaPlot or plotting spirals in general will be greatly appreciated!!!
     
  2. jcsd
  3. Mar 19, 2010 #2
    Hi,
    i hope this link might be useful
    http://www1.webng.com/economics/logspiral.txt [Broken]

    source Mishra
     
    Last edited by a moderator: May 4, 2017
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