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MathGroup Archive 2007

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Re: show residuals of circle fit

  • To: mathgroup at smc.vnet.net
  • Subject: [mg79951] Re: [mg79900] show residuals of circle fit
  • From: DrMajorBob <drmajorbob at bigfoot.com>
  • Date: Thu, 9 Aug 2007 05:26:15 -0400 (EDT)
  • References: <4787716.1186570751288.JavaMail.root@m35>
  • Reply-to: drmajorbob at bigfoot.com

Data points:

angleRange = {-\[Pi]/4, \[Pi]};
points = Table[theta = RandomReal@angleRange;
    {2*Cos[theta] + 1 + .05*Random[],
     2*Sin[theta] - 3 + .05*Random[]}, {30}];

Nonlinear fit and plot:

Clear[func]
func[x_] = Fit[points, {1, x, x^2, x^3}, x]

-1.26986 + 0.743417 x - 0.366836 x^2 - 0.0257307 x^3

Show[Graphics[
   Plot[func[x], Evaluate@Flatten@{x, Sort[points][[{1, -1}, 1]]}]],
  ListPlot[points, PlotStyle -> {Red, PointSize[Small]}], Axes -> True,
   AspectRatio -> Automatic]

(no need to copy/paste the limits!)

Residuals:

ListPlot[points /. {x_, y_} :> {x, y - func[x]}]

Circle fit and plot:

hypersphereFit[data_List] := Module[{rhs, qq, rr, x, y, r},
   {qq, rr} = QRDecomposition[Append[#, 1] & /@ data];
   rhs = #.# & /@ data;
   {x, y, r} = {1/2, 1/2, 1} LinearSolve[rr, qq.rhs];
   {{x, y}, Sqrt[r + x^2 + y^2]}]
{center, r} = hypersphereFit@points
Show[Graphics[Circle[center, r, angleRange]], ListPlot@points,
  AspectRatio -> Automatic]

{{1.02827, -2.97834}, 2.00092}

Circular residuals:

ListPlot[points /. {x_, y_} :> {ArcTan[x, y],
     Norm[{x, y} - center] - r}]

Directly minimizing the sum of squared residuals gives a nearly identica=
l  =

circle:

Clear[error, r]
error[xc_, yc_, r_] = #.# &[
    points /. {x_, y_} :> Norm[{x, y} - {xc, yc}] - r];
Minimize[{error[x, y, r], r > 0}, {x, y, r}]

{0.00679177, {r -> 2.00117, x -> 1.02822, y -> -2.97891}}

Bobby

On Wed, 08 Aug 2007 03:53:39 -0500, will <willpowers69 at hotmail.com> wrot=
e:

> Dear Mathforum,
>
> I am having a problem with displaying residuals after fitting a circle=
 to
> a set of points. I am using some code that I found on the mathgroup
> site  =

> (see:http://forums.wolfram.com/mathgroup/archive/2003/Jan/msg00019.htm=
l)
>  to generate the points and fit the circle:
>
> points generation and display:
>
> points = Table[theta = Random[];
>   {2*Cos[theta] + 1 + .05*Random[],
>    2*Sin[theta] - 3 + .05*Random[]}, {30}]
>
> plot = ListPlot[points, AspectRatio -> Automatic]
>
> circle fitting and display with points:
>
> fit circle:
>
> hypersphereFit[data_List] :=
>  Module[{mat, rhs, qq, rr, params, cen},
>   mat = Map[Append[#, 1] &, data];
>   rhs = Map[#.# &, data];
>   {qq, rr} = QRDecomposition[mat];
>   params = LinearSolve[rr, qq.rhs];
>   cen = Drop[params, -1]/2;
>   {Sqrt[Last[params] + cen.cen], cen}]
>
> display circle (using Graphics[Circle[]] inputing the results from the=
  =

> above
> function) and points:
>
> Show[Graphics[
>   Circle[Part[hypersphereFit[points], 2],
>    Part[hypersphereFit[points], 1], {0, 1}]], plot]
>
> now i really want to be able to display the residuals for the points, =
 =

> but can't
> figure out how to.
>
> If it is any help i use the following code when displaying residuals f=
rom
> a quadratic curve fit:
>
> quadratic fit:
>
> func = Fit[points, {1, x, x^2}, x]
>
> define range of plot:
>
> Insert[Part[Sort[points], {1, -1}, 1], x, 1]
>
> out: {x, 2.1305, 3.04856}
>
> copy and paste this output into the range for the Plot function:
>
> Show[Graphics[
>   Plot[func, {x, 2.1305, 3.04856}]],
>  ListPlot[points, PlotStyle -> {Red, PointSize[Small]}], Axes -> True,=

>   AspectRatio -> Automatic]
>
> this shows the quadratic fitted line with the points
>
> then calculate and display the residuals for the quadratic fit:
>
> res = Transpose[{First /@ points,
>     Last /@ points - (func /. x -> First /@ points)}];
> ListPlot[res]
>
> I would really, really like to be able to do this residuals calculatio=
n
> and display for the circle fit.
>
> Any help would be much appreciated,
>
> William Parr
>
>



-- =

DrMajorBob at bigfoot.com


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