fitting circle to data

[Gastón Araguás]

Hello,

i need to extract the radius and centre points of a circle fitting a
set of datas. I was trying with this way

xt = %data
yt = %data

F = @(x,p) sqrt( p(3).^2 -(x-p(1)).^2)+p(2)
pin = [1;1;1]

[f, p] = leasqr(xt,yt,pin,f);

testing it with xt = 2+4*cos(thetas) and yt = 3+4*sin(thetas) works well, i get
p =

   2.0000 + 0.0000i
   3.0000 + 0.0000i
   4.0000 - 0.0000i

but using the real data i get
p =

   -2.23630 -  0.62042i
   12.24122 - 64.02140i
    0.34923 + 59.63389i

and should be aprox. p = [0, 300, 290]

If i use pin =  [0; 300; 290] get
p =

    -2.2316 -   1.5828i
    12.2604 - 162.9057i
     0.3318 + 161.1838i

is that right? how souhld i use this comples result?
Thank you

PD: here a few xt and yt datas
xt =

   29.94340
   28.45820
   27.19490
   25.85900
   24.73510
   23.54580
   21.88660
   19.90880
   17.77030
   15.64780
   13.04540
   10.67190
    8.43594
    6.35659
    4.31690
    2.28645
    0.19964
   -1.74150
   -3.66550
   -5.75025
   -7.75397
...
 yt =

   13.277
   13.079
   12.952
   12.747
   12.670
   12.638
   12.475
   12.225
   12.059
   11.977
   11.773
   11.676
   11.556
   11.526
   11.454
   11.415
   11.394
   11.449
   11.444
   11.491
   11.556
...



-- 
Gastón Araguás
______________________________________________________
CIII - Centro de Investigación en Informática para la Ingeniería
Univ. Tecnológica Nacional Facultad Regional Córdoba. Argentina