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\section{Circle parameters from three points}
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\label{App:SciFiThreePointCircle}
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A circle in the plane $z=0$ may be parameterised as:
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( x - X_0 )^2 + ( y - Y_0 )^2 = \rho^2 \, ;
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where $(X_0, Y_0)$ is the position of the centre of the circle and $\rho$
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(x^2+y^2) - 2 X_0 x - 2 Y_0 y = \rho^2 -( X_0^2 + Y_0^2 ) \, ;
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\frac{(x^2+y^2)}{\rho^2 -( X_0^2 + Y_0^2 )} -
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\frac{2 X_0 x}{\rho^2 -( X_0^2 + Y_0^2 )} -
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\frac{2 Y_0 y}{\rho^2 -( X_0^2 + Y_0^2 )} = 1 \, .
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The circle may be parameterised:
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\alpha(x^2+y^2) + \beta x + \gamma y + \kappa = 0 \, ;
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\alpha & = & \frac{1}{\rho^2 - ( X_0^2 + Y_0^2 )} \, ; \\
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\beta & = & -2 X_0 \alpha \, ; \\
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\gamma & = & -2 Y_0 \alpha \, ; \\
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These equations are readily inverted to yield:
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X_0 & = & \frac{-\beta}{2 \alpha} \, ;
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Y_0 & = & \frac{-\gamma}{2 \alpha} \, ;
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\frac{\beta^2 + \gamma^2}{4 \alpha^2}
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- \frac{\kappa}{\alpha}
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The equation of a circle passing through three points $(x_i,y_i)$,
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where $i=1,2,3$ can be found from:
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x^2+y^2 & x & y & 1 \\
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x_1^2+y_1^2 & x_1 & y_1 & 1 \\
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x_2^2+y_2^2 & x_2 & y_2 & 1 \\
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x_3^2+y_3^3 & x_3 & y_3 & 1
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which can be re-written as:
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x_1^2+y_1^2 & y_1 & 1 \\
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x_2^2+y_2^2 & y_2 & 1 \\
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x_1^2+y_1^2 & x_1 & 1 \\
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x_2^2+y_2^2 & x_2 & 1 \\
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x_1^2+y_1^2 & x_1 & y_1 \\
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x_2^2+y_2^2 & x_2 & y_2 \\
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x_3^2+y_3^3 & x_3 & y_3
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Comparing this relation with equation \ref{Eq:CrclPrm}:
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x_1^2+y_1^2 & y_1 & 1\\
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x_2^2+y_2^2 & y_2 & 1\\
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x_3^2+y_3^3 & y_3 & 1
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x_1^2+y_1^2 & x_1 & 1\\
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x_2^2+y_2^2 & x_2 & 1\\
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x_3^2+y_3^3 & x_3 & 1
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x_1^2+y_1^2 & x_1 & y_1 \\
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x_2^2+y_2^2 & x_2 & y_2 \\
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x_3^2+y_3^3 & x_3 & y_3
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(x + \frac{\beta}{2 \alpha})^2 +
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(y + \frac{\gamma}{2 \alpha})^2 =
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\left( \sqrt{\frac{\beta^2 + \gamma^2}{4 \alpha^2} -
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\frac{\kappa}{\alpha} }
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the position of the centre of the circle, $(X_0, Y_0)$ and its radius,
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$\rho$, are given by equations \ref{Eq:Param1} to \ref{Eq:Param3}.