# poly2D fit terms: secret decoder ring

Hi,

When doing a poly2D fit of order n, how do I make the correlation to a specific term in W_Coef wave.

For example if a 3 order fit, I would get a wave with 10 points and I understand that k0 is the constant term. But how do the other points correspond to the terms in my fit?

CurveFit/NTHR=0/TBOX=768 poly2D 3, RealX /X={ImageX,ImageY} /D

Is there a standard way to assign the outpoint points in the w_coef wave?

Thanks

Andy
Have a look at the details section of
`displayhelptopic "poly2d"`
for the in- and output of the parameters as well as to generate the 'fit result'.
HJ
Hi,

Thanks for the insight on poly2d, that solved my immediate problem. However, the problem with solving the immediate problem is that you get a follow up problem.

I did the fit below and when I use poly2D with the coefficient wave I can get the predicted values and compare to the actuals. This is good.

Now for coefficients K0-K5 which of the polynomial term do they represent? Which are the linear terms, the squared terms, the cross term?

CurveFit/TBOX=768 poly2D 2, D268_Temperature /X={Air_Temperature,Solar_Radiation} /D
fit_D268_Temperature= poly2D(W_coef,x,y)
W_coef={61.639,-2.9922,-0.076167,0.064866,0.0026198,-1.1798e-05}
V_chisq= 3448.81;V_npnts= 728;V_numNaNs= 0;V_numINFs= 0;
V_startRow= 0;V_endRow= 727;
W_sigma={13.7,0.893,0.0142,0.0144,0.000432,3.88e-06}
Coefficient values ± one standard deviation
K0 =61.639 ± 13.7
K1 =-2.9922 ± 0.893
K2 =-0.076167 ± 0.0142
K3 =0.064866 ± 0.0144
K4 =0.0026198 ± 0.000432
K5 =-1.1798e-05 ± 3.88e-06

Thanks
hegedus wrote:

Coefficient values ± one standard deviation
K0 =61.639 ± 13.7
K1 =-2.9922 ± 0.893
K2 =-0.076167 ± 0.0142
K3 =0.064866 ± 0.0144
K4 =0.0026198 ± 0.000432
K5 =-1.1798e-05 ± 3.88e-06

Quoting the help file that was suggested above:

"Among coefficients for a given degree, those for terms having higher powers of X are first. Thus, poly2D returns, for a coefficient wave cw:
f(x,y) = cw[0] + cw[1]*x + cw[2]*y + cw[3]*x^2 + cw[4]*x*y + cw[5]*y^2 + ..."

So, your fit means f(x,y) = 61.639 - 2.9922*x - 0.076167*y + 0.064866*x^2 + 0.0026198*x*y - 1.1798e-5*y^2.
Hi,

Here is an idea: Can the data label for the W_Coef contain the term of the fit? Thus when I send the W_coef to a table with dimension labels it is straight forward to know which is which.

Andy