MATLAB Answers

0

Why do my coeffvalues not produce a sensible result?

Asked by Nicholas Turton on 28 Aug 2019
Latest activity Commented on by Torsten
on 28 Aug 2019
I have a script that uses the fit fuction on some data using a poly55 fit type. I then use this to get the coeffvalues for the fittted curve and try to replot the surface using the coeffvalues but the result produced is orders of magnitude different from the original data.
Script attached as well as some example data.
Can anyone explain what is happening or recommend a way forward?

  0 Comments

Sign in to comment.

Products

2 Answers

Answer by Torsten
on 28 Aug 2019
 Accepted Answer

If you use the 'Normalize','on' fitting option, your polynomial has powers of (x-meanx)/stdx and (y-meany)/stdy instead of x and y.

  2 Comments

Hi Torsten,
Can you elaborate. I am new to matlab and not following fully what you mean
I have used
[Psi_d_xData, Psi_d_yData, Psi_d_zData] = prepareSurfaceData( id_ind, iq_ind, Psi_d);
[Psi_d_fitresult, Psi_d_gof] = fit( [Psi_d_xData, Psi_d_yData], Psi_d_zData, ft, 'Normalize', 'on' );% Fit model to data.
Psi_d_coeffvals = coeffvalues(Psi_d_fitresult);
curvefit.png
which when produced numel(Psi_d_coeffvals) = 21
Using the poly55 eqation
f(x,y) = p00 + p10*x + p01*y + p20*x^2 + p11*x*y + p02*y^2 + p30*x^3 + p21*x^2*y
+ p12*x*y^2 + p03*y^3 + p40*x^4 + p31*x^3*y + p22*x^2*y^2
+ p13*x*y^3 + p04*y^4 + p50*x^5 + p41*x^4*y + p32*x^3*y^2
+ p23*x^2*y^3 + p14*x*y^4 + p05*y^5
...I have then called up each of these coeffvals as well as my desired x and y values to try and reproduce the curve but only could manage the following
reproduced curve.png
Are you saying I need to calculate the mean and std values of x and y and use these in the calculation?
if you switch the mentioned option off, you can use the polynomial you defined in the calculation.
Otherwise you will have to insert (x-meanx)/stdx for x and (y-meany)/stdy for y in the calculation for your polynomial value:
f(x,y) = p00 + p10*(x-meanx)/stdx + p01*(y-meany)/stdy + p20*((x-meanx)/stdx))^2 + ...

Sign in to comment.


Answer by Nicholas Turton on 28 Aug 2019

Thanks Torsten, it worked perfectly
function output_fit = fittingcurve(x_data, y_data, coeffvals)
x_mean = mean(x_data(1,:));
x_std = std(x_data(1,:));
y_mean = mean(y_data(:,1));
y_std = std(y_data(:,1));
output_fit = ...
coeffvals(1) ...
+ coeffvals(2).*((x_data-x_mean)./x_std) ...
+ coeffvals(3).*((y_data-y_mean)./y_std) ...
+ coeffvals(4).*((x_data-x_mean)./x_std).^2 ...
+ coeffvals(5).*((x_data-x_mean)./x_std).*((y_data-y_mean)./y_std) ...
+ coeffvals(6).*((y_data-y_mean)./y_std).^2 ...
+ coeffvals(7).*((x_data-x_mean)./x_std).^3 ...
+ coeffvals(8).*((x_data-x_mean)./x_std).^2.*((y_data-y_mean)./y_std) ...
+ coeffvals(9).*((x_data-x_mean)./x_std).*((y_data-y_mean)./y_std).^2 ...
+ coeffvals(10).*((y_data-y_mean)./y_std).^3 ...
+ coeffvals(11).*((x_data-x_mean)./x_std).^4 ...
+ coeffvals(12).*((x_data-x_mean)./x_std).^3.*((y_data-y_mean)./y_std) ...
+ coeffvals(13).*((x_data-x_mean)./x_std).^2.*((y_data-y_mean)./y_std).^2 ...
+ coeffvals(14).*((x_data-x_mean)./x_std).*((y_data-y_mean)./y_std).^3 ...
+ coeffvals(15).*((y_data-y_mean)./y_std).^4 ...
+ coeffvals(16).*((x_data-x_mean)./x_std).^5 ...
+ coeffvals(17).*((x_data-x_mean)./x_std).^4.*((y_data-y_mean)./y_std) ...
+ coeffvals(18).*((x_data-x_mean)./x_std).^3.*((y_data-y_mean)./y_std).^2 ...
+ coeffvals(19).*((x_data-x_mean)./x_std).^2.*((y_data-y_mean)./y_std).^3 ...
+ coeffvals(20).*((x_data-x_mean)./x_std).*((y_data-y_mean)./y_std).^4 ...
+ coeffvals(21).*((y_data-y_mean)./y_std).^5;
reproduced curve.png

  0 Comments

Sign in to comment.