CODEWITHSUNDEEP
matlabcurve-fitting
Ono
Ono

Reputation: 1357

polyfit() not following the data

Here is my code:

clear all
close all
clc


%% abdomen

apAbdWat = [5.7 7.4 11.2 14.9 18.6 22.4 26.1 29.8 33.6];
latAbdWat = [7.7 10 15    20   25    30   35 40    45];

apTisAbd = [8.9 11.4 13.9 15.9 18.4 22 24.9 30.7];
latTisAbd = [10.6 14 18 20.6 24 30 32.4 38.9];

apAnthAbd = [20.8];
latAnthAbd = [28.3];

figure
plot(apAbdWat, latAbdWat, '-+'); hold on;
plot(apTisAbd, latTisAbd, 'r-o'); hold on;
plot(apAnthAbd, latAnthAbd, '-k*')

xlabel('AP diameter (cm)')
ylabel('LAT diameter (cm)')
title('AP diameter vs. LAT diameter (abdomen phantom) ')
legend('water abdomen', 'tissue abdomen', 'anthrop abdomen');

x = [apAbdWat apTisAbd apAnthAbd];
y = [latAbdWat latTisAbd latAnthAbd];

[p,S,mu] = polyfit(y,x,1);
hold on; t = 1:40;
plot(t,p(1)*t+p(2), 'g-')

Here is what I have got: enter image description here

Shouldn't the green line follow the rest of all the points pretty well? I cannot figure out where I did wrong. thanks.

Upvotes: 1

Views: 1105

Answers (2)

NKN
NKN

Reputation: 6424

your getting the wrong variable, you may edit your code as follows:

p= polyfit(y,x,1);
t=1:1:40;
hold on;plot(x,polyval(p,x),'r');

enter image description here

Upvotes: 1

Luis Mendo
Luis Mendo

Reputation: 112659

You have two issues:

  • x and y are interchanged in your line containing polyfit.
  • The three-output version of polyfit doesn't give a polynomial in x. To obtain a polynomial in x you use the two-output version.

From the documentation (format added to highlight the two above issues):

POLYFIT Fit polynomial to data.

P = POLYFIT(X,Y,N) finds the coefficients of a polynomial P(X) [...]

[P,S] = POLYFIT(X,Y,N) returns the polynomial coefficients P and a structure S [...]

[P,S,MU] = POLYFIT(X,Y,N) finds the coefficients of a polynomial in XHAT = (X-MU(1))/MU(2) where MU(1) = MEAN(X) and MU(2) = STD(X) [...]

So, change your line

[p,S,mu] = polyfit(y,x,1);

to

[p,S] = polyfit(x,y,1);

Resulting image:

enter image description here

Upvotes: 1

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