How to plot a 3D cfit funktion in a 2d plot with isolines.

Hi i have the following problem.

I fitted a curve with fit:

it worked and that is my result:

     Linear model Poly23:
     sf(x,y) = p00 + p10*x + p01*y + p20*x^2 + p11*x*y + p02*y^2 + p21*x^2*y 
                    + p12*x*y^2 + p03*y^3
     Coefficients (with 95% confidence bounds):
       p00 =       23.25  (-4.782, 51.29)
       p10 =      -13.85  (-28.85, 1.159)
       p01 =    -0.02275  (-0.0584, 0.01289)
       p20 =       5.615  (-2.615, 13.85)
       p11 =    0.006377  (-0.005635, 0.01839)
       p02 =   7.943e-06  (-6.068e-06, 2.195e-05)
       p21 =   -0.002272  (-0.004391, -0.000152)
       p12 =    7.47e-07  (-1.605e-06, 3.099e-06)
       p03 =  -5.769e-10  (-2.288e-09, 1.134e-09)

       

So now i want to plot the funtion. In 3D its easy (just plot(sf)),

but i want to have it in 2D with isolines:

       sf(x,y)=1200
       sf(x,y)=1660
       etc.

How can i do that.

contours seems not to work and plot3 either.

in the attachement is the sfit 1x1 for sf.

Greetings from Germany

Answer 1

[Divyajyoti Nayak]

Hi @Christian Beigl,

From what I understand, you are trying to plot contours of different values for the surface defined by ‘sf.mat’. Like you mentioned, the ‘contour’ function cannot be used directly on ‘sf’, but we can recreate the fitted polynomial using the coefficients stored in ‘sf’ and then use the ‘contour’ function to get the isolines. Here’s a simple function to do this:

clc
clear

load 'sf.mat'
figure(1)
plot(sf);
isolines([20,30,40,50],sf)
function isolines(vals,sf)
    X = linspace(0,4);
    Y = linspace(0,4000);
    [x,y] = meshgrid(X,Y);
    %Recreating the fitted surface
    z = sf.p00 + sf.p10*x + sf.p01*y + sf.p20*x.^2 + sf.p11*x.*y + sf.p02*y.^2 + sf.p21*x.^2.*y + sf.p12*x.*y.^2 + sf.p03*y.^3;
    figure(2)
    contour(x,y,z,vals);
end

Here are the results: