Plotting contour lines at specific value

I have two functions f1 and f2 defined as follows:

f1(x) = x_1^2 + x_2^2 - 4

f2(x) = 5(x_1 - 1)^2 - x_2 - 3

where x = (x_1,x_2)^T.

Now I want to plot the contour lines of these functions at value 0, that is, I want to plot the functions

x_1^2 + x_2^2 - 4 = 0

5(x_1 - 1)^2 - x_2 - 3 = 0

I tried using contourplot, but no luck there. Is there a smarter way, for example using Linspace?

Answer 1

[Matt Fig]

Please re-paste your code and use the code format button so we can read it. I see a bunch of ? marks in the code.

When using the CONTOUR function, you can provide an argument which tells the function where to draw the contours.

Here is a way to get a surface and contours in one plot:

[x_1,x_2] = meshgrid(-10:.1:10,-10:.1:10);
f1 = @(x_1,x_2) x_1.^2 + x_2.^2 - 4;
f2 = @(x_1,x_2) 5*(x_1 - 1).^2 - x_2 - 3;
subplot(2,1,1)
surfc(x_1,x_2,f1(x_1,x_2))
subplot(2,1,2)
surfc(x_1,x_2,f2(x_1,x_2))

To get just the surface, use the SURF function.

[David]

I can only see contour(X,Y,Z,n), where n is the number of lines, not there specific z-value.

[Matt Fig]

From the documentation:

CONTOUR(Z,V) and CONTOUR(X,Y,Z,V) draw LENGTH(V) contour lines

at the values specified in vector V. Use CONTOUR(Z,[v v]) or

CONTOUR(X,Y,Z,[v v]) to compute a single contour at the level v.

[David]

I have no idea how I missed that option. Is there a simpler way to do this, if for example if I just wanted to plot the curve

x_1^2 + x_2^2 - 4 = 0

Is it possible to plot the curve, without using contourplot?

[Matt Fig]

See my answer above.

[Mohammad Sohail]

@David try this

syms x y

ezplot(x^2+y^2-4)

[Avinash Kumar]

thanks

Answer 2

[kanekal kullayappa]

hello sir, I need the function to obtain the plots in the joint tdoa and aoa location algorithm http://www.jseepub.com/EN/article/downloadArticleFile.do?attachType=PDF&id=11834.