Matlab:Make a contour plot with 3 vectors

Question

I have 3 vectors of data, X (position), Y (position) both of which are not regularly spaced, and Z(value of interest at each location). I tried contourf, which doesn't work because it needs a matrix for Z input.

Solution 1

You can also use griddata.

%Generate random data
x = rand(30,1);
y = rand(30,1);
z = rand(30,1);

%Create regular grid across data space
[X,Y] = meshgrid(linspace(min(x),max(x),n), linspace(min(y),max(y),n))

%create contour plot
contour(X,Y,griddata(x,y,z,X,Y))

%mark original data points
hold on;scatter(x,y,'o');hold off

Solution 2

For a contour plot you actually need either a matrix of z values, or a set (vector) of z-values evaluated on a grid. You cannot define contours using isolated Z values at (X,Y) points on the grid (i.e. what you claim you have).

You need to have the generating process (or function) provide values for a grid of (x,y) points.

If not, then you can create a surface from nonuniform data as @nate correctly pointed out, and then draw the contours on that surface.

Consider the following (random) example:

N = 64; % point set
x = -2 + 4*rand(N,1); % random x vector in[-2,2]
y = -2 + 4*rand(N,1); % random y vector in[-2,2]

% analytic function, or z-vector
z = x.*exp(-x.^2-y.^2);

% construct the interpolant function
F = TriScatteredInterp(x,y,z);

t = -2:.25:2; % sample uniformly the surface for matrices (qx, qy, qz)
[qx, qy] = meshgrid(t, t); 
qz = F(qx, qy);

contour(qx, qy, qz); hold on; 
plot(x,y,'bo'); hold off

The circles correspond to the original vector points with values (x,y,z) per point, the contours on the contours of the interpolant surface.