The GRIDDATA function is trying to use the data in x,y,vv to fit a surface and then interpolate this surface at the points specified in X and Y.
The problem here is that x and y were created with RAND, so they are on the range 0<x<1 and 0<y<1. The points specified in X and Y are on the range 1<X<100 and 1<Y<100. So the range over which we are trying to interpolate has no defined values from the range that the surface was calculated.
A quick way to get some reasonable results would be to modify line 2 to be:
x = 100*rand(10000,1); y = 100*rand(10000,1);
