The triangulation of unit disk is not consistent with that of rectangular area,

for example

mesh Th=square(64,64,[-4+8

x,-4+8y]),

the spatial step size is

8/64=1/8,

So the error convergence order is

log(errorh)-log(errorh/2) /log2;

However, according to the division of the circle

real r=1;

real a=0,b=0;

border C(theta=0, 2pi){x=a+rcos(theta); y=b+r*sin(theta);}

// Build the mesh from boundary

mesh Th = buildmesh(C(10));

How can we get the error convergence order without knowing the size of the spatial step?