function err = err_fit(y,u,D,G,N,n,var)

%used in var_spher.m

%INPUTS: y = [r,a] variables
%        u = unit vector
%        D = Distance values
%        G = Variogram estimates
%        N = bin counts
%        n = number of distinct pairs
%      var = sample variance

%calculate Theta

r = y(1);

a = y(2);

theta = min(D,r)/r ;

THETA = N'*theta/n ;

v = a + max([0,(var - a)])*theta/THETA ;

%v = spher(x,D);

err = sqsum(u,G,v) ;