function S=sphermod(x,p)
% SPHERMOD a m-function to calculate the semivariance using a spherical
%       model. This function is used in conjunction with NLLEASQR.M to
%       provide a non-linear least square fit to emprically derived 
%       semivariance estimates (from the data).
%
% SYNTAX:
%       S=sphermod(x,p)
%
% The parameter order is as follows:
%       p(1)=nugget
%       p(2)=sill
%       p(3)=range
%
% Started 1999:05:12 D. Glover, WHOI
% Modif'd 1999:05:13 DMG to improve the "help" instructions
% Modif'd 1999:05:14 SCD correct "roll-over" and typos
% Modif'd 1999:08:27 DMG & SCD correct S_far

S_near=p(1)+(p(2)-p(1))*((1.5*x/p(3))-(0.5*(x/p(3)).^3));
S_far = p(2);

S = S_near.*(x<p(3)) + S_far.*(x>=p(3));