function a = av2(a)
%AV2	grid average function.  
%       If A is a vector [a(1) a(2) ... a(n)], then AV2(A) returns a 
%	vector of averaged values:
%	[ ... 0.5(a(i+1)+a(i)) ... ]  
%
%       If A is a matrix, the averages are calculated down each column:
%	AV2(A) = 0.5*(A(2:m,:) + A(1:m-1,:))
%
%	TMPX = AV2(A)   will be the averaged A in the column direction
%	TMPY = AV2(A')' will be the averaged A in the row direction
%
%	John Wilkin 21/12/93

[m,n] = size(a);
if m == 1
	a = 0.5 * (a(2:n) + a(1:n-1));
else
	a = 0.5 * (a(2:m,:) + a(1:m-1,:));
end