if ~exist(monterey_map)
    load('Monterey_Topo');
end;

[ii jj] = size(monterey_map);

m = monterey_map;
dx = m(:, 1:(jj-2)) - m(:, 3:jj);
dy = m(1:(ii-2), :) - m(3:ii, :);
ds = dx(2:(ii-1), :) + dy(:, 2:(jj-1));

pcolor(dx);
colormap(bone);
shading interp