栅格法建立地图环境

MATLAB R2014B:
a = [0 1 0 0
    0 1 1 1
    0 0 0 1
    0 1 1 1];
colormap([0 0 0;1 1 1])
imagesc(a)
set(gca,'XTick',0.5:size(a,2)+0.5,'YTick',0.5:size(a,1)+0.5,...
    'XTickLabel','','YTicklabel','','dataaspect',[1 1 1],...
    'XGrid','on','YGrid','on','GridColor','k','GridAlpha',1)

如果用pcolor：
a = [0 1 0 0
    0 1 1 1
    0 0 0 1
    0 1 1 1];
b = a;
b(end+1,end+1) = 0;
colormap([0 0 0;1 1 1]),pcolor(b)
axis image ij off

每个格的中点分别是1.5 2.5 3.5 4.5 。怎么让它显示1 2 3 4
a = [0 1 0 0
    0 1 1 1
    0 0 0 1
    0 1 1 1];
b = a;
b(end+1,end+1) = 0;
colormap([0 0 0;1 1 1]),pcolor(0.5:size(a,2)+0.5,0.5:size(a,1)+0.5,b)
set(gca,'XTick',1:size(a,2),'YTick',1:size(a,1))