How can I draw the contour/rectangle on the plot?

Hi,

I have grid files a and b both have 23*15 size. I wanna plot b over a to show the boundary of the b data. I wrote below code for this but the contour based on b data show over a is not seems correct. Can anyone please help me to solve this issue?

b(b < 0.417) = 0;
b(b > 1.832) = 0;
figure ('Name', 'b plot over a')
pcolor(a);
axis tight;
cptcmap('test.cpt','mapping','scaled');
shading interp;
colorbar('eastoutside');
caxis ([0 6]);
hold on
contour(b,[0.417 1.832],'LineStyle','--','LineWidth',1.5,'LineColor','k');
view(0,90);

I attached the mat file and the figure with this post.Note that I use above command for the attached figure.

Thanks